Rafter Size Calculator
Rafter size means the lumber dimension — 2x4 through 2x12 — needed to span your roof without excessive deflection. This calculator returns rafter length and angle from your span and pitch; the reference table below shows typical dimension ranges by span.
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Matching Lumber Size to Rafter Span
Rafter size is driven by span, spacing, species/grade, and roof load — a longer unsupported span or heavier snow load needs a deeper member (more resistance to bending) or closer spacing to compensate. Common residential rafters range from 2x6 for short spans to 2x10 or 2x12 for wide, heavily loaded spans.
Rafter depth resists bending (deflection) under load, while width mainly affects nailing surface and stability — this is why increasing depth (going from a 2x8 to a 2x10) has a much bigger effect on allowable span than increasing width.
The American Wood Council (AWC) publishes span tables cross-referencing species, grade, spacing, and load against maximum allowable rafter span for each dimension — always confirm your exact rafter size against the current span table or a stamped design for your load and grade.
Reference span table — No.2 grade, 20 psf live load, 10 psf dead load, L/180 deflection limit, ceiling not attached to rafters (a common baseline IRC condition; values are compiled from published AWC-derived span data and rounded to the nearest inch):
| Species (No.2) | Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|---|
| Douglas Fir-Larch | 2x6 | 14'11" | 12'10" | 10'7" |
| 2x8 | 19'8" | 15'0" | 12'4" | |
| 2x10 | 21'3" | 18'2" | 14'11" | |
| 2x12 | 24'6" | 21'1" | 17'3" | |
| Southern Pine | 2x6 | 15'7" | 13'6" | 11'2" |
| 2x8 | 20'6" | 15'10" | 13'0" | |
| 2x10 | 22'4" | 19'2" | 15'7" | |
| 2x12 | 25'8" | 22'0" | 18'0" | |
| Spruce-Pine-Fir | 2x6 | 14'2" | 12'2" | 10'0" |
| 2x8 | 18'8" | 14'3" | 11'8" | |
| 2x10 | 20'2" | 17'3" | 14'1" | |
| 2x12 | 23'3" | 20'0" | 16'4" | |
| Hem-Fir | 2x6 | 13'9" | 11'10" | 9'9" |
| 2x8 | 18'2" | 13'10" | 11'4" | |
| 2x10 | 19'8" | 16'10" | 13'9" | |
| 2x12 | 22'8" | 19'6" | 15'11" |
This single table covers one specific load case only. Snow load, wind exposure, deflection limit (L/240 or L/360), lumber grade (Select Structural, No.1, No.3), and whether a ceiling is attached all change the allowable span — for your exact combination, use the AWC span table tool or your local IRC/IBC rafter span tables directly.
Frequently Asked Questions
What size rafters do I need for a 20 ft span?
At 16" o.c. spacing with a No.2 grade and no snow load, a 20 ft span typically needs a 2x10 (most species) or a 2x12 for lighter species like Hem-Fir. Heavier snow loads or wider spacing push this up to a 2x12 — always verify against your local span table or an engineer's design before final sizing.
Does rafter depth or width matter more for strength?
Depth matters far more — a deeper rafter (like a 2x10 vs a 2x8) resists bending much more effectively than a wider one, since bending resistance increases with the square of the depth.
Which species allows the longest rafter span?
Among common framing species, Southern Pine No.2 generally allows the longest spans (highest bending strength, Fb ≈ 1,000 psi), followed by Douglas Fir-Larch, then Spruce-Pine-Fir, then Hem-Fir — though actual required size always depends on your local load conditions too.