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There are a great many styles of roofs, and many roof systems. Some differ remarkably in both appearance and construction, while others vary only in relatively minor details. In fact, it would take an entire book just to discuss roofs. In theory, just about any kind of roof can be applied to a log building. In practice, how ever, the ones actually used are few in number and their construction is comparatively simple and straightforward. The more important ones will be considered here.
The slope or slant of a roof above a horizontal line is called the pitch of the roof. Pitch is the number of inches the roof rises divided by the number of feet of run that the roof covers. The rise is the vertical distance between the wall top and the roof peak. The run is the distance between the lowest supporting point and the highest supporting point. For example, consider a 20-foot-wide building with a shed roof.
The run of the roof is the distance from the out side of the front wall to the outside of the back wall (eave overhangs don’t count), and the roof run is therefore 20 feet. If the leading edge of the roof is elevated 20 inches above the rear edge, this is called a 20-inch rise. Because the 20-inch rise takes place over a run of 20 feet, it is a 1-inch-per-foot or 1-inch-in-i-foot rise, which is called a one-in-one pitch or simply a one-pitch (Fig. 8-1).
For comparison, consider a 20-foot-wide building with a gable or peaked roof, and the ridge running along the centerline of the building. Here the roof run is not 20 feet, but only 10 feet, the distance between the lowest and the highest supporting points. If the roof peak is elevated 20 inches above the lower edges, or the wall tops (again, eaves don’t count), this is still a 20-inch rise but over a run of only 10 feet. Thus, the amount of rise is 2 inches for every foot of run, and this roof would be designated a two-in-one pitch or a two-pitch (Fig. 8-2).
Fig. 8-1. A roof with a one-in-one pitch.
Fig. 8-2. A roof with a two-in-one pitch on each side.
If the roof peak were 40 inches above the wall level, the rise would be 40 inches over a run of 10 feet for a four-in-one pitch, or simply four-pitch. If the roof were perfectly flat, a style seldom if ever used on log buildings, it would be a zero-pitch roof.
The business of roof pitch is important from several standpoints. One is style and appearance, because the arrangement and slope of the roof sections is one of the major architectural design features of a building. More importantly, however, there are some purely practical matters involved. The degree of pitch determines the rapidity and ease with which a roof can shed rainwater or snow, which in turn affects weather-ability and tightness. Pitches from one-in-one to about three-in-one shed water fairly well but hold snow. From four-in-one up, water runs off rapidly, and from about eight-in-one or ten-in-one on up, snow will usually zip right off as well, depending upon the composition of the roof surface, the makeup of the snow, and the weather.
The pitch of a roof also determines to some degree the manner in which it can be built. A flat roof, for instance, must be timbered and braced to withstand the heavy loads and stresses imposed upon it, including its own massive weight. The steeper the roof, the less of a problem this becomes. Steeply sloping roofs can be built with economical beams and/or truss bracing to add strength. They shed loads more rapidly and easily, and strains and stresses are partially transmitted to the remainder of the structure rather than being sustained by the roof frame and supporting members.
On the other hand, the steeper the roof the greater the cost of construction, though there are various balance points depending upon exactly what materials are used and how, and upon what the loading factors are. A flat roof, for instance, usually has just about the same area as the floor area beneath it. A flat roof over a 1000-square-foot floor area, for example, would be just about 1000 square feet itself, or perhaps a little more if there were slight over hangs around the building perimeter. But as the roof pitch increases, so does the roof area— therefore the amount of material needed to build the roof frame and cover it. A twelve-in- one simple gable roof is almost 1½ times as large as a flat roof on the same building, disregarding overhangs.
There is another factor involved, too, and that is the cost of labor. Or, if you are building the roof yourself, the time and effort required. From zero pitch to about four-in-one, a roof is easy to work on. You can walk around, and tools and materials don’t skitter off to the ground when you set them down. Above four- in-one the work becomes increasingly harder, until finally the workers must move slowly and cautiously and proceed with the aid of elaborate scaffolding, roof jacks, bosun’s chairs, slings, safety lines, and material-feed systems.
Another point to keep in mind is that the steeper the roof and the larger the area, the more exposure there is to the weather. This means several things to the homeowner. First, the roof is a huge radiator that throws off heat from the inside during the cold weather, and absorbs heat into the structure in the warm weather. The larger the roof, the greater the insulating problems, and the greater the heating and cooling loads. Second, the roof is the part of the house that suffers the most wear and tear from weather. After a time the roof surface will actually wear out, sooner or later depending upon the material used. The larger the roof, the greater the expense will be to replace the weather surface, and the greater the amount of possible maintenance needed in the meantime. Third, the larger and more complex the roof, the greater will be the wind loads and the susceptibility of the roof to wind damage. The smaller the roof, the less resistance it offers. And last, the larger the total roof area, whether steep or shallow, the greater the likelihood of mechanical damage and leaks.
The pitch of the roof also determines to some extent the type of finish material—the weather surface—that can be put on it. For in stance, slate or tile is used only on steeply pitched roofs. Some kinds of metal roofing can be used on pitches as low as two-in-one; others are suitable for four-in-one and up, but not be low. Shingles of any kind should not be used below a four-in-one pitch. Double-coverage roll roofing can be used down to two-in-one pitch. Flat and low-pitched roofs must be covered with cross-lapped layers of roofing felt bonded together with hot tar and topped with roofing stone.
DIMENSION-STOCK ROOF FRAME
This type of roof used on log houses is no different than is found on conventional platform-framed houses. The entire framework is made box-frame fashion from dimension stock, usually 2 x 8s, 2 x 10s, or 2 x 12s, de pending upon the span, pitch, and loading factors of the roof. Rafter spacing is usually 16 or 24 inches on centers.
About the simplest roof construction is involved where only one unbroken roof section is needed, with no attached additional roof sections going off in different directions. All that is required is a continuous series of rafters ex tending from the ridge or peak at the top to the supporting plate resting atop the walls (Fig. 8-3). These full-length rafters are called common rafters, and there are two methods of in stalling them.
