Roof Pitch Calculator — Slope, Rafter Length & Roof Area
Free tool · Works offline · Degrees, percentage & X:12 · Metric & imperial
What is roof pitch and how is it measured?
Roof pitch is the steepness of a roof expressed as a ratio of vertical rise to horizontal run. In the US and UK it's written as X:12 — meaning X inches of rise for every 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every foot of run, giving a 26.6° angle. In Europe pitch is often expressed as a percentage (rise ÷ run × 100) or in degrees. This calculator converts between all three instantly.
What is the most common roof pitch?
Most residential roofs in the US and UK fall between 4:12 and 6:12 (33%–50%, 18°–26.6°). This range balances weather resistance, aesthetics and ease of construction. Low-slope roofs (under 3:12) need special waterproofing membranes. Steep roofs (over 9:12) require safety equipment during installation and are more expensive to build. The most popular single pitch is 6:12 — it sheds water and snow well, looks proportional on most house styles and is straightforward to frame.
Minimum roof pitch by material
Different roofing materials have minimum pitch requirements set by manufacturers and building codes. Asphalt shingles: minimum 2:12 (with extra underlayment), recommended 4:12+. Metal roofing panels: can go to 1:12 with standing seam. Clay/concrete tiles: minimum 4:12. Natural slate: minimum 4:12. Flat roofing (EPDM, TPO, bitumen): under 2:12. Always check the manufacturer's specification for your chosen material — installing below minimum pitch voids the warranty.
How to calculate rafter length
Rafter length uses the Pythagorean theorem: rafter = √(run² + rise²). For a building 6m wide with a 6:12 pitch: run = 3m (half the span), rise = 1.5m (3m × 6/12). Rafter = √(3² + 1.5²) = √(9 + 2.25) = √11.25 = 3.35m. Add the overhang (typically 300–600mm) and you have the total rafter length. The rafter tab in this calculator does this automatically once you enter the pitch and building width.
How to calculate roof area for shingles
Roof area is not the same as footprint area — it's the actual sloped surface. Multiply your building's footprint by the roof pitch factor for your slope. A 6:12 pitch has a factor of 1.118, so a 100 m² footprint gives 111.8 m² of actual roof. Shingles are sold by the roofing square (100 sq ft / 9.3 m²) and come 3 bundles per square for standard 3-tab. Add 10–15% for waste and cuts. The area tab calculates all of this automatically.
Frequently asked questions
What roof pitch is best for snow?
A pitch of 6:12 (26.6°) or steeper is recommended in heavy snow areas — it allows snow to slide off rather than accumulate. Below 4:12, snow load can become a structural concern and specialist snow guards may be needed.
How do I measure my roof pitch without going on the roof?
From inside the attic: place a spirit level horizontally against a rafter, measure 12" along the level from the rafter, then measure vertically down to the rafter — that measurement is the rise. From outside: use a smartphone pitch app or hold a level against the rake board.
What is a 4:12 pitch in degrees?
A 4:12 pitch is 18.4°, equivalent to 33% slope. It's the minimum recommended for asphalt shingles without additional underlayment.
How many bundles of shingles for a 2000 sq ft roof?
A 2000 sq ft footprint at 6:12 pitch gives about 2,236 sq ft of actual roof area (× 1.118 factor) = 22.4 squares × 3 bundles = 67 bundles. Add 10% waste = 74 bundles.