Frequently Asked Questions
How long is a rafter for a 12-foot run at 6:12 pitch?
Length = 12 x sqrt(1 + (6/12)^2) = 12 x 1.118 = 13.42 ft. With a 2-ft overhang: (12+2) x 1.118 = 15.65 ft.
What is the roof angle for a 6:12 pitch?
atan(6/12) = atan(0.5) = 26.6 degrees.
How many rafters for a 40-foot building at 16 OC?
ceil(40 / (16/12)) + 1 = ceil(30) + 1 = 31 rafter positions per side.
What is the pitch multiplier for a 5:12 roof?
sqrt(1 + (5/12)^2) = sqrt(1.1736) = 1.083. Multiply the run by 1.083 to get rafter length.
What is the run versus the span?
Span is the full width of the building from outside wall to outside wall. Run is half the span (for a simple gable with a centered ridge). Enter the run, not the span, in this calculator.
How many rafters for a 24-foot building at 16 OC?
Two sets of rafters meet at the ridge: ceil(24 / (16/12)) + 1 = 19 rafter positions per side, so 19 pairs (38 rafters total) plus a ridge board.
Do hip and valley roofs use the same formula?
No. Hip rafters and valley rafters have different geometry (they run diagonally across the plan). This calculator is for common (perpendicular to ridge) rafters only.
What length lumber should I buy?
Round the total rafter length up to the nearest standard lumber length (8, 10, 12, 14, 16, 18, 20 ft). Add 5-10% waste factor for bird's-mouth cuts and trim.
Provided by AllCalculators.io
Free online calculators for everyday. No registration required.
Estimates for informational purposes only.
Important Disclaimer: Estimates for informational purposes only.
This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.