How to Cut Shed Rafters

Reader MICHAEL in MILACA writes:

“Building a Loafing type pole shed, I have a question how to cut the rafters? I am building a 24 x90 loafing shed and I am using a 1-3/4″ x 24″ wide x 28 foot long LVL for the rafters. I do I figure the right field cut for this? I am using a 2/12 roof pitch.”

This excerpt from Hansen Pole Buildings’ Construction Manual should get Michael going:

Chapter 40: Cutting Rafters

Most Common Mistakes:

1. Failing to learn to use a framing square. 
2. Ignoring adage to measure twice before cutting.
3. Overlooking cutting so rafter crown is up when installed.

Using a Framing Square to Mark Rafter Slope

With rafter crown up, place framing square on wide face. See Figure 40-1 

“12” location will be a constant for any roof slope. Adjust other “leg” of framing square to match roof slope needed.

Figure 40-1: Using A Framing Square to Mark Rafter Slope

To determine rafter length, multiply distance traveled horizontally by rafter, by appropriate slope factor from Table 40-1

For other slope factors – multiply slope by itself and add 144. Take square root (use a calculator) of this number and divide by 12.

Example to calculate slope factor for 3.67/12:  [3.67 X 3.67] + 144 = 157.47. Square root of 157.47 = 12.549. Divided by 12 = 1.0457.

Table 40-1                               COMMON SLOPE FACTORS

Example below: 36’ wide pole and raftered “grid” style barn with a 4/12 roof slope.

See Figure 40-2

Figure 40-2: Column Spacing For Grid Barns

Here rafter has an 18’ horizontal distance (1/2 of 36’ building width) times 4/12 slope factor 1.0541 = 18.9738’ (18’ 11-11/16”) See Table 40-2

Table 40-2