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:
- Failing to learn to use a framing square.
- Ignoring adage to measure twice before cutting.
- 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
| Roof Slope | Slope Factor | ||
| 2/12 | 1.0138 | ||
| 3/12 | 1.0308 | ||
| 4/12 | 1.0541 | ||
| 5/12 | 1.0833 | ||
| 6/12 | 1.118 | ||
| 7/12 | 1.1577 | ||
| 8/12 | 1.2019 |
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
| DECIMAL OF A FOOT TO INCH CONVERSION | |||
| Feet | Inches | Feet | Inches |
| 0.9167 | 11 | 0.0781 | 15/16 |
| 0.8333 | 10 | 0.0729 | 7/8 |
| 0.75 | 9 | 0.0677 | 13/16 |
| 0.6667 | 8 | 0.0625 | 3/4 |
| 0.5833 | 7 | 0.0573 | 11/16 |
| 0.5 | 6 | 0.0521 | 5/8 |
| 0.4167 | 5 | 0.0469 | 9/16 |
| 0.3333 | 4 | 0.0417 | 1/2 |
| 0.25 | 3 | 0.0365 | 7/16 |
| 0.1667 | 2 | 0.0313 | 3/8 |
| 0.833 | 1 | 0.0260 | 5/16 |
| 0.0208 | 1/4 | ||
| 0.0156 | 3/16 | ||
| 0.0104 | 1/8 | ||
| 0.0052 | 1/16 | ||
Table 40-2
