Setting out, explained

What CutRoof calculates and what stays your judgement

The geometry the app is responsible for, the decisions it hands back to you, and the roofs it will not attempt.

CutRoof calculates geometry from your dimensions. Structural design and timber sizing are the responsibility of a competent person.

That line is on the schedule for a reason, and this article is the longer version of it.

What the app is responsible for

Give it a span, a plate length, a pitch, sections, an overhang and centres, and it works out every member on a gable, a hipped or a lean-to roof: commons, hips, jacks, crown rafters and the ridge board.

For each one it gives true lengths, plumb and seat cuts, edge bevels, birdsmouth depth with its plumb and seat lengths, height above plate, hip drop or backing bevel, and quantities. Every figure is shown with the working that produced it, deductions as separate steps, so a schedule can be checked against a square rather than believed.

If a number in the app is wrong, that is the app's fault and it is a bug worth reporting.

What it assumes about your building

The geometry it works is a clean one. Square corners, equal pitch on every face, straight plates, level plates, and the span measured across the outside faces.

Buildings depart from it all the time. A plan that is 20mm out of square at one corner, a plate that dips, a gable wall that leans: the app cannot see any of it, and its figures will be exactly right for a building that is not the one you are standing on. Check the plates for square and level before you cut a set, and take the span at both ends.

Decisions it hands back to you

The conventions are the obvious ones. Where the run ends, how the birdsmouth is sized, whether the hips are backed or dropped, how bevels are stated. The app has UK defaults for all of them and prints what it used, in words, on every output. It never guesses quietly.

Rounding is yours too. The 5mm site rounding option always rounds lengths down, and notch depths are never rounded up, so an error goes in the direction you can recover from at the bench.

Warnings are advisory. A pitch outside common UK practice, or a seat cut shortened to fit the wall plate, still gives you a schedule. The app tells you what it did and leaves the decision with you.

What it does not do

It does not size timber, check a section for span, or tell you whether a rafter will carry what is going on it. It does not schedule purlins, struts, collars, binders, bracing, straps or fixings. It does not check building regulations or produce anything a building control officer should be handed instead of a structural design.

It also does not attempt every roof. Valleys, T and L plans, half hips and unequal pitch are outside this version. When one of those turns up, the app is the wrong tool for that part of the job, and it would rather say so than give you a number that looks like an answer.

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

Try the rafter calculator

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