Fig. 8-3. In a simple pitched-roof construction only common rafters are used.
For the first method, place a ridge board in. position, establishing the longitudinal centerline of the roof peak. This consists of a 1- x -8 or wider board stood on edge and running from the highest point of one end wall to the other. Several boards are usually needed to make up the full length, and they are propped and se cured in place by whatever means are handiest. Stand up the first rafter, with both ends cut to the appropriate angles, and with one end resting on the plate and the other placed flat against the ridge board. The rafter is aligned by means of marks previously made, and nailed in place at top and bottom. The opposite rafter is raised in the same manner, nailed at the bottom and toe-nailed to the ridge board and into the opposing rafter tip (Fig. 8-4).
Fig. 8-4. The rafters are attached to the ridge board using this method.
The second method dispenses with a ridge board and the rafters are placed in sets or pairs. Cut the proper mating top angles first, as well as the bottom angles where the rafters rest upon the plate. Join the rafter tips together and securely nail them while they are lying on the second-level floor, or up on sawhorses. Temporarily nail a couple of boards across the rafters to form a large A. Drag the pair of rafters into position so that the bottom ends rest in approximately the right spots on the plates, and tip the A up to a vertical position. It is aligned and the bottom ends are nailed to the plates, with the top held in place by means of temporary props. When the second set is swung up into position, it is held in alignment with its neighbor by nailing two or three boards across the outside faces of the rafters (Fig. 8-5). As subsequent sets are erected, they too are held by temporary boards. As the roof sheathing is put on, the temporary boards and props are removed.
There are several methods of attaching the rafter bottoms to the plates. The most common is to cut a bird’s-mouth notch at an appropriate point on the underside of each rafter. The angles of the notches are calculated to rest exactly upon the plate for a given roof pitch. The plate must be flat on the top and outside surfaces, which in log construction means the plate log must be two-flailed (Fig. 8-6), or suit able notches must be cut in a round log. The portion of the rafter that extends outward behind the wall is called the tail, and this length can be varied to suit the desired amount of roof overhang. This could be as little as 6 inches, allowing just enough room to trim out, or could be as much as 3 or 4 feet.
Fig. 8-5. One method of erecting rafter sets made without a connecting ridge board.
Fig. 8-6. A bird’s mouth cut into the rafter allows it to sit squarely upon the top plate.
Fig. 8-7. This shave-cut rafter seats fully on a flatted plate log.
Another method of attaching rafters is to make half-cuts or shaves at the rafter tails, with a flat angle-cut which rests on the plate (Fig. 8-7). Toe-nail the rafter to the plate just as though a bird’s mouth were used, and there is no diminution of strength or structural effectiveness. The only appreciable difference is that the apparent thickness of the roof and the trim board of fascia that runs along the ends of the rafters and closes them off, is reduced by the amount of the shave cut.
If the rafter does not continue beyond the plate and is “tail-less,” as would be the case if a porch roof were to be later attached to form a continuous roof line, different methods of rafter seating are used. This involves making bobtail cuts in one fashion or another (Fig. 8-8) so that the rafter ends are flush with or slightly inset from the exterior face of the wall upon which they rest.
Fig. 8-8. Bobtail rafter cuts can be made in these three ways.
Fig. 8-9. Mockup of a typical roof frame construction shows the various component parts.
Where another section of roof angles off in a different direction, the framing construction gets a bit more difficult. To make an outside corner, a hip rafter must be installed, and in making an inside corner a valley rafter must be installed. These rafters run from appropriate points at the ridge to the corner locations at the plates. Hip jack rafters extend from the plate to the hip rafter, while valley jack rafters run from the valley rafter to the ridge board. As usual, the common rafters extend clear from the ridge board to the plate wherever there are straight-through runs (Fig. 8-9). These different rafters are attached top and bottom in the same way as common rafters, with their ends cut to whatever angles (sometimes they are compound) are necessary for a flat, tight fit.
Fig. 8-10. One way to construct a lookout ladder that will provide an extended roof rake.
Fig. 8-11. A collar tie beam, which is usually a board rather than an actual beam, is set in the upper third of the rafter peak.
Occasionally a roof structure is cut off short so that it extends beyond the outer surface of the end walls by an amount equal to only the thickness of a trim board or two. More often, however, a substantial overhang is desired, sometimes as great as that of the eaves. In a conventional roof frame this is accomplished by building a lookout ladder (Fig. 8-10).
To make a lookout ladder, a series of short pieces of dimension stock called lookouts are secured at right angles to the rafters. These form a ladder extending beyond the end wall of the building, providing the overhang or ex tended rake. Lookouts are often made from 2 x 4s which are attached at one end to the first inboard rafter, and are toe-nailed in place where they pass over the top plate of the end wall. The lookouts must be flush with the rafter tops, and the end-wall plate must be calculated to lie at the correct height. Wide overhangs might require the use of heavier stock, and either notching or shimming at the end-wall plate can be done if necessary to properly align the lookout ladder. An outside rafter of the same size as the common rafters is then nailed to the outboard ends of the lookouts. In some cases, a lighter piece of nominal 1-inch stock is substituted for the outside rafter, to act as a trim board.
In some designs it might be necessary or desirable to stiffen up the roof frame, especially if the rafters span a considerable distance or if the roof is expected to carry a heavy snow load. This can be easily done with very little expense or effort by adding collar-tie beams. These beams, which are actually made of dimension stock, are nailed to the faces of the rafters at opposite ends, and are placed horizontally in the upper third of the triangle created by the rafter pair (Fig. 8-11). In an unfinished attic 1- x -6 boards are adequate, installed on each third or fourth rafter set. In a finished attic, make the collar-tie beams of 2 x 4s or 2 x 6s, depending upon the span, and place them high enough to provide sufficient headroom. Here they should be installed on each rafter set, to serve later as the structural framework for a finished ceiling. In effect, they become joists.
