Gable, Gambrel, and Shed Roofs Unit 47 Gable, Gambrel, and Shed Roofs Gable Roofs • Gambrel Roofs • Shed Roofs • Dormers

A gable roof slopes in only two directions. Next to the shed roof, which has only one slope, a gable roof is the simplest type of pitched roof to build because it slopes in only two directions. See Figure 47‑1.

Framework for a gable roof includes ridge board, common rafters, and gable studs. The basic structural members of a gable roof are the ridge board, common rafters, and gable studs. See Figure 47‑2.

A bird’s mouth is formed by the heel plumb cut line and seat line. Plumb cuts are made at the ridge, heel, and tail of a common rafter. A seat cut, or level cut, is made where the rafter rests on the top wall plates. The notch formed by the seat and heel plumb cut lines is often referred to as a bird’s mouth. See Figure 47‑3.

When calculating true rise when there is a roof overhang, the total rise and stand are first added. The drop at the ridge is then subtracted. The amount of stock remaining above a seat cut, referred to as the stand or HAP (height above plate), is added to the total rise. The true rise is the total rise if there is no seat cut. The stand should be two-thirds the length of the seat cut. However, this distance will vary depending on the width of the rafter. When calculating true rise, an allowance is made for the drop at the ridge. See Figure 47-4.

The rafter table on a framing square can be used to calculate rafter lengths for pitched roofs with 2² to 18² unit rises. Framing squares typically have a rafter table printed on the face side of the blade. See Figure 47‑5. The rafter table makes it possible to determine the lengths of all types of rafters for pitched roofs with 2″ to 18″ unit rises.

Common rafter length is calculated by multiplying the total run by the length of the common rafter per foot of run. The roof has a 7″ unit rise and a 16′ total span. See Figure 47‑6. Look at the first line of the rafter table on the framing square to find the “length of common rafters per foot of run.” Since the roof in this example has a 7″ unit rise, locate the number “7” at the top of the square. Directly beneath the number “7” is the figure “13.89,” which means that a common rafter with a 7″ unit rise will be 13.89″ long for every foot of run.

The actual length of a common rafter is calculated by deducting one‑half the ridge thickness perpendicular to the plumb line. Rafter length found by any of the methods previously discussed is the measurement from the heel plumb cut line to the center of the ridge. This is known as the theoretical length of the rafter. Since a ridge board, usually 1 1/2″ thick, is placed between the rafters, one‑half of the ridge board thickness (3/4″) must be deducted from each rafter. This calculation is known as shortening the rafter, and is done when the rafters are laid out. The actual length of a rafter is the distance from the heel plumb line to the shortened ridge plumb line. See Figure 47‑7.

A framing square is used to lay out the plumb and seat cuts. Before rafters can be cut, the angles of the cuts must be laid out. Layout consists of marking the plumb cuts at the ridge, heel, and tail of the rafter, and the seat cut where the rafter will rest on the top wall plates. The angles are laid out with a framing square or Speed® Square. A pair of square gauges is useful when using a framing square. One square gauge is secured to the tongue of the square next to the number that is the same as the unit rise. The other square gauge is secured to the blade of the square next to the number that is the same as the unit run, which is always 12″. When the framing square is placed on the rafter stock, the plumb cut is marked along the tongue (unit rise) side of the square. The seat cut is marked along the blade (unit run) side of the square. See Figure 47‑8.

A framing square or a Speed® Square can be used to lay out a common rafter for a gable roof. The roof in this example has an 8² unit rise and a 10² overhang. The same procedure is used regardless of the layout tool being used. Rafter layout also includes marking off the required overhang and making the shortening calculation. A procedure for laying out a common rafter is shown in Figure 47‑9. A Speed® Square can be used to lay out the angled cuts on a common rafter. Rafter length tables are typically included in a book of rafter tables. The procedure for laying out common rafters using a Speed® Square is similar to the procedure using a framing square.

The step-off method combines the procedure of laying out the rafters with a procedure of stepping off the length of the rafter. In this example, the roof has an 8² unit rise and a total run of 5¢‑9². The step‑off method for rafter layout is an older, but still practiced, method. The step-off method combines the procedure for laying out the rafters with a procedure for stepping off the length of the rafter. Figure 47‑10 shows the step-off method. Extreme care must be used using the step-off method for calculating after length.

When constructing a gable roof, common rafters are precut and then fastened to the ridge board and wall plates in a continuous operation. The major part of gable roof construction is setting the common rafters in place. See Figure 47‑11. The most efficient method of constructing gable roofs is to precut all common rafters and then fasten them to the ridge board and the wall plates in a continuous operation.

When ceiling joists are spaced 16² OC and rafters are spaced 24² OC, every other rafter will tie into the joists. Proper layout ensures that rafters and joists will tie into each other wherever possible. Rafter locations should be marked on the top wall plates when the positions of ceiling joists are laid out. Proper roof layout will ensure the rafters and joists tie into each other wherever possible. Roof rafters are often spaced 24″ OC and ceiling joists are commonly spaced 16″ OC. An example of this type of layout is shown in Figure 47‑12.

Ridge board layout is copied from markings on wall plates Ridge board layout is copied from markings on wall plates. Butt joints for ridge boards must occur at the center of a rafter. The ridge board, like the common rafters, should be precut. Rafter locations are then copied onto the ridge board from the markings on the wall plates. See Figure 47‑13. The ridge board should be the length of the building plus the overhang at the gable ends.

When only a few carpenters are present on a job site to set the ridge board, the ridge board is secured in position with props and braces. When only a few carpenters are present on a job site, the most convenient procedure is to set the ridge board to its required height (total rise) and hold it in place with temporary props and braces. See Figure 47‑14. The rafters can then be nailed to the ridge board and the top wall plates.

