Conversion Factors for Rafters

---

Directions: Multiply the factors below by the horizontal distance from the fascia to the roof ridge.

If you use the horizontal distance from the exterior wall to the ridge, you will exclude the roof overhang.

You must include the roof overhang to get the rafter length.

You may also use the factors below to obtain square feet of roof deck and roofing.

Roof Slope Factor

Flat 1.0

1:12 1.003

2:12 1.014

3:12 1.031

4:12 1.054

5:12 1.083

6:12 1.118

7:12 1.158

8:12 1.202

9:12 1.250

10:12 1.302

11:12 1.357

12:12 1.414

13:12 1.474

14:12 1.537

15:12 1.601

16:12 1.667

17:12 1.734

18:12 1.803

19:12 1.873

20:12 1.943

21:12 2.015

22:12 2.088

23:12 2.162