1. Basic Roof Types and Roof Theory
Unit 46
Basic Roof Types and Roof Theory
Basic Roof Types • Roof Layout Principles • Structural Factors in Roof Design • Roof Sheathing

2. Shed, gable, and hip roofs are the basic types of pitched roofs.
Basic types of pitched roofs are shed, gable, and hip roofs. See Figure 46‑1. Shed, gable, and hip roofs are traditional roof designs that have proven practical over hundreds of years and are still used in all types of construction. Flat roofs are most often used for buildings of modern design.

3. Gambrel and mansard roofs are similar to gable and hip roofs except that they have double slopes on each side.
Two other types of roofs are gambrel and mansard roofs. See Figure 46‑2. Both roof types provide additional living space directly underneath. A gambrel roof is similar to a gable roof except that the slope of a gambrel roof is broken near the center of the roof, making a double slope on each side. A mansard roof is similar to a hip roof except that it has a double slope on each of its four sides.

4. Butterfly and monitor roofs are variations of the shed roof design
Butterfly and monitor roofs are variations of the shed roof design. Continuous‑slope gable roofs are a variation of the gable roof design.
Other roof styles, such as the butterfly, monitor, and continuous-slope gable roofs, are variations of the shed, gable, and hip roofs. See Figure 46‑3.

5. An intersecting roof is formed when two roofs are combined
An intersecting roof is formed when two roofs are combined. Among the many possible variations are the L‑shaped gable and L‑shaped hip intersecting roofs, and the T‑shaped hip‑and‑gable intersecting roof.
The basic types can also be combined in various ways, producing intersecting roofs. See Figure 46‑4.

6. The shape of a gable roof is based on two right triangles
The shape of a gable roof is based on two right triangles. A common roof rafter forms the third side (hypotenuse) of each triangle.
A roof that slopes in two or more directions is based on the shape of two or more right triangles. A shed roof, which slopes in one direction, is based on the shape of one right triangle. A gable roof slopes in two directions and is similar to the shape of two right triangles placed together. See Figure 46-5.

7. The unit rise of a roof is the number of inches the rafter will rise vertically for every foot of run.
The unit rise is the number of inches that the rafter rises vertically for every foot of unit run. See Figure 46‑6. As the unit rise of the roof increases, the slope of the roof becomes steeper. Unit rise is specified on the vertical leg of the slope diagram.

8. The total rise of a roof is based on unit rise and total run.
Figure 46-7 shows two examples of calculating total rise of a roof. The examples shown and explained in this unit demonstrate theoretical roof dimensions that do not include roof overhangs and ridge boards. The inclusion of an overhang and ridge board will affect the actual total rise, and is explained in detail in Unit 47.

9. Purlins and braces help support spans of longer rafters.
A roof must withstand a great deal of weight and stress. To guarantee structural strength, the dead load and live load that a roof will bear must be considered in roof design and construction. Roof rafters, ceiling joists, and bracing such as collar ties and purlins are factors in roof strength. See Figure 46‑8.

10. The allowable span is the distance from the ridge to the outside wall plates. Rafter spacing and roof slope determine the allowable span. For example, 2 × 4 rafters spaced 16″ OC have an allowable span of 8′‑0″ if the unit rise of the roof is less than 4″. The allowable span is 9′‑0″ if the unit rise is 4″ or more.
Dead and live loads have a direct effect on the allowable span of rafters used in a roof. The allowable span is the distance from the ridge to the outside wall plates. Examples of allowable spans for roof rafters are given in Figure 46‑9. Rafters for low‑pitched roofs must be able to support greater live loads than rafters for steeper roofs.

11. A slope cut must be made on the ends of ceiling joists.
Lumber used for ceiling joists is often wider than lumber used for roof rafters. In this case, a slope must be cut at the end of the joist. A framing square can be used to mark the angle of the slope, as shown in Figure 46‑10.

12. As high-velocity wind passes over a pitched roof an uplift force is created, pulling on the roof surface.
Roof and other structural damage may occur as a result of a roof separating from its supporting walls. Extreme weather conditions, such as hurricanes and tornados, may generate enough force to significantly damage a roof or possibly remove the roof from the building. As high-velocity winds pass over a pitched roof, an uplift force exerts pressure on the roof surface. See Figure If roof sheathing is not adequately fastened to the rafters or trusses, the sheathing panels may become dislodged. If the roof sheathing performs as designed and stays attached, the uplift force is transferred to the rafters or trusses and their connections to the building walls. If the rafter-to-wall or truss-to-wall connection is successful, the uplift force is transferred to the wall-to-foundation connections.

13. Rafter and truss anchors tie rafters and trusses to the building walls.
Metal connectors, such as rafter and truss anchors, are vital in connecting the wall frame to the roof rafters or trusses. Rafter and truss anchors are available in a variety of designs. Rafter anchors are nailed to the rafter and into the plates or the studs below. See Figure Truss anchors are embedded in grout in the lintel course of a CMU wall, fastened to a CMU or concrete wall with screws, or fastened to a wood-framed wall with nails. Always refer to the manufacturer instructions for proper installation and to the local building code for wall-to-roof connection requirements.

14. Panel products such as plywood and OSB are typically used for roof sheathing. Panels are attached to roof rafters using 8d common nails or screws.
Sheathing should be nailed to a roof as soon as framing is completed. Roof sheathing serves as a base for the finish roof material and also strengthens the roof structure. See Figure 46‑13.

15. Panel clips are placed at the unsupported edges of roof sheathing panels and eliminate the need for blocking between rafters. Panel clips are made of galvanized steel and accommodate all panel thicknesses used for roof sheathing.
For certain applications, rafters may be spaced farther apart than the allowable distance between rafters for the thickness of the panels. In these situations, blocking is nailed between the rafters, or panels with tongue-and‑groove edges are used. Panel clips may be used instead of blocking or tongue‑and‑groove panels. See Figure 46‑14.