Fig. 8-12. Frieze blocks are installed between rafters and on top of the plate to close the gaps.
In most construction methods the rafters lie atop the plates and the rafter tops extend above the plate tops by several inches. When the roof sheathing is applied, a large gap re mains between the plate top and the sheathing undersurface, between each pair of joists. If the underneath portion of the eaves is to be left open and uncovered, these gaps must be filled in. Even if the roof overhang will be boxed in with soffit panels, the gaps are best closed off and sealed to help keep out cold drafts and in sects. This means that a series of frieze blocks must be fitted carefully into each opening. With closed eaves, pieces of dimension stock can be cut and fitted with beveled or angle-cut top edges to match the roof line, and toe-nailed into place (Fig. 8-12). Run a full bead of caulk around either inside or outside, or both, to seal the blocking off. Lay another bead of caulk along the top surface of the blocking as the roof sheathing is laid, to seal that portion.
If the eaves are open to view, dimension- stock frieze blocks can also be used and will be just as effective, but certainly won’t present the same appearance as the log wall. In this case, cut and fit short sections of log, flatted on the bottom to match the plate log and beveled off on the top to match the roof pitch.
A roof frame made up entirely of log rafters can be made in almost the same fashion as a dimension-stock frame. The minimum rafter diameter should be 6 inches, and larger logs must often be used. The spacing commonly chosen is either 24 or 30 inches, though in small and steeply pitched roofs this might be stretched to 36 inches. Full flatting is not essential, but cutting at least a narrow flat (2 inches or so) along the top makes for easier laying of roof sheathing. Likewise, if the under side of the rafters is to be later covered with some sort of sheathing or finish materials, a narrow flat is helpful here.
Log rafters can be installed one by one against a ridge board using the same method as with the dimension-stock roof frame, or can be erected in sets or pairs without benefit of the ridge board. Installation at the plate end is also the same, using either the bird’s-mouth notch, half-cut method, or bobtail. There is also another method that can be used, though it doesn’t have the strength of the others. This involves leaving the rafter log round and cutting an angled saddle notch into the plates. This method is sometimes used with fully round logs in small buildings. Each rafter log is spiked directly to the plate log.
The other aspects of a log-rafter roof frame are also pretty much the same as for a dimension-stock frame. For an extended rake, build a lookout ladder of logs extending from the first inboard rafter across the end-wall plate to whatever distance beyond the outer wall surface is desired. The lookouts can be made from small logs or be the same size as the rafter logs or the wall logs, and spaced according to your liking. They might rest on the end-wall top, be notched in, or actually extend through the end wall. The underside of the extended rake can later be enclosed or not, and a trim board can be attached to the outer lookout ends or not— as you wish.
Collar-tie beams of saplings, poles, full logs, split logs, or dimension stock can be in stalled as necessary. All gaps between plate logs and the roof undersurface, whether along sidewalls or at the end walls, should be filled with frieze blocks of logs or dimension stock and sealed with caulk. For a tight fit, notch the blocking into the log rafters or lookout sides.
The purlin roof is a traditional old construction once widely employed but nowadays seldom seen except on log houses. The purlin roof presents an attractive appearance when left open to view from inside the building. It is also a strong and simple construction that can be put together in several different ways.
The simplest type is a small and short roof assembly using purlins only. The purlins run lengthwise of the roof and are supported by the end walls (Fig. 8-13). In most log construction they are notched into or pass through the end walls to form the basis for an extended rake. They are spaced 3 or 4 feet apart up the roof slope, and in 10-inch diameter can span as much as 20 feet, longer for greater diameters. A ridge log is located at the peak, and that’s all there is to the frame. The roof sheathing is applied directly to the purlin and ridge log tops. There are no plate logs as such, and the top logs of the sidewall courses are bevel- flatted to match the roof pitch. The sheathing is nailed directly to them. Boards are most often used for this purpose, running at right angles to the purlins; a double layer is sometimes laid for extra strength. End-wall logs are treated in the usual way, with their ends cut to match the roof pitch and the sheathing nailed down tight, sealed with caulk.
As the roof becomes larger and the spans longer, some sort of support is needed for the purlins. One way of providing support is to position tie logs at right angles to the purlins and attach them to the sidewalls at ceiling height. These tie logs can be spaced approximately every 10 or 12 feet. The next step is to secure small logs from the tie beam tops directly up ward to the purlin bottoms. The best method of attachment is with large mortise-and-tenon joints at each end, secured through the middle with a pin, peg, or bolt. A similar support post can be run from the tie beams to the ridge log (Fig. 8-14). This arrangement works nicely where the tie beam span is not too great, say 20 feet or less, depending upon the diameter of the tie beams.
Fig. 8-13. Log purlins and ridge can be set into gable ends and projected to provide an extended roof rake.
Fig. 8-14. Side-supported tie logs hold support poles for the purlins and ridge log in this construction.
Where the spans are greater the positions of the upright supporting logs can be shifted somewhat and additional members added to form trusses. There is quite a variety of possible configurations, depending upon the number of purlins and the spans involved. One of the simplest is a large W (Fig. 8-15), where the tops of the outer arms of the W bisect the roof and the bottom ends trisect the tie beam. Another possibility is an M-truss, and the center- post truss is also widely used (Fig. 8-16). The Y-truss (Fig. 8-17) is another configuration, and the post-and-tie truss arrangement (Fig. 8-18) is effective and relatively easy to build.
Truss systems of this sort can be built with either logs or beams, and are generally installed in open-plan houses where they are a visible part of the interior design (Fig. 8-19). They can also be made of extremely large members and spaced 25 or 30 feet apart, allowing upper-level living quarters to be constructed between and around the heavy timbering. Smaller and more closely spaced trusses obviously eliminate the possibilities of having usable upper-level living space and so are relegated to cathedral ceiling spaces or unfinished attics.
Fig. 8-15. A typical W-truss arrangement.
