Engineering and Construction

The generic roof shapes for wood light frame buildings are shown in Figure 5.50. These are often combined to make roofs that are suited to the covering of more complex plan shapes and building volumes.

Figure 5.50 Basic roof shapes for wood light frame buildings.

For structural stability, the rafters in gable and hip roofs must be securely tied together at the top of the supporting walls by well-nailed ceiling joists to make what is, in effect, a series of tri- angular trusses. If the designer wishes to eliminate the ceiling joists to expose  the sloping underside of the roof as  the finished ceiling surface, a beam or bearing wall must be inserted at the  ridge or a system of exposed horizontal  ties must be used in place of the ceiling joists. Sometimes a designer wishes  to raise the ceiling joists or exposed ties to a higher elevation than the tops of the wall plates. This greatly increases  the bending forces in the rafters and should be done only after consultation with a structural engineer.

Although a college graduate  ar  chitect or engineer would find it difficult  to lay out all the cuts of the rafters for a sloping roof using trigonometry, a  skilled carpenter, without resorting to  mathematics, has little problem making the layout if the  pitch (slope) is  specified as a ratio of rise to run. Rise is  the vertical dimension and run is the  horizontal. In the United States, pitch  is usually given on the architectÕs drawings as inches of rise per foot (12 inches) of run. A skilled carpenter uses  these two figures on the two edges of  a framing square to lay out the rafter as  shown in Figures 5.52 and 5.58. The  actual length of the rafter is never figured, nor does it need to be, because all the measurements are made as  horizontal and vertical distances with  the aid of the square. Today, many  carpenters prefer to do rafter layout  with the aid of tables that give actual  rafter lengths for various pitches and  horizontal distances; these tables are  stamped on the framing square itself  or printed in pocket-size booklets. Also available are hand-held calcula- tors that are specially programmed to find dimensions of rafters.

Hips and valleys introduce another level of trigonometric complexity  in rafter layout, but the experienced  carpenter has little difficulty  even  here: Again, he or she can use published tables for hip and valley rafters or do the layout the traditional way, as illustrated in Figure 5.56. The  head carpenter lays out only one rafter of each type by these procedures. This then becomes the  pattern rafter from which other carpenters trace  and cut the remainder of the rafters (Figure 5.59).

In areas that are subject to hurricanes, the building codes may require  that rafters be attached to their sup- porting walls with sheet metal rafter anchors. The type, size, and spacing of the nails that attach the roof sheathing to the rafters are also closely controlled.  The intent of both of these measures is to reduce the likelihood that the roof  will be blown off in high winds. Similar to requirements for bridging in floor joists, the International  Residential Code requires bridging  for ceiling joists and rafters where the  depth of these members exceeds their  thickness by a ratio of 6 to 1 based on   nominal dimensions (for conventional solid wood framing, members deeper  than 2 12 or 38 286 mm). Where  the undersides of the joists or rafters  remain unfinished, wood strapping  nailed perpendicular to the bottoms of the joists or rafters may be used instead of solid blocking or crossbracing installed between framing members.

Figure 5.52 Roof framing details and procedures: The lettered details are keyed to Figure 5.53. The remainder of the page shows how a framing square is used to lay out a pattern rafter, reading from the fi rst step at the lower end of the rafter to the last step at the top.

Figure 5.53 Step Seven: framing the attic fl oor and roof. The outer ends of attic fl oor joists are not usually headed off, but instead are face nailed to the rafter pairs that overlap them.

Figure 5.54 A summary of the major details for the structure shown in Figure 5.55, aligned in relationship to one another. The gable end studs are cut as shown in A and face nailed to the end rafter.

Figure 5.55 Step Eight: The sheathed frame is completed. The letters A, B, and C indicate portions of the framing that are detailed in Figure 5.54.

Figure 5.56 Framing for a hip roof. The diffi  cult geometric problem of laying out the diagonal hip rafter is solved easily by using the framing square in the manner shown. Toward the bottom of the rafter shown in the lower part of this fi  gure is a birdsmouth cut, an angled notch in the rafter that allows the rafter to seat securely on the top plate of the wall.

Figure 5.57 Framing for an overhanging rake.

Figure 5.58 A framing square being used to mark rafter cuts. The run of the roof, 12 inches, is aligned with the edge of the rafter on the blade (the wider leg) of a square, and the rise, 7 inches in this case, is aligned on the tongue (the narrower leg) of the square. A pencil line along the tongue will be perfectly vertical (a plumb cut) when the rafter is installed in the roof, and one along the blade will be horizontal (a level cut). True horizontal and vertical distances can be measured on the blade and tongue, respectively. The layout of these types of cuts can also be seen in Figures 5.52 and 5.56.

Figure 5.59 Tracing a pattern rafter to mark cuts for the rest of the rafters. The corner of the building behind the carpenters has let-in corner braces on both fl oors, and most of the rafters are already installed.