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Roof Pitch Calculator
Pitch, angle, rafter length and true roof area.
What it does. Roof pitch is the ratio of rise to run, expressed as x-in-12 in the US, as a ratio elsewhere, or in degrees. Rafter length comes from Pythagoras, and the true roof area is always larger than the building footprint — by 12% at a 6-in-12 pitch and 41% at 45 degrees.
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How to use Roof Pitch Calculator
- Enter the pitch as rise-in-run, a ratio or degrees.
- Add the span and length for rafter and area figures.
- Read the converted pitch, rafter length and true area.
Three ways to say the same thing
| Pitch (x-in-12) | Degrees | Ratio | Description |
|---|---|---|---|
| 2 : 12 | 9.5° | 1 : 6 | Low slope, minimum for many tiles |
| 4 : 12 | 18.4° | 1 : 3 | Common shallow pitch |
| 6 : 12 | 26.6° | 1 : 2 | Very common residential |
| 9 : 12 | 36.9° | 1 : 1.33 | Steep |
| 12 : 12 | 45° | 1 : 1 | Equal rise and run |
| 18 : 12 | 56.3° | 1 : 0.67 | Very steep, alpine |
The x-in-12 convention is US practice; UK and European work usually quotes degrees. Confusing "4 in 12" with "4 degrees" is a real error and a large one — 18.4 degrees against 4.
True area is not footprint area
A sloping roof covers more surface than the rectangle it sits on. The multiplier is 1 divided by the cosine of the pitch angle.
At 6-in-12 that is about 1.12, so a 100 square metre footprint has roughly 112 square metres of roof. At 45 degrees it is 1.41.
Ordering roofing material against footprint area is a classic and expensive mistake — and it gets worse as the pitch increases, which is exactly when the material is most awkward to fetch more of.
Why pitch is constrained
Roof coverings have minimum pitches below which wind-driven rain gets under them. Plain clay tiles typically need 35 degrees or more; interlocking concrete tiles manage around 17.5; standing-seam metal and single-ply membranes go much lower.
The covering is chosen to suit the pitch, not the other way round.
If you are calculating pitch to choose a material, check the manufacturer's minimum for the specific product. These are product-level figures, not general rules, and going below them voids the warranty as well as leaking.
Frequently asked questions
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run. A 6-in-12 pitch is 26.6 degrees, 4-in-12 is 18.4, and 12-in-12 is exactly 45.
Why is my roof area bigger than the floor area?
Because the roof slopes. Multiply the footprint by 1 divided by the cosine of the pitch angle — about 1.12 at 6-in-12 and 1.41 at 45 degrees.
How do I calculate rafter length?
Pythagoras: the square root of rise squared plus run squared, measured from the ridge to the wall plate. Add for the eaves overhang separately.
What is the minimum pitch for roof tiles?
It depends on the product. Plain clay tiles typically need 35 degrees or more, interlocking concrete tiles around 17.5, and metal or membrane roofs much less. Always check the manufacturer's figure.
What does 4 in 12 mean?
The roof rises 4 units for every 12 units of horizontal run — 18.4 degrees. It is US convention; UK and European work usually quotes degrees, and confusing the two is a large error.