1. Hip Rafters • Hip Jack Rafters • Constructing Hip Roofs
Unit 48
Hip Roofs
Hip Rafters • Hip Jack Rafters • Constructing Hip Roofs

2. A hip roof has four sloping sides
A hip roof has four sloping sides. Four hip rafters run at a 45° angle from the exterior walls. Hip jack rafters frame the space between the hip rafters and the top of the exterior walls.
A hip roof has four sloping sides. Four hip rafters run at a 45° angle from the corners of the building to the ridge board. Hip jack rafters frame the space between the hip rafters and the tops of the exterior walls. See Figure 48‑1.

3. Side cuts are required at the ridge and tail of a hip rafter.
A hip rafter travels at a diagonal (45° angle on a plan view) to reach the ridge board and is longer than a common rafter. Hip rafters differ from common rafters in other ways. In addition to plumb cuts at the ridge, heel, and tail, a hip rafter requires side cuts where it meets the ridge. Side cuts are also necessary at the tail in order for the overhang of the hip rafters to align with the overhang of the common rafters. See Figure 48‑2. The procedure for marking rafter side cuts is explained later in this unit. Layout is usually not done until the rafter lengths are calculated.

4. The unit run of a hip rafter is 17″, which is the 45° diagonal of a 12″ square.
The unit run of a hip rafter is 17″, compared with 12″ for a common rafter. Since a hip rafter runs at a 45° angle to the common rafter, its unit run is calculated using the diagonal of a 12″ square. The diagonal of a 12″ square is 16.97″, which rounds up to 17″. A hip rafter must run 17″ in order to reach the same height that a common rafter reaches in 12″. See Figure 48‑3.

5. To calculate the actual hip rafter length, one‑half the diagonal thickness of the ridge board that fits between the rafters is deducted from the theoretical length. When shortening the hip rafters, make the measurement perpendicular to the plumb line, not along the length of the roof member.
All methods for calculating hip rafter length discussed in this text use the theoretical length rather than the actual length. The theoretical length is the distance from the heel plumb cut line to the center of the ridge board. To determine the actual length, one‑half the diagonal thickness of the ridge board that fits between the rafters must be subtracted from the theoretical length. For example, the diagonal thickness of a 1 1/2″ thick ridge board is 2 1/8″. Therefore, 1 1/16″ is subtracted from the theoretical length (2 1/8″ ÷ 2 = 1 1/16″) at a right angle to the plumb line. Hip roofs may or may not be framed with king common rafters. If king common rafters are used, one-half the diagonal thickness of the common rafter must be deducted from the hip rafter. The procedure for shortening hip rafters is shown in Figure 48‑4.

6. When laying out a hip rafter, the angles for side cuts and ridge plumb cuts are marked first.
The layout procedure for hip rafters begins with marking the ridge plumb and side cuts. Next, the seat and heel plumb cuts are marked, followed by the lines showing the overhang. A procedure for laying out hip rafters using a framing square or Speed® Square is shown in Figure 48-5.

7. The rafter table on a framing square can be used to determine the exact angle for the side cut of a hip rafter. The roof in this example has a 7″ unit rise.
For a 45° hip roof, a circular saw is always set to 45°. This blade angle will produce the proper 45° angle cut in plan view. When viewed perpendicular to the edge of the rafter, the angle will lay out at less than 45°. The rafter table on a framing square can be used to determine the exact angle for the side cut of a hip rafter when laid out on the top edge of the rafter. The procedure for determining the proper angle is as follows:
1. Locate the unit rise (in this example, 7″) along the blade above the rafter tables.
2. Follow the column down to the last line, which indicates the “Side Cut Hip or Valley.” The number is 11 1/16″.
3. Using the 11 1/16″ on the tongue and 12″ on the blade, place the framing square along the edge of a piece of the rafter.
4. Mark the side cut along the 12″ side. See Figure 48-6.

8. A hip rafter overhang runs 1. 42″ for every 1″ of common rafter run
A hip rafter overhang runs 1.42″ for every 1″ of common rafter run. In this example, the run of the common rafter is 10″. To find the run of the hip overhang, multiply 1.42″ by 10. The result is 14.2″, or 14 1/8″.
Since a hip rafter runs at a diagonal, its overhang is longer than the common rafter overhang. The run of a hip rafter overhang is 1.42″ for every 1″ of common rafter overhang. Therefore, to find the run of the hip rafter overhang, multiply 1.42″ by the run of the common rafter overhang. See Figure 48‑7.

