There are calculators that are made specifically for assisting with construction math. These are very helpful in finding rafter lengths. Construction Master IV® is one available calculator, which we will refer to and use in this chapter to demonstrate the process of finding rafter® lengths. These calculators make it easy to do the complicated math, working in feet and inches. The sequence of buttons takes a little time to master, but once you are familiar with them, you will never go back to pencil and paper.
The best way to learn cutting of rafters is to completely work through the actual process. The following example uses the diagonal percent method of finding rafter lengths. The example shows how to find the lengths of the parts of a relatively difficult roof. If you are able to work through this example and understand the processes, you should be able to figure out how to cut and stack rafters.
This example starts by organizing the whole process. Some of this information was presented earlier in the chapter, but it’s important to review for every job. We’ll start by outlining the important parts of cutting rafters.
Considerations for Cutting Rafters
When cutting rafters, you need to consider the following four factors:
1. Figuring rafter length
2. Figuring the Adjustment to rafter length at top and bottom
3. Finding the angle cuts at the top, bottom, and at the bird’s mouth
4. Finding the bird’s mouth height
Construction calculator designed to assist in construction math.
Figuring Rafter Length Using Diagonal Percent was shown earlier in the chapter with the six ways to figure rafter lengths. (See "F.") When you use the diagonal percent, the most difficult part of figuring rafter length is finding the length of the run. The adjustments that need to be made at the top and bottom of the rafter should be added and subtracted from the run before the rafter length is calculated.
In finding the run, it is best to start with the full run distance from the outside of the bearing wall to the framing point of any connecting framing member. Use the framing point for consistency, and then make adjustments from there.
Figure the Adjustments to Rafter Length at Top and Bottom
Because there are so many different types of connections for rafters, it helps to establish certain standard ways to connect, and measure them in order to find the proper adjustments to length for the top and bottom. Following are some standard connections and their adjustments. They will not apply to every situation, but they will work for the most common roofs.
Adjustments for Common Rafters
1. Subtract half the thickness of the ridge board at the top.
2. At the bottom measure to the outside of the wall framing (not the sheathing).
1. Subtract half the 45° thickness of the ridge board. (See “Connection # 1 [close-up] illustration later in this chapter.)
2. At the bottom, measure to the outside corner of the two connecting walls.
Adjustments for Jack Rafters
1. Measure to the framing point where it meets the hip or valley it is connected to. (See “Adjusting the Top Length for Jack Rafters" illustration later in this chapter.)
2. Subtract half the 45° thickness of the valley rafter.
3. When the rafter rests on an exterior wall, measure to the outside of the wall framing.
Adjust Length for Top of Common Rafter
IV2" Ridge board ^ 12
6
22V2" 24
■ Span 14-0"- Adjustment:
Subtract V2 the thickness of the ridge board. This example = %"
This example:
Plumb cut height at bird’s mouth =
Run diagonal = 6ЧІ/4" x 1.118 =
Overhang diagonal = 22Уг" x 1.118 = 2′-1 J/ls"
Rafter length = Run diagonal + Overhang diagonal = 9′-10/іб"
1. Measure to the framing points where it meets the hip or valley it is connecting to. (See “Adjusting the Top Length for Jack Rafters" illustration later in this chapter.)
2. Subtract half the 45° thickness of the hip or valley rafter at each end.
Adjustments for Miscellaneous Connections Between Hips, Valleys, Ridges, and Rafters
1. Find the combination of cuts that provides the greatest number of standard cuts and still provides a sound structural connection.
2. Measure to the framing point for making adjustments.
3. Connection #2 is an example of a miscellaneous connection where a ridge board, a common rafter, a hip rafter and a valley rafter connect. (See “Connection #2" illustration later in chapter.)
Finding the Angle Cuts at the Top, Bottom, and at the Bird’s Mouth
It is easy to figure the angle cuts if you break them down into two separate angles. The first is the pitch angle, and the second is the connection angle.
The pitch angle is either a common or a hip/valley. If you use a speed square, you don’t even have to calculate it. If you are cutting a rafter that is not a hip or a valley, then use the common scale on a speed square for the pitch of your roof and draw your pitch angle line on the rafter. If you’re cutting a hip or valley rafter, then use the hip-val scale on the speed square. The pitch angle line will be your cut line for your saw cut.
The connection angle depends on a lot of factors, but 45° and 90° are the most commonly used angles. Basically, you will be setting the angle of your saw at the connection angle and cutting the cut line created by the pitch angle. For 90° corners on hips and valleys, the connection angle for jack rafters will be 45°. For standard, common rafters, the top connection angle is 90°.
Finding the Bird’s Mouth Height
The height of the bird’s mouth will affect the height of the roof and possibly the interior design of the ceiling. The most common detail for a bird’s mouth has the bird’s mouth cut starting at the inside corner of the wall.
On a hip and valley, the inside corner won’t align with the wall. Since the height of the hip and valley bird’s mouth must be the same as the common rafter bird’s mouth, you can simply measure the common rafter height and transfer it to the hip and valley bird’s mouth. The chart on page 103 shows some common bird’s mouth heights.
On hip rafters, you measure the height to the outside edge of the hip, whereas on valley rafters, it’s a little tricky. You need to measure to the center of the valley, which is slightly higher than the outside edge.
