Fidamen

Roof Pitch Calculator

Use this calculator to find roof pitch (rise over run and in degrees), the rafter length per 12" of horizontal run, and an estimated rafter length for a given building span. It supports common carpentry workflows such as converting measured rise/run to standard 'X in 12' pitch and estimating rafter lengths for cutting.

Results assume simple gable geometry (symmetric roof, practical for single-span gable roofs). For complex roof types (hips, valleys, dormers, compound slopes), consult a structural engineer or your local building code official.

Updated Nov 7, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 137ab28a0b4f • Reviewed by Fidamen Standards Committee

Inputs

Results

Updates as you type

Slope (rise / run)

0.5

Pitch (rise per 12 inches)

PER 6.00

Slope (degrees)

—

Rafter length per 12" of run (inches)

13.4164

Rafter length for half-span (feet)

11.1803

OutputValueUnit
Slope (rise / run)0.5—
Pitch (rise per 12 inches)PER 6.00inches per 12 inches
Slope (degrees)—degrees
Rafter length per 12" of run (inches)13.4164inches
Rafter length for half-span (feet)11.1803feet
Primary result0.5

Visualization

Methodology

Pitch is defined as the vertical rise divided by the horizontal run. Common carpentry convention expresses pitch as inches of rise per 12 inches of run.

This tool uses elementary trigonometry: slope = rise / run; degrees = arctangent(slope) × 180/π. Rafter (hypotenuse) is computed from the Pythagorean theorem.

When estimating rafter length for a given span, the calculator treats the span as the overall building width and uses half the span as the horizontal run for the rafter (common for centered ridge gable roofs). It does not include overhang, birdsmouth cuts, trimming allowance, or plumb-cut allowances—add those per on-site practice.

F.A.Q.

What units should I measure in?

Measure consistently. This tool accepts rise and run in inches and span in feet. If you measure in metric, convert to inches (1 in = 25.4 mm, 1 ft = 0.3048 m) before entering values or use an internal conversion workflow.

How accurate are the rafter length estimates?

Calculations are mathematically precise for idealized geometry (no overhangs or cut allowances). On-site cutting requires adding allowances for birdsmouth and plumb cuts, and confirming with a framing square. For final cutting, measure and recalibrate on the roof or truss manufacturer data.

Can I use this for any roof shape?

This calculator covers straight (single-pitch) rafters on symmetric gable roofs. For hips, valleys, multi-plane roofs, or engineered trusses, use dedicated design/engineering tools or consult a licensed structural engineer.

Is there a safety/regulatory note I should know?

Follow fall-protection and ladder safety rules when measuring on roofs. OSHA provides guidance for roofing work; consult local building codes for loading, ventilation, and insulation requirements before making structural changes.

Why show degrees and rise/run both?

Carpenters commonly use rise per 12 for layout (framing squares), while architects and solar installers often use degrees. Providing both helps match field tools and technical documentation.

How do I calibrate field measurements?

Confirm your reference plane (true horizontal) with a level. When measuring on an installed roof, measure short, repeatable runs (for example 2–3 ft) to average out local irregularities; use a digital inclinometer for quick degree readings and cross-check with rise/run measurements.

Sources & citations

Further resources

Versioning & Change Control

Audit record (versions, QA runs, reviewer sign-off, and evidence).

Record ID: 137ab28a0b4f

What changed (latest)

v1.0.0 • 2025-11-07 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Public QA status

PASS — golden 25 + edge 120

Last run: 2026-01-23 • Run: golden-edge-2026-01-23

Engine

v1.0.0

Data

Baseline (no external datasets)

Content

v1.0.0

UI

v1.0.0

Governance

Last updated: Nov 7, 2025

Reviewed by: Fidamen Standards Committee (Review board)

Credentials: Internal QA

Risk level: low

Reviewer profile (entity)

Fidamen Standards Committee

Review board

Internal QA

Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person

Semantic versioning

  • MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
  • MINOR: New features or fields that do not change existing outputs for the same inputs.
  • PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.

Review protocol

  • Verify formulas and unit definitions against primary standards or datasets.
  • Run golden-case regression suite and edge-case suite.
  • Record reviewer sign-off with credentials and scope.
  • Document assumptions, limitations, and jurisdiction applicability.

Assumptions & limitations

  • Uses exact unit definitions from the Fidamen conversion library.
  • Internal calculations use double precision; display rounding follows the unit's configured decimal places.
  • Not a substitute for calibrated instruments in regulated contexts.
  • Jurisdiction-specific rules may require official guidance.

Change log

v1.0.0 • 2025-11-07 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: b47bb487267c