Fig. 8-16. One type of center-post truss assembly as used on wide-span buildings.
Another method of setting a purlin roof involves both purlins and log rafters. This consists of first erecting a series of log rafters that are larger in diameter and spaced farther apart than ordinary common rafters would be, but in stalled in the same way. They can be cut to project past the plate logs if desired, resting upon the plate logs with a bird’s-mouth notch, or can be cut to mate flush with the plate logs and the outside wall surface. The diameter of these logs can run from 10 inches up with a spacing of 6 or more feet, depending upon span and size, and the diameter of the purlins that will be used. The purlins are then set at right angles across the main rafters and lock-notched at each crossing point (Fig. 8-20). The purlins can be as small as about 5 inches in diameter if the spans are very short, but 7 to 8 inches would be considered more adequate. If the spans are long the diameter should be greater. A spacing of 3 to 4 feet is about right, and the roof sheathing can be applied directly to the purlins.
Fig. 8-17. A typical Y-truss arrangement.
Fig. 8-18. A typical post-and-tie truss arrangement.
Fig. 8-19. A beam truss arrangement used with a purlin log roof frame.
Fig. 8-20. Heavy log main rafters and ridge with smaller purlin logs.
Fig. 8-21. A log purlin truss assembly with a log ridge and double tie beam chord.
For larger roofs this system can be modified. Instead of running the main rafters out to the sidewall plates, they are shifted into a truss configuration. This involves placing one or a pair (one above the other) of tie beams from wall to wall. The main rafters are then set upon the tie beams just to the inside of the walls, making the tie beams the bottom chord of the truss. A center post is erected from the center of the tie beams to the peak of the rafters. The purlins are then attached lengthwise to the rafters. A ridge log is placed at the peak (Fig. 8-21).
Fig. 8-22. Log purlins with log or dimension-stock rafters above.
Yet another method of building a purlin roof is to first install large purlin logs from end wall to end wall, supported or trussed or not as necessary. These are large-diameter logs, and in small roofs there might be only one on each side, with a ridge log at the peak. Rafter logs then are run from plate to ridge log, notched and spiked at crossing points (Fig. 8-22). These rafter logs should be a minimum 6-inch diameter and spaced about every 3 to 4 feet, depending upon the spans involved.
Note that any of these constructions can be strengthened in a number of ways. Diameters can be increased, spans shortened, or both; ridge logs or main rafters can doubled; trusses can be beefed up with additional members; rafter spacing can be increased or decreased, as can their size; and where purlins are the uppermost members, another full complement of rafter logs can be attached to them as closely spaced as necessary.
Dormers and log houses seem to complement one another, and they are frequently incorporated in 1½- or 2-story designs having gable, gambrel, or other pitched styles of roofs. They are sometimes made with flat or shed-type roofs, and sometimes with gable roofs. Either type can originate at the main roof peak, or in steeply pitched main roofs, at some intermediate point.
A dormer can be as narrow as 3 feet, just enough to frame in a window, or in a shed-roof style might run almost the full length of the house. The main purpose of one or more small dormers is to admit light and afford ventilation in an under-roof living area—such as would be found in a story-and-a-half structure—while at the same time increasing the available usable floor space and overall spaciousness to a small extent. Large dormers, especially the full- length variety, perform the same functions to a much greater degree and also can expand the living quarters to virtually a full-sized second floor.
In log house construction, dormers are commonly framed with dimension stock materials, but they can also be log-framed. This is largely a matter of individual preference as to appearance, both interior and exterior. In either case, the process starts with a framed hole of appropriate size and shape in the main roof structure. This framing is carried out in the usual manner. Then the dormer itself is framed above the opening. Figure 8-23 shows typical dimension-stock dormer framing in gable- roofed style.
By eliminating the header and shortening the jack rafters, the ridge of the dormer could tie directly to the main ridge; this would move the entire assembly back up the roof. Or, the side and front studs could be made taller and the top plates extended, making a higher and longer dormer. The dormer ceiling could follow the gable roof line, or joists could be run across from plate to plate and a lower ceiling attached to them. The design is completely flexible. In a shed-roofed dormer, a series of rafters would originate at the ridge, or at the header of an opening lower down the roof, and extend to the front top plate of the dormer face.
Fig. 8-23. A typical framing arrangement for a gable-roofed dormer built with dimension stock.
The roof construction methods for a dormer are the same as for any other roof. To frame with logs, simply use logs or poles of appropriate diameter for the spans involved, and follow the same general principles for placement of the various structural members. To build up the walls of a dormer from logs, use the framed- out main roof opening as a starting point and build up the walls in the same fashion as the main walls of the structure. For both convenience and appearance, logs of smaller diameter than the main wall logs are often used for this purpose.
Once the roof frame is complete you can go about the task of laying the roofing. This consists of four principal elements: the sheathing, underlayment, flashing, and the weather or finish surface. There are several methods of applying each, and numerous materials that can be employed.
Fig. 8-24. Flat-sawn roofing boards or planks should be placed heart side up; shrinkage and cupping is less of a problem in this position.
In a standard roof (as opposed to a built-up roof, which will be discussed later), the sheathing is applied first, directly to the rafters or purlins. The old method of sheathing with boards is still very much in use, and is the most practical method for one person working alone.
Relatively inexpensive 1- x -6 S3S stock is often used for the purpose of sheathing, laid at right angles to the rafters and pushed tightly together. Tongue-and-groove boards can also be used, as can wide boards, though they are more susceptible to cup warping. Regardless of the board width or kind, they should be applied with the heart side up, because cupping of flat-sawn boards takes place invariably in the opposite direction (Fig. 8-24). This helps to re duce the amount of cupping and also eliminates the series of sharp ridges that would appear if the boards were laid heart side down.
