Pitch three ways: degrees, rise:run, total rise

Three ways of stating the same angle, how CutRoof converts between them, and the one measurement people get the wrong way round.

Pitch turns up on site in three forms, depending on who wrote it down. All three describe the same angle, and CutRoof takes whichever one you have rather than making you convert it first.

Degrees

The commonest statement on a UK drawing, and the one the app works in internally. Type 35 and there is nothing to convert.

Rise to run

A ratio saying how much the roof climbs for a given horizontal distance. Written 1:2, or 1 in 2, meaning one up for two along.

A 1:2 roof works out at 26.6 degrees. The ratio is what a lot of older setting-out is built on, because it is what a framing square works in, and it scales to any size of roof.

The trap is which way round the two numbers go. One up for two along is a shallow roof at 26.6 degrees. Two up for one along is a steep one at 63.4. Both get written 1:2 by someone in a hurry.

Total rise

The vertical from the top of the wall plate to the apex, measured on the building.

This is the figure people arrive with when the ridge height is already fixed, by a planning drawing or by the head height in a loft conversion. It is also the one with a catch: the rise is measured over the half span, not the full span.

On a 4800 span, a total rise of 1200 gives 26.6 degrees, the same roof as 1:2. Feed the same 1200 in as a rise over 4800 and you would get half the angle and a roof that does not match the drawing. CutRoof reads it over the half span, which is the run of a common rafter, so a total rise of 1680 on that span comes out at 35.0 degrees.

They all end up in the same place

Whichever way you enter it, the app converts to one angle and works everything from that. Enter 1:2 on a 4800 span or a total rise of 1200 on the same span and you get identical schedules: 26.6 degrees, and a common rafter at 2665.

The working prints the pitch it resolved to. If a roof came in as a ratio or a rise, check that angle against the drawing before you cut anything.

The limits

CutRoof calculates between 5 and 75 degrees. Outside that it will not compute, because the geometry stops describing anything a cut roof does.

Between 5 and 15, and above 50, it computes and warns. Those roofs exist and the figures are right for the pitch you entered, but they are outside common UK practice and worth a second look before the timber is ordered. A pitch that surprises you usually means a span or a rise went in wrong, and it is cheaper to find that on the phone than on the deck.

Every figure above is worked out by the same engine that produces CutRoof's cut schedules, and is checked against it on every build.