Roof Pitch, Rise, Span, and Truss Spacing Explained
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Run is half the span. Rise = run × pitch ÷ 12. Angle = arctan(pitch ÷ 12). Sloping length = run × √(1 + (pitch ÷ 12)²). Truss count with one at each end = ceil(length ÷ spacing) + 1.
Truss quotes and roofing estimates use the same handful of geometric terms. Getting them straight prevents ordering errors — but geometry is not structural design, which belongs to the truss manufacturer's engineering.
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Terms
| Term | Meaning |
|---|---|
| Span | Horizontal distance across the outside of bearing walls |
| Run | Half the span for a symmetrical gable |
| Pitch | Inches of rise per 12 inches of run (e.g. 6/12) |
| Rise | Vertical height from wall line to peak |
| Overhang | Horizontal projection of the eave beyond the wall |
| Sloping length | Length measured along the roof surface |
| Spacing | Center-to-center distance between trusses |
Worked example
A 28 ft span at 8/12 has a 14 ft run and rises 14 × 8 ÷ 12 = 9.333 ft. Angle = arctan(0.6667) = 33.69°. Slope factor = √(1 + 0.4444) = 1.2019, so wall-to-peak is 16.826 ft. A 16 in horizontal overhang adds 16 × 1.2019 = 19.23 in along the slope, for 18.428 ft per side. A 36 ft building at 24 in on center has 432 ÷ 24 = 18 spaces and 19 trusses.
Common mistakes
- Measuring the overhang along the slope and treating it as horizontal
- Using pitch (x/12) where an angle in degrees is required
- Forgetting the extra truss at the end of the count
Assumptions and limitations
The formulas assume a symmetrical gable and measure to geometric lines. They do not determine member sizes, loads, bracing, connector plates or code compliance; metal-plate-connected trusses are designed under industry standards maintained by TPI and produced by suppliers such as SBCA members.