Rafter Length Calculator

How It Works

1. Enter Geometry Prompts

   Enter roof pitch, building span, ridge-board thickness, horizontal overhang, rafter spacing, rafter depth prompt, and building length.

2. Review Unit Length

   Common rafter unit length uses sqrt(rise-squared + 144) inches per foot of run. A square-plan hip/valley prompt uses sqrt(rise-squared + 288).

3. Review Total Geometry

   The app multiplies unit length by the common run after the ridge-thickness prompt and adds the horizontal overhang converted to slope length.

4. Resolve Source Gaps

   Use the warnings to resolve adopted code, span sizing, notch/bearing detail, roof assembly, material order, cutting, and roof-work safety outside the app.

Built For

• Framers checking local common rafter geometry before a pattern-rafter and code/field review
• Carpenters comparing square-plan hip or valley geometry prompts before detailed roof layout
• Estimators screening roof-area geometry before a verified material takeoff and supplier order
• Designers documenting unresolved span, code, and roof-plan assumptions for qualified review
• Inspectors or owners separating geometry prompts from structural adequacy and permit acceptance
• Shed or outbuilding planners identifying when roof pitch, overhang, span, or safety review needs escalation

Features & Capabilities

Common and Hip/Valley Geometry Prompts

Computes unit length, common run, rafter length, tail length, and square-plan hip/valley prompts while keeping jack rafters and compound cuts explicit gaps.

Bird's Mouth Source Boundary

Shows a one-third notch-depth comparison as a review prompt, not code compliance, bearing approval, or cutting authorization.

Roof Area Geometry Prompt

Converts plan area to sloped gable area using the pitch slope factor, then warns that material order quantities require separate verification.

Source and Safety Warnings

Surfaces IRC/AWC/OSHA/NIST source pointers and residual gaps for structural design, roof assembly, field layout, and work-at-height safety.

PDF Export

Exports the same source-aware geometry prompts, assumptions, source warnings, and source pointers used on screen.

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).
• The one-third bird's mouth comparison is a local review prompt, not a compliance or cutting decision.
• 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, species, grade, loads, bearing, bracing, ties, connectors, uplift, or ridge support.
• Does not determine adopted-code compliance, permit approval, inspection acceptance, AHJ interpretation, or engineered design.
• Does not solve compound hip and valley intersections, unequal pitches, jack rafters, backing/drop, fascia details, HAP, or pattern-rafter fit.
• Does not verify roofing product requirements, underlayment, low-slope waterproofing, sheathing layout, waste, flashing, ventilation, or material order quantities.
• Does not approve roof access, fall protection, scaffold/ladders, training, competent-person inspection, rescue, weather, or employer safety procedures.

References

• ICC-IRC-2024-CH8-ROOF-CEILING-SOURCE - 2024 IRC Chapter 8 roof-ceiling construction source pointer
• AWC-SPAN-TABLES-JOISTS-RAFTERS-2012 - AWC span tables for joists and rafters source pointer
• AWC-NDS-2024-SOURCE - AWC National Design Specification source pointer
• OSHA-1926-501-FALL-PROTECTION-2026 - OSHA construction fall-protection duty source pointer
• OSHA-1926-451-SCAFFOLDS-2026 - OSHA scaffold general requirements source pointer
• NIST-SP811-B8 - unit conversion and rounding context

Frequently Asked Questions