In most roof constructions the boards are laid horizontally in rows up the roof. However, in roofs where the purlins are uppermost, the boards run at right angles to the purlins, in the vertical direction. This is an advantage in that full-length boards often can be used to span the roof top to bottom, eliminating joints and in creasing strength and stiffness. Boards can also be applied diagonally, on either rafters or purlins, for increased structural rigidity (Fig.
Fig. 8-25. Roofers can be laid horizontally or diagonally on rafters (left), but are laid vertically on purlins (right).
The method of applying sheathing boards tucked tightly together is called closed sheathing. Open sheathing is another method that is used in some parts of the country, often where the weather surface is composed of cedar shingles or shakes. This involves laying the first three rows of boards, starting at the eave edge, tightly together. These are usually nominal 1- x -6 stock. Subsequent boards may be of the same width but are more often 1- x -4 stock.
These are laid with a gap between each board equal to the amount of shingle surface left ex posed to the weather in each course (Fig. 8-26).
Closed sheathing may also be comprised of nominal 1- x -6 shiplap or tongue-and-groove boards. These fit tightly together and lock into place, forming a solid and rugged surface that is somewhat less susceptible to cupping. Also, when shrinkage occurs, cracks are less likely to open up between the boards. In the instances where the underside of the sheathing will be visible from the interior of the building and doubles as a ceiling, nominal 2- x -6 tongue- and-groove decking planks can be fitted with the best face down, just as with a decked floor. This makes an exceptionally strong roof assembly, and the planks can be applied over a frame- work of widely spaced joists or purlins without loss of strength or stiffness.
Perhaps the most widely used sheathing material nowadays is plywood. The large sheets can be laid rapidly and efficiently by a crew of workers, and an entire roof can be covered in short order. Plywood sheets are laid with the face grain at right angles to the sup porting members, with all sheets offset from each other (usually by halves) so that no joints are coincidental. A 1/16-inch joint space should be left at all end joints and a Vs-inch gap at all edge joints.
An exterior grade of plywood is generally used for the entire roof surface, but in fact an exterior grade could be used along only the eave and rake edges, with interior-grade panels laid everywhere else. Where the rafter spacing is 16 inches or less on centers, a plywood thickness of ¾-inch is adequate and allowable under most building codes. However, most builders prefer the ½-inch thickness. As far as mini mums are concerned, that thickness is also suitable for rafters on 24-inch centers. Either ½- or ¾-inch thicknesses can be used on rafters with 32-inch centers, and ¾-inch on 42-inch centers. For 48-inch centers, either ¾-inch or 7 plywood is suitable. Rafters spaced up to 6 feet can be handled by special 1¼-inch sheets, provided additional blocking is in stalled to support all free edges.
Nailing of panels that are up to ½ inch thick is done with either 6d common smooth nails or ring-shank nails. Up to a 1-inch thickness, nail with 8d nails of either type. Nails should be spaced 6 inches apart around the perimeter and 12 inches apart at intermediate supports.
Fig. 8-26. When applying roofers with the open method, lay the first three or four planks up from the eave edge abutted together.
With the sheathing all nailed down, the next step is to apply the underlayment. Roofing underlayment is a dry felt material that has been saturated with asphalt and comes in roll form. It is called roofing felt, and is designated by its weight per square (100 square feet). A number of weights are available, but those most often used for underlayment purposes are 15-pound and 30-pound. If the roof pitch is four-in-one or more, either weight of roofing felt is applied in continuous horizontal rows, starting at the eave edge and progressing up the roof, with a 2-inch top overlap at each seam. The felt is nailed down at all edges with roofing barbs (nails) of suitable length and spaced about every 6 inches (Fig. 8-27).
Fig. 8-2 7. Apply roofing felt underlayment in this manner when the roof pitch is four-in-one or steeper.
Fig. 8-28. Apply roofing felt underlayment in this manner when the roof pitch is less than four-in-one.
If the roof pitch is less than four-in-one, either weight of roofing felt (but preferably the 30-pound) is laid up in much the same arrangement but with a greater overlap. The laying starts with a 19-inch-wide strip along the eave edge. This is followed by a full-width (36-inch) row of felt that completely covers the starter strip. Succeeding strips are laid with a 19-inch top overlap until the sheathing is fully covered (Fig. 8-28). In all cases, side-laps should be a minimum of 4 inches wide and there is no harm whatsoever in making them a foot or more. The nailing pattern is the same as for the steeper pitches.
The next step in roof-building involves the installation of a metal drip edge along the eaves. You might find this material available at your local lumberyard, or you might have to have it made up at a sheet metal shop. Drip edge is a narrow band of metal (the width varies), about 2 or more inches of which rest upon the roof surface, with a narrower strip bent downward and outward at the eave edge, forming a lip to direct dripping water out and away (Fig. 8-29). The drip edge is aligned and nailed in place with as few short nails as will hold it safely in place. For extra insurance, you can coat the entire underside of the drip edge that bears on the roof with roofing tar in order to seal it down firmly, and then daub each nail head with more tar.
Fig. 8-29. Metal drip edge should be installed along the leading edge of the roof sheathing.
For roof pitches of four-in-one or less, yet another layer should be added. This is some times called the selvedge, and consists of a strip of smooth-surface mineral roll roofing, which comes in 36-inch-wide rolls and is generally used in a 90-pound weight. Nail this strip in place along the eave edge, aligned with the outer edge of the drip edge. Place one row of nails about 6 inches apart at the up-roof edge first, and another row spaced the same but back about 8 inches from the drip edge. The same treatment, incidentally, can be applied to roofs of steeper pitches for a bit of additional protection against water and ice backup at the eaves.
Metal drip edge is also sometimes applied along the rake edges. There is another method, however, that is perhaps easier, more attractive, and less expensive. It involves nailing a standard cedar or redwood clapboard along the rake edge with the thick edge of the clapboard aligned with the roof edge and the thin edge inboard (Fig. 8-30). This diverts water from the rake edges and channels it back down the roof to drip from the eaves.
