This method is probably the easiest to use. To find the lengths of rafters, you make an actual size drawing of the rafter on the floor and then measure the length.
All the information you need to use this method should be on the plans. First you will need the pitch. (See Method C mentioned previously.) Second is the span, which is the distance from the support on one side of the rafter to the support on the other side. Third is the width of the rafter, the length of the roof overhang, and the size of the exterior wall.
Steps (shown in Line Chalking Sequence illustration):
1. Chalk a straight line longer than the length of your rafter, which would represent the bottom of the ceiling joist if there were a ceiling joist.
2. Chalk two lines perpendicular to the first line to represent the exterior wall.
3. From the point where your inside exterior wall line crosses the ceiling joist line, measure out and up according to your pitch xp2. For this example let your pitch equal 6г2. Therefore, for every 12” you measure out, you measure 6” up. The longer the distance out, the greater your accuracy. Make sure that the line up is exactly perpendicular or square. You can use surrounding walls that are square to measure from or measure a 3-4-5 triangle.
5. Measure the distance for the span along the ceiling joists line, then make a perpendicular line up and mark the ridge board.
6. Measure the distance of the roof overhang and draw in the fascia board.
With these lines in place, you can measure all the lengths you will need to cut your rafters.
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Line Chalking Sequence |
4. Chalk a line for the thickness of the rafter.
Rafter Pr°fiies E. Computer Software
Using the methods described on previous page to find the lengths and angles for cutting rafters is not easy, but it is at least organized—and with a calculator that works in feet and inches and that figures the diagonals automatically, the process is straightforward. However, the easiest method is to use the computer. There are software programs currently available that will do all the work for you and produce a sketch of each rafter. Solid Builder is one of these programs.
The illustrations on this page were done in Solid Builder. “Roof Production" identifies the type of roof parts, the quantity, lumber, and strength. “Rafter Profiles" illustrates the individual rafters with the balance of information you will need for cutting the rafters.
The hardest part of producing these computergenerated diagrams is learning the software and then entering the information needed for
each structure in order to generate the diagrams. However, for the architect who has already drawn up the plans, or the builder who is working with computer-generated plans, it is an easy task to produce these rafter profiles. If computer-generated rafter profiles were prepared and attached to plans, it could really make framing roofs a breeze.
