Rafter Length Calculator: Common, Hip, and Valley Rafters

How It Works

1. Enter Roof Parameters

   Input roof pitch (rise per 12 inches of run), building span (total width), and eave overhang. Select rafter type: common, hip, or valley.

2. Calculate Unit Length

   Common rafter unit length = sqrt(rise-squared + 144) inches per foot of run. Hip/valley unit length = sqrt(rise-squared + 288) because they run at 45 degrees in plan.

3. Determine Total Length

   Multiply unit length by run in feet (half the span for common rafters). Add overhang length. Subtract half the ridge board thickness measured along the slope.

4. Review Cuts and Area

   See bird's mouth cut dimensions (seat cut depth and plumb cut height) for the plate connection. Total roof area uses the slope factor times plan area for material ordering.

Built For

• Framers cutting common rafters for a gable roof and marking plumb and seat cuts
• Carpenters calculating hip rafter length for a hip roof addition
• Roofing contractors converting flat roof area to sloped area for shingle and underlayment ordering
• Timber frame builders determining rafter lengths for post-and-beam structures
• Home inspectors checking whether existing rafters are properly sized for the span
• Shed builders calculating simple rafter layouts for small outbuilding roofs
• Estimators figuring total roof area for bid proposals on re-roofing jobs

Features & Capabilities

Common, Hip, and Valley Types

Calculates unit length and total length for all three rafter types. Hip and valley use the diagonal factor of 16.97 inches per foot.

Bird's Mouth Cut Dimensions

Shows seat cut depth and plumb cut height for the rafter-to-plate connection. Seat cut should not exceed one-third of rafter depth.

Ridge Height Output

Calculates ridge height above the top plate. Accounts for HAP (height above plate at the bird's mouth).

Slope Factor for Roof Area

Converts plan area to actual sloped area using sqrt(pitch-squared + 144) / 12. A 6/12 pitch adds 11.8% to the plan area.

Overhang and Ridge Adjustment

Adds overhang length to the rafter and subtracts the ridge reduction. Both measured along the slope, not horizontally.

PDF Export

Export rafter layout as a branded PDF for job site reference or material ordering.

Assumptions

• Roof framing is conventional stick-built with a ridge board (not a ridge beam or structural ridge).
• Common rafter unit length uses sqrt(rise-squared + 144); hip/valley uses sqrt(rise-squared + 288).
• Bird's mouth seat cut does not exceed one-third of the rafter depth per standard framing practice.
• Overhang is measured horizontally from the outside face of the wall to the fascia line.

Limitations

• Does not perform structural analysis of rafter size, spacing, or species/grade for the given span and loads.
• Compound hip and valley roof intersections with unequal pitches are not calculated.
• Jack rafter lengths (common jacks off a hip or valley) are not individually computed.
• Does not account for dropped or raised plate conditions at different wall heights.

References

• IRC Section R802 — Wood Roof Framing (span tables and construction requirements)
• Carpentry and Building Construction, 6th Edition — Rafter Layout Methods
• Machinery's Handbook — Right Triangle Calculations (unit length derivation)
• Larry Haun — The Very Efficient Carpenter (practical rafter cutting techniques)

Frequently Asked Questions