The next consideration is flashing. Flashing is a method of completely sealing roof joints that probably would open up with time and commence to leak if only caulked or plastered over with a sealant. The material used most often is sheet metal, but in some instances a heavy asphalt-impregnated paper or felt may be laid. Some types of flashing can be installed entirely before the weather surface is put on, while others are installed concurrently as the point of application is reached. Either way, flashing details are best sorted out ahead of time.
The chief points that require flashing are valleys where down-sloping roof sections join to form a natural watercourse, chimneys, pipes of any sort, vent stacks or hoods, skylights, trap doors or windows, and the junction points of dormer or other walls. Eave edges are also sometimes flashed with metal from the edge upward for 3 to 5 feet, with the weather surface beginning from 1 to 3 feet from the eave edge.
Asphalt roofing materials can be used to flash closed valleys, but all other installations where any part of the flashing is exposed to the weather should be metal or flexible membrane, such as butyl rubber or neoprene. Lead is seldom used any more but copper is, despite its cost. This is the longest-lived and most easily worked of all the flashing materials. Galvanized steel sheeting is probably the most popular, with aluminum running close behind.
Fig. 8-30. A clapboard installed between the underlayment and the weather surface, along the rake edge, channels runoff down to the eave.
Whichever metal you choose, stick with it all the way. Sheets of different metals should not be laid overlapping one another because they will quickly corrode from a process called electrostatic action. For the same reason, cop per nails must be used with copper flashing, and aluminum nails with aluminum flashing. Galvanized nails can be used with zinc, galvanized steel, or lead.
Valley flashing is done one of two ways. The first is open, where the center of the valley is exposed and the edges of the weather surface are set back 2 or 3 inches or more on each side of the valley centerline. The other is closed, where the weather surfaces are butted tightly together along the centerline of the valley, or perhaps interleaved, and the valley flashing is covered and invisible.
Closed valley flashing can be applied using 90-pound mineral-surfaced roll roofing. Cut an 18-inch-wide strip and nail it length wise up the centerline of the valley, 9 inches to one side and 9 inches to the other. The nails should be roofing barbs on 4-inch centers and 1 inch in from the outside edges. Paint a 3-inch stripe of roofing cement down each outer edge of this strip, and cover with a 36-inch-wide strip of the same material. Nail as before, and add a second row of nails 4 inches in from the outside edges, also on 4-inch centers but staggered from the first row (Fig. 8-31). If the flashing is applied over the underlayment, paint a 3-inch-wide strip of roofing cement under the outside edges of both strips as they are laid.
Metal flashing, whether for an open or a closed valley, is applied in the same manner but with a single layer of metal. In an open valley a single long strip of metal is fitted (or several top-lapped strips) and extends about 2 feet to either side of the centerline of the valley (Fig. 8-32). Overlaps should be about a foot and preferably 18 inches. Be sure that each overlap is a top-lap, and that you don’t inadvertently make a bottom-lap that would funnel water inside the roof.
Fig. 8-31. A method of flashing a valley with roofing felt.
Fig. 8-32. A valley flashed with metal strips.
Fig. 8-33. Valley flashing can be interwoven with the shingles as laying proceeds.
Fig. 8-34. A typical roof jack installation.
Closed valley flashing is done the same way for solid or sheet-type weather surfaces, but if desired can be done differently when any of the various forms of shingles are applied. Individual pieces of flashing 18 inches wide, and as long as the diagonal dimension of the shingle when cut to meet the valley centerline, are cut and bent to an appropriate V-angle to match the valley angle. These individual pieces are then interwoven with the course of shingles as they are nailed up (Fig. 8-33). The shingle nails hold the flashing in place and the material, if trimmed properly, is invisible.
Most items such as roof vents and ventilating units have their own built-in flashing bases that are interwoven with the shingles or other materials as work progresses. The object is to have the upper two-thirds or so of the flashing plate lie beneath the weather surface, sealed in place with a layer of roofing cement, and the lower portion above the weather surface for proper water-shedding. Pipes can usually be fitted with standard flashing units called flashing sleeves or roof jacks. These consist of a lower flashing plate attached to a tall collar that fits snugly around the pipe and is sealed to it by means of a curled lead sleeve, silicone caulking, or a special expandable jacket of neoprene or a similar substance (Fig. 8-34). Round stainless or galvanized steel appliance chimneys so often used nowadays with wood stoves and fireplaces have special flashing sleeves and storm collars made to fit each chimney diameter and are adjustable for roof pitch. All of these sleeves and jacks are installed so the flashing plate lies with the upper two-thirds or more below the weather surface, and the remainder above.
There are a number of methods of flashing masonry chimneys, used concurrently with the laying of the weather surface. Sometimes a special saddle or cricket is installed, and some times pieces of flashing are interwoven with the bricks as the chimney is made and later with the shingles as they are laid up. Water proof roofing cement is used liberally, and a base of heavy asphalt roll roofing is sometimes installed first. As usual, the object is to shed all moisture away from the chimney joints and down over the weather surface of the roof. However, unlike a standard flashing unit as used in frame construction, a slip joint must be provided. Typically this is accomplished with a two-part system, a base flashing and a cap flashing. The two are not joined and are free to slip by one another (Fig. 8-35).
Fig. 8-35. One method of installing chimney flashing with a slip joint to allow for settling.
Fig. 8-36. Flashing must be applied at all roof/wall intersections.
Flashing is also required where the edge of a roof meets a higher sidewall. Here individual pieces are cut and interleaved with the shingles as the courses are laid up, and attached to the sidewall sheathing before the finish siding is applied. If the weather surface is a solid material such as roll roofing, a continuous strip of flashing is installed against the roof surface and up against the sheathing of the sidewall (Fig. 8-36). The flashing should ex tend at least a foot in each direction. Dormer sides are handled in exactly the same way, and the dormer front is flashed with a continuous strip of metal laid against the roof surface and bent up against the dormer sheathing with a long wrap around the corners. The dormer front should be flashed first, with the corners sealed completely with roofing cement. The side flashing is then wrapped around the front in downward folds.