9. A framing square can be used to calculate the run of the hip rafter overhang. In this example, the run of the roof overhang is 10″.
A framing square can also be used to calculate the run of the hip rafter overhang. Take a diagonal measurement from two points that are equal to the sides of a square formed by the common rafter overhang. See Figure 48‑8.

10. Backing a hip rafter allows roof sheathing to lie in the same plane as common and jack rafters.
Chamfering the top edges of a hip rafter is called backing the rafters. See Figure 48‑9. Backing prevents roof sheathing from being higher where it covers hip rafters than where it covers common and jack rafters.

11. Dropping a hip rafter is faster than backing a rafter and accomplishes the same purpose.
Another method to prevent roof sheathing from being higher over hip rafters is dropping the hip rafters. See Figure 48‑10. The seat cut is enlarged, causing the rafter to drop. Consequently, the sheathing rests on the top corners of the rafter and is in line with the roof. Most carpenters use the dropping method because it is faster than the backing method.

12. When calculating lengths of hip jack rafters for layout beginning from the common rafter at the end of the ridge, the common length difference must first be determined. In this example, the roof has a 6″ unit rise and a 20′‑8″ span. The length of common rafters is 11′-6 5/8″. The hip jack rafters are spaced 24″ OC. The common length difference is 26 13/16″.
As shown in Figure 48-11, layout for hip jack rafter placement may begin from a common rafter at the end of the ridge board.

13. Hip jack rafter layout can begin from a common rafter located away from the end of the ridge board. In this example, the roof has a 5 unit rise and 23′-0″ span. The hip jack rafters are spaced 24″ OC. The common length difference is 2′-2″.
Layout may also begin from a common rafter located at some point other than the end of the ridge board. See Figure 48‑12.

14. When calculating hip jack rafter length beginning from the corner of a building, the length of each succeeding hip jack rafter length is increased by the common length difference. In this example, the roof has a 6″ unit rise. The hip jack rafters are spaced 24″ OC. The 26 13/16″ or 2′‑2 13/16″ common length difference shown in the rafter table is the length of the first hip jack rafter placed 24″ OC from the corner of the building.
In a third hip jack rafter layout method, layout begins at the corner of the building. See Figure 48‑13.

15. Hip jack rafters require a single side cut where they fasten to the hip rafter. In this example, the roof has a 4″ unit rise.
Hip jack rafters require a single side cut where they fasten to the hip rafter. A framing square can be used to find the angle of the side cut as viewed and laid out perpendicular to the top edge of the rafter. See Figure 48‑14.

16. A hip jack rafter has plumb cuts where it fastens to the hip rafter as well as cuts at the heel and tail.
A procedure for laying out the cuts on a hip jack rafter using a framing square or Speed® Square is shown in Figure 48‑15.

17. When calculating the theoretical ridge board length of a hip roof, subtract the total span from the total length of the building. In this example, the total span of the roof is 22′ and the total length of the building is 45′.
The main framing members for hip roofs should be precut prior to constructing the roofs. The ridge board for a hip roof must be precut to its exact length before it can be set in place. A procedure for finding the theoretical length of the ridge board is shown in Figure 48‑16.

18. Different calculations are used when a common rafter is installed at the end of a ridge board than when a common rafter is not installed at the end.
The actual length is affected by the framing method used at the end of the ridge. As previously mentioned, one method places common rafters at the end of the ridge board, whereas a second method does not. See Figure 48‑17.

19. When constructing a hip roof, the main framing members should be precut before construction begins.
Conditions that require the use of purlins, braces, and collar ties are described in Unit 46. Collar ties may be placed at every second or third pair of rafters. Purlins may be used to support longer rafters and are placed beneath the rafters at an intermediate point between the roof ridge and the exterior wall. Braces extending to the nearest interior partition support the purlins. General construction procedures for erecting hip roofs include the following and are shown in Figure 48‑18.
1. Install common rafters at two ends of ridge board.
2. Install hip rafters at corners.
3. Install hip jack rafters and remaining common rafters.