How to Work Out Roof Pitch

What you need

A tape measure, a spirit level, a pencil and paper. You will measure in the loft, where the roof structure is visible and your feet are on a floor — not on the roof itself. If you cannot get into the loft safely, measure from the ground instead (Step 1 covers both ways).

Step 1 — Measure the rise and the run

In the loft, pick a rafter and mark a level line 12 in out from it along the horizontal — that 12 in is the run. Measure straight up from the end of that line to the rafter — that is the rise. The two numbers must be in the same unit: inches with inches, or millimetres with millimetres. If you cannot reach the loft, measure the building's width from the ground and halve it for the run, then measure the ridge height above the eaves for the rise — but the loft method is more reliable, because the ground method depends on the eaves being level.

Step 2 — Work out the pitch

Divide the rise by the run and multiply by 12: that is the pitch, written as a number over 12. The roof pitch calculator does this and gives you the slope angle and the slope factor too — the factor is what turns the flat footprint of the roof into its real sloping area. A 6/12 pitch rises 6 in for every 12 in it runs, which is a 26.57° angle.

Step 3 — Work out the roof area

Multiply the footprint (length × width) by the slope factor. The roof area calculator does this for up to three roof sections, each with its own pitch, and tells you the area in squares (one square is 100 ft²) — the unit roofing is sold by. Ordering by the footprint alone under-orders, because a sloping roof is always bigger than its footprint.

Step 4 — What the pitch tells you

Steeper roofs shed water faster and need more material; shallow roofs hold water longer and need materials made for low pitches. Every roofing material has a minimum pitch the manufacturer states — check it before you order, because a material laid below its minimum pitch will leak. The pitch also decides how the roof is walked on and maintained, which is a job for a roofer, not a homeowner with a ladder.

Worked example

A roof with a rise of 6 in over a run of 12 in has a pitch of (6 ÷ 12) × 12 = 6/12, a slope angle of atan(6 ÷ 12) = 26.57°, and a slope factor of √(1 + 0.25) = 1.118. For a 30 ft × 40 ft footprint (1,200 ft²), the roof area is 1,200 × 1.118 = 1,341.64 ft² — 13.42 squares. Those are exactly the numbers the roof pitch calculator and the roof area calculator give you.

Common mistakes

• Measuring the slope as the run. The run is horizontal, not along the slope. Measuring along the rafter gives a smaller pitch than the roof really has.
• Mixing units. Rise in inches and run in feet gives a nonsense pitch. Use the same unit for both.
• Ordering by the footprint. The roof area is the footprint times the slope factor. Ordering by the footprint leaves you short — by 12% on a 6/12 pitch.

Safety

This guide is about measuring only. Do not climb onto a roof to measure it — measure from the loft or the ground. Roof work is for a qualified roofer: falls from roofs cause serious injuries, and a wet, steep or damaged roof is never safe to walk on. If the roof needs repair, replacement or even a proper inspection, hire a roofer and let them work from proper access equipment.

Try these calculators

• Roof Pitch Calculator — pitch, angle and slope factor from rise and run.
• Roof Area Calculator — roof area and squares for up to three sections.
• Roofing Calculator — roofing area and materials for a roof.