After all of this preliminary work, the finish roofing or weather surface seems almost an afterthought. Don’t treat it as such because it is important. The weather surface should be care fully selected to provide you with a long-lived surface that will need a minimum of maintenance and also complement the structure in the way of color, shadow-lines, pattern, and overall appearance. Pay the price and buy only top-grade, high-quality materials, preferably with a guarantee or bond.
Today you have a wider selection than ever from which to choose. The traditional roofing for a log house is cedar shakes, and cedar shingles look nearly as nice. Many log houses are fitted with sheet-metal roofing, of which there are many varieties and colors in both steel and aluminum; the newer baked-on finishes hold up very well. Single- or double- coverage mineral-surface roll roofing is another possibility, but is more often used on vacation camps than primary residences, perhaps be cause of its plain appearance. Probably the most popular of all roofing materials are the various kinds of three-tab asphalt shingles. There is a broad range of colors available, as well as numerous patterns and weights. The weight is calculated in terms of so many pounds per square (100 square feet), and the shingles are so designated. The 235-pound weight is a common one; the heavier the shingle, the longer it is likely to last. They are easy to apply, especially for one person working alone.
There are several other choices, too, that are less common but well worth looking into, especially if you would like to have something a little different. Slate, for example, is still available, but requires a well-engineered roof structure to hold the weight. You might choose vinyl or aluminum shingles, which look much like three-tabs but are longer lived. Clay tile roofing, which is extremely long-lived and available in numerous colors and configurations, is staging something of a comeback and is popular in the Southwest. Like slate, it needs a rugged roof structure. One expensive but handsome and very long-lived alternative is porcelainized steel shingles. They are finished with a fused-on ceramic coating—glass, actually—and are available in several shapes and colors. There are new roofing materials coming along all the time that also merit attention, such as the composite wood fiber shingles that look like hand-riven shakes and come attached to long backing strips for ease of installation.
Pre-installation requirements can be obtained from the manufacturers, and complete instructions for installation are usually included with the products. For best results and to keep warranties valid, follow all of the requirements to the letter. Suppliers usually have specifications and instructions on hand that you can investigate before buying.
There are two distinctly different kinds of built- up roofs widely used in the construction industry today. The older type is a weather surface of a roof that has zero or low pitch, consisting of multiple cross-lapped layers of heavy roofing felt and hot tar, topped with a layer of stone chips, or some similar variant of this construction. The newer definition applies to roofs usually of four-in-one pitch or greater that are built up above the sheathing layer with one or successive layers of thermal insulating material and spacers, with the weather surface lying on top.
Because of the high value put on thermal insulation today in both cold and hot parts of the country, this latter type of roof has become most important. It is primarily used wherever the underside of the roof sheathing is open to view and is actually the finish ceiling. Or, there might be a thin finish ceiling layer applied, with logs or beams left mostly visible, which allows no room for the substantial thickness of insulation so often needed.
Of all the housing styles, log houses probably most often have roof/ceilings open to view in all or at least part of the building. Add to this the fact that very few areas in the country can get by without heating or cooling, and it becomes obvious that many log houses must have built-up roofs of one sort or another.
There are a number of ways to go about making a built-up roof, depending upon the amount of thermal insulating value needed and the type of insulation chosen. One type is actually a cross between a standard roof and a built-up roof, in that it makes use of large squares of thick, edge-matched fibrous insulating roof decking applied directly to the rafters. The underside is prefinished for the ceiling effect, or a separate finish ceiling covering can be applied. The weather surface can be applied directly to the decking in some instances, while in others nailing strips are necessary.
Most built-up roof construction, however, begins on top of the roof sheathing, which may be either boards, plank decking, or plywood. The first system discussed here starts with a layer of sheathing, which in this case becomes sub-sheathing, laid over the rafters or purlins. A series of dimension-stock nailers is then in stalled horizontally or in a grid across the roof, spaced at 24-inch intervals. They are stood on edge and the width of the nailers must be equal to or greater than the total thickness of the thermal insulation required. Roll or ball fiberglass or mineral wool insulation is bedded in the spaces, and the whole roof is covered with another layer of sheathing. The remainder of the roofing materials are applied in the usual fashion (Fig. 8-3 7). If purlins are involved, the nailers must be run vertically.
Another method makes use of rigid thermal insulation, which is available in large sheets of various thicknesses. This material is lightweight and a little fragile, and is glued or tacked carefully to the roof sheathing. Another layer of sheathing is added over the rigid insulation and nailed through to the rafters (Fig. 8-38). The sheets should be filled tightly together, and preferably glued with construction or other recommended adhesive along the joints. The outer edges of the insulation are protected with nailer strips laid around the perimeter of the roof. The remainder of the roofing materials are installed in the usual way.
With both of these built-up roofs, a vapor barrier should be installed over the subsheathing and under the nailers and/or insulation. The most effective material for this is thin plastic sheeting from 3 to 6 mils thick, often called construction plastic. Either the black or the transparent is fine. Note, however, that plastic sheeting is terribly slippery when dry and absolutely lethal when wet. Use extreme caution when climbing around and working on a plastic-covered pitched roof. And remember, the more slits and punctures there are in the plastic, the less effective it is as a vapor barrier. Carry a roll of duct tape right along with you, and make repairs as you spot any damage.
The top layer of sheathing in both styles of built-up roof or the insulating decking roof can be closed and solid or can be open, with the sheathing boards spaced the same distance apart as the extent of shingle that will be ex posed to the weather. The sheathing boards can be nominal 1-inch stock, but 1 x 3s are generally adequate in most installations, and are the least costly.