When several carpenters are present to raise a gable roof, the method shown is used to set the ridge board and nail rafters for a gable roof. A faster system that can be used when a larger crew is present is shown in Figure 47‑15.

Many carpenters prefer to cut the overhang after rafters are fastened to the ridge board and wall plates. A chalk line is snapped along the rafters and a sliding T-bevel is used to lay out the tail plumb lines. Common rafter overhang can be laid out and cut before the rafters are set in place. However, carpenters may prefer to cut the overhang after rafters are fastened to the ridge board and wall plates. A chalk line is snapped from one end of the building to the other and the tail plumb lines are marked with a sliding T‑bevel. See Figure 47‑16. The rafters are then cut with a circular saw. Cutting common rafter overhang after the rafters are set in place ensures the line of the overhang will be perfectly straight, even if the building is not.

A gable end overhang can be framed with lookouts and a fascia rafter A gable end overhang can be framed with lookouts and a fascia rafter. The fascia rafter is nailed to the ridge board and fascia board. Blocking rests on the end wall and is nailed between the fascia rafter and the rafter next to it. An overhang is further strengthened when the roof sheathing is nailed to it. Another overhang is formed over each gable end of the building. The main framing members of the gable end overhang are fascia rafters, or barge rafters. Fascia rafters are fastened to the ridge board at one end and to the fascia board at the lower end. Fascia boards are often nailed to the tail ends of the common rafters to serve as a finish piece at the edge of a roof. By extending past the gable ends of the house, fascia boards also help to support the fascia rafters. Figures 47‑17 and 47‑18 show different methods used to frame gable end overhangs.

A gable end overhang can be framed with a fascia rafter supported by the ridge board, outrigger, and fascia board. Two common rafters are placed directly over the gable ends of the building. The fascia rafters are installed between the ridge board and the fascia boards. The gable studs are notched to fit against the rafter above. Another overhang is formed over each gable end of the building. The main framing members of the gable end overhang are fascia rafters, or barge rafters. Fascia rafters are fastened to the ridge board at one end and to the fascia board at the lower end. Fascia boards are often nailed to the tail ends of the common rafters to serve as a finish piece at the edge of a roof. By extending past the gable ends of the house, fascia boards also help to support the fascia rafters. Figures 47‑17 and 47‑18 show different methods used to frame gable end overhangs.

Gable studs decrease in length from the ridge section toward the walls. At each gable end, vertical members called gable studs are placed. Gable studs decrease in length from the ridge section toward the exterior side walls. A method for finding the common length difference for gable studs is shown in Figure 47‑19.

The lengths of gable studs can be calculated with a framing square. Another method, using a framing square, is shown in Figure 47‑20. The common length difference can also be calculated mathematically by dividing the gable stud spacing by the unit run and then multiplying by the unit rise ([16 ÷ 12] × 8 = 10.67″ or 10 11/16″).

A framing square can be used to find the angle of the top cut on gable studs. Combine the unit rise and 12² and mark the cut along the unit rise side. Gable studs also require an angle cut where they fit beneath a top plate or rafter. The framing square can be used to lay out the angle. See Figure 47‑21.

Gable studs provide gable wall structure and support rafters along their bottom edges. A finished gable stud will appear similar to the one shown in Figure 47-22.

A gambrel roof is essentially a gable roof with a double slope on each side. Note the walls supporting the meeting points of the upper and lower rafters. The length of the rafters for the lower portion of the roof is based on a 20² unit rise and a 4¢‑6² run. The length of the rafters for the upper section is based on a 4² unit rise and an 8¢‑0² run. Figure 47‑23 shows a section view of a gambrel roof. The meeting points of the upper and lower rafters in this gambrel roof are supported by walls.

Common rafters for the upper and lower portion of a gambrel roof are laid out differently. The cuts laid out in this illustration are based on the unit rises and runs shown in Figure 47-23. A procedure is shown in Figure 47‑24 for laying out common rafters for the gambrel roof shown in Figure 47‑23. Angles for plumb cuts are marked at the ridge, tail, and heel of the common rafters. A seat cut is marked where the rafter rests on the wall. Finally, the overhang cut is marked.

When constructing a gambrel roof, the lower portion of the roof is constructed first. If the attic space is to be used for a living area, the subfloor should be installed before the roof is constructed. If the attic space provided by a gambrel roof is to be used for a living area, the subfloor should be installed before roof framing begins. The two outside rows of subfloor panels should be held back or cut so the seat of the rafters can be easily nailed to the top of the wall plates. One recommended procedure for framing a gambrel roof is shown in Figure 47‑25.

A shed roof has common rafters with overhangs at each end. A shed roof has only one slope. See Figure 47‑26. Common rafters for a shed roof are marked on each end for seat cuts where the rafters will rest on the two opposite walls of the building. The rafters are also marked for overhang cuts on each end.

When laying out common rafters for a shed roof, the length of common rafters is based on the unit rise and total run of the roof. The length of the common rafters is based on the unit rise of the roof and the total run. The total run for a shed roof is the width of the building minus one wall thickness. A procedure for laying out a shed roof is shown in Figure 47‑27.

The construction of a gable dormer is similar to that of a gable intersecting roof. A gable dormer has a level ridge board. Most dormers are of gable or shed design. The construction of a gable dormer is similar to the construction of a gable intersecting roof. The gable dormer consists of a ridge board, common rafters, valley rafters, and valley jack rafters. See Figure 47-28.

The front wall of a shed dormer is usually directly over the exterior wall below. The rafters extend from the main ridge board. The rafters must be pitched enough to shed water and snow. The front wall of a shed dormer is usually directly over the exterior wall of a building. The dormer rafters extend from the main ridge. See Figure 47-29. They must be pitched enough to shed water and snow.