How to measure roof pitch

Measuring roof pitch does not always require walking the roof. For many homes you can get a usable rise-over-run reading from the loft with a spirit level and a tape measure.

What you are measuring

Roof pitch is steepness, usually written as rise over run. In the US customary system a pitch of 6/12 means 6 inches of vertical rise for every 12 inches of horizontal run. The same idea works in metric if you keep the run consistent (for example rise over 30 cm).

From rise and run you can derive the angle in degrees and an area multiplier that converts plan (footprint) area into approximate roof surface area.

Method from inside the loft

1. Find a clear underside of a main rafter on the slope you care about.
2. Place a spirit level against the rafter and adjust until the bubble shows level (horizontal).
3. From the contact point, measure exactly 12 inches (or 30 cm) along the level.
4. At that mark, measure vertically down to the rafter surface. That vertical distance is the rise.
5. Express pitch as rise/12 (or scale metric readings to a familiar X/12 style if you prefer).

Enter rise and run in the free calculator on this site to get angle, multiplier, area and rafter length estimates.

If you measure outside

The same geometry applies on the roof surface: level horizontal, fixed run, vertical rise. Outside measurement adds slip, edge and weather risk. Many homeowners never need to leave the loft for a planning estimate.

On complex roofs (hips, valleys, dormers, unequal pitches) measure each major plane. A single pitch number is only valid for the plane you measured.

Common mistakes

• Measuring along the slope instead of using a true horizontal run
• Using a short run (errors multiply when you scale up)
• Assuming every face of the house has the same pitch
• Treating a calculator estimate as a structural or permit document

For buying materials or hiring work, confirm pitch and area with a professional when the project is large or the roof is complex.

Free roof pitch tools

Calculate pitch, angle, roof area, rafter length and material estimates from rise, run and building size.

Open the free calculators →

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