SKYLIGHTS AND ROOF WINDOWS
There is nothing new about skylights—they have been used in houses all over the country for decades. But for some reason the have recently taken the house-building public by storm and have become all the rage. There are two important things to know about skylights. One is that they can dispense expensive heat to the outdoors, and conversely, gather unwanted heat in hot country. The other is that they can be potential trouble spots; if not in stalled exactly right, they are bound to leak sooner or later. On the other hand, properly arranged skylights can admit comforting solar heat in cold weather, and they can indeed be trouble-free if installed correctly. And, they can be most attractive and frequently serve to in crease ventilation, visibility, and available light. It is worth noting, too, that there are several brands of specially built full windows that can be installed in many types of roofs, provided the pitch is not too low.
The best approach to skylights or roof windows is no to cobble up your own, but to purchase top-quality brand name units. Then follow the manufacturer’s installation instructions to the letter. Better yet, let a professional do it; this is the course most likely to give you a permanent and trouble-free installation.
Fig. 8-37. A cross-section of a built-up roof filled with blanket insulation.
Fig. 8-38. A cross-section of a built-up roof thermally protected with rigid insulation.
If you do want to make your own skylights, this is basically how it’s accomplished: First, a suitable opening must be framed into the roof frame. This is done in just the same way as making an opening in a floor frame, as discussed in Chapter 5. Then a box-like sleeve must be built up around the interior of the opening, projecting a minimum of 6 inches above the roof weather surface, preferably about a foot if you live in snow country. The projecting sleeve must then be completely flashed and sealed into the roof weather surface, much like a pipe or vent is, so that leaks are virtually impossible. On a low sleeve the flashing should be wrapped completely up and over the top rim of the sleeve. On tall sleeves, run the flashing up at least halfway and cover the entire outside of the sleeve with an appropriate exterior siding material. Make liberal use of roofing cement and/or silicone or other currently recommended caulk. The skylight it self is fastened to the top of the sleeve securely and with a weathertight seal. The unit can be flat glass of one sort or another, or a plastic dome or “bubble”—check with your glass sup plier for recommendations on locally available units or materials.
Various modifications can be made to a simple skylight, such as making it openable, installing double or even triple glazing to re duce heat loss, and setting in a lower pane of frosted glass to reduce the effects of direct sun light but still admit a good quantity of diffused light. You can also use any of the several kinds of special coated glazing materials, or install a vent fan within the skylight assembly. You might also cover the bottom of the skylight well with frosted or opalescent glass or plastic, with a design painted on the upper side and lights installed above. Thus the skylight transmits light by both day and night, and also serves as an unusual accent point in the interior decor.
Under most circumstances proper venting is a necessary part of the building construction. Ventilation in both soffit and attic areas serves to eliminate condensation in winter and also helps with cooling in the summer. Proper ventilation in an unheated attic can reduce or eliminate water backup from ice dams forming along the eaves. Ventilation is also required by FHA regulations and many building codes as well.
The usual requirement is that there be 1 square foot of ventilating area for every 150 square feet of ceiling lying below the attic floor. If the ceiling is covered with a full vapor barrier, the requirement can be reduced by half to 1 square foot for every 300 square feet. The venting area can be provided by placing ventilators or louvers in the gables or end walls, or by installing roof vents. Exhaust fans can be added for a forced draft.
The soffit areas are sometimes open into the attic area, and screened vents or louvers can be installed in them to work in conjunction with gable or roof ventilators. Where the soffits are closed off from the attic area they should be vented for their own protection from condensation buildup. This can be accomplished with small screen vents or louvers, or by making the soffit of perforated hardboard or metal panels made for the purpose. Plug louvers, which are tiny round louver heads made to be pushed into a 1-inch-diameter drilled hole, also work well if you install enough of them. The rule of 1 square foot of ventilation area to 150 square feet of ceiling area doesn’t really apply in this case, though you can if you wish substitute soffit area for ceiling area in the formula. The usual practice is simply to install enough vents so that there is obvious free air movement through all the soffits all the time.
Fig. 8-39. One method of trimming out the eave edge of a built-up roof.
The finish trim is the final step for completion of the roof assembly, and can be put on last, or all or part can be installed as the roof construction proceeds if it happens to be convenient to do so. There is no point in setting up ladders or scaffolding just to install a bit of trim, if it can be done earlier.
Most of the trim work takes place around the eaves and rakes, and the different possibilities are practically endless. The projecting rafter tails can be left completely alone with no further work done except for making sure that all the joints are fully sealed. If there is a built- up roof atop the rafters, a trim board is installed along the bottom edge to close the roof section. An additional strip of molding might be added as well at the top of the trim board and directly beneath the sheathing to dress out a slight edge projection (Fig. 8-39). The rafters remain open.
Whether the roof is built-up or not, you could also nail a fascia of 1-inch stock all the way across the faces of the rafter tails. The tails themselves can be cut at right angles, perpendicular to the ground, or with beveled bottom cuts (Fig. 8-40). The bottom of the fascia might be square, or cut in scallops or some other con figuration. The underside of the rafters might be covered with soffit panels to form a completely boxed cornice. The soffit panels can be attached directly to the rafter bottoms so that the soffit pitches upward at the same angle as the roof, or the panels can run straight across and attach to a nailing strip mounted on the wall. Molding strips can be added at the wall/soffit joint, at the soffit/fascia joint, or at the fascial roof sheathing joint (Fig. 8-41). Some of these treatments get pretty fancy and include carved bulls-eyes, ornate Swiss chalet-type or namentation, gingerbread trim reminiscent of the Victorian era, notched-and-toothed edge trim, and a thousand other odds and ends.
The situation is basically the same at the rakes or roof ends. If there is no appreciable roof overhang, there might be only a single- thickness trim board applied along the wall at the wall/roof sheathing joint. An extended rake can be left open, and often is where log purlins protrude through the end walls. Or, the ex tended roof can be boxed in completely, or just faced with a trim board across the open ends. Again, you can carry out whatever decorative trim-work you like.
Fig. 8-40. Three popular ways of trimming rafter ends.
Fig. 8-41. Two methods of enclosing rafter ends and the under-eave area. There are dozens of possible trimming-out details that could be added.All DIY Log Home/Cabin articles