Roof pitch calculator
Degrees, a rise in twelve, a one in n, or a percentage — a bare “6” on a drawing means 26.57° or 9.46° depending on whose convention it is. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Minimum pitches for tiles and slates are manufacturers' warranty conditions, not building regulations. They vary by product and they change — read the datasheet for the covering you are actually buying.
“Pitch” means four different things
And a bare number on a drawing does not say which. The same “6” is:
| 6 in 12 | 26.57° — the American convention, rise per twelve of run |
|---|---|
| 1 in 6 | 9.46° — the British convention, rise to run |
| 6° | 6° — the angle |
| 6% | 3.43° — the grade |
The first two differ by a factor of 2.8 in the angle. That is the difference between a roof a tile will stay on and one it will not. If a drawing says “6” and you do not know whose convention it is, ask — and this page converts all four so the ambiguity is at least visible.
The rafter is longer than the span suggests
Obvious once stated and routinely forgotten when ordering. At 30° the rafter is 1.155× the run; at 45° it is √2, or 1.414. So a building with a 100 m² footprint and a 30° roof needs 115 m² of covering, not 100 — before any allowance for laps, verges and waste.
The same factor applies to the roof area, the felt, the battens and the labour. It is the single most common reason a tile order comes up short.
Why pitch is chosen
Almost entirely by what the covering needs. Every tile and slate has a minimum pitch below which wind-driven rain gets under it, and that minimum is a manufacturer's warranty condition rather than a building regulation. Interlocking concrete tiles go down to about 12.5° with special underlays; plain tiles want 35° and more; slate sits in between and depends on the size of the slate and the lap.
So the honest order is: decide the covering, read its minimum pitch from its own datasheet, then design the roof. Not the other way round, and never from a general table on a website — including this one.
What it will not do
- It will not tell you what your roof can be covered with. That is the manufacturer's minimum pitch for the specific product, and it is a warranty condition.
- It will not do hips, valleys or a roof whose two sides have different pitches — the hip rafter sits at a different angle again.
- It will not account for laps, verges, ridge, waste or the fact that tiles come in whole numbers.