Wooden Shed Roof Rafter Calculator
Calculate lumber quantity, board feet, and structural suitability for single-pitch wooden roofs. Includes span estimates based on wood species.
About
In timber framing, the "Common Rafter" serves as the primary load-bearing element of the roof system. This tool functions as a carpenter's assistant, estimating the volume of lumber required based on On-Center (O.C.) spacing logic. Spacing directly influences the load distribution; tighter spacing (12" O.C.) allows for smaller dimensional lumber over longer spans, whereas standard (16" or 24" O.C.) spacing is more material-efficient but requires stiffer timber.
Material estimation goes beyond simple counting. This calculator computes Board Feet - the industry standard for volume pricing - and checks the proposed span against a simplified heuristic of the International Residential Code (IRC) tables for common softwood species like Douglas Fir and Southern Pine. Note that deflection limits (typically L/240) dictate the maximum span to prevent visible sagging under live loads (snow/rain).
Formulas
The number of rafters (N) is determined by the roof width (W) and spacing (s):
Total Board Feet (BF) is calculated using nominal dimensions:
Reference Data
| Species | Grade | Modulus (E) | Typical Max Span (2x6 @ 16" OC) |
|---|---|---|---|
| Douglas Fir-Larch | #2 | 1.6 M psi | 9ft 9in |
| Southern Pine | #2 | 1.4 M psi | 9ft 4in |
| Hem-Fir | #2 | 1.3 M psi | 8ft 11in |
| Spruce-Pine-Fir | #2 | 1.2 M psi | 8ft 9in |