Roof Span Calculator
Calculate maximum allowable roof span based on lumber size and species
:12
in o.c.
Max Horizontal Span
19.2 ft
Max Rafter Length21.5 ft
Live Load Capacity20
RecommendationAdequate for standard residential roofing
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How to Use the Roof Span Calculator
Structural calculations require precision — an undersized rafter or beam can be a safety hazard. Use the Roof Span Calculator as a starting point, but always have a licensed engineer verify critical structural members.
- Identify the span and spacing of your structural members.
- Determine the design loads: dead load (roofing materials) plus live load (snow, maintenance).
- Enter dimensions, species, and grade of lumber where applicable.
- Review the calculated member size and compare against your local building code tables.
- Have results reviewed by a licensed structural engineer before construction.
Formula & Methodology
The Roof Span Calculator uses the following established formulas:
Rafter Length = √(Rise² + Run²) [Pythagorean theorem]
Total Load = Dead Load + Live Load + Snow Load (psf)
Tributary Area = Rafter Spacing × Rafter Span
Total Load on Rafter = Tributary Area × Total Load (psf)
Structural calculations are governed by local building codes. Always verify with a licensed professional.
Frequently Asked Questions
Rafters are individual framing members cut and assembled on-site. Trusses are engineered, factory-built triangular frames. Trusses are generally more economical for simple roof shapes; rafters offer more flexibility for complex designs.
Yes, in most jurisdictions any structural work — including new roof framing — requires a building permit. Permits ensure code compliance and protect your home's insurability and resale value.
Ground snow loads vary by location from under 10 psf to over 100 psf in mountain regions. Your local building code specifies the design ground snow load. Roof snow loads are typically 0.7 × ground snow load for standard-pitch roofs.