Measure a roof from the ground, in about the time it takes to walk around it.

One length you know, and the rest follows

Tracing a roof over an aerial photo gives you a shape with no size. What turns it into measurements is calibration: you click one wall you actually know the length of — off the plans, off a tape, off the driveway — and type that length in. Everything else on the drawing scales off it.

Get that one number right and every measurement after it is right. Get it wrong and they are all wrong by the same proportion, which is why RoofMap asks for it as a deliberate step rather than guessing a scale from the image.

The roof lines draw themselves

The moment an outline closes, RoofMap asks the two things it cannot work out on its own: the roof shape and the sheet material. It suggests the shape from the footprint — on an L-shaped plan it will say hip and valley, and tell you that a straight gable would leave the wing without a ridge — so the usual case is one click.

Then it generates the ridges, hips, valleys and barges for that shape, in the right places, with their lengths. Five shapes are covered: hip and valley, straight gable, gable with a hip and valley corner, dutch gable, and mono-pitch. Pick the wrong one and Change roof type redraws it.

Pitch changes every length on the roof

A roof drawn on a photo is a plan view — flat. The real roof is longer than the plan wherever it slopes, and the difference is not small. At 25°, a rafter is about 10% longer than its plan length; at 35°, about 22%. Sheet runs, valley lengths, hip lengths and area all carry that factor.

RoofMap holds a pitch per roof, applies it to everything that slopes, and leaves the things that do not slope — the ridge, the gutter line — alone. That is the arithmetic most spreadsheets get wrong once and never notice.

Every roof on the property, drawn together

A house with a lean-to, a veranda and a garage is four roofs, not one. RoofMap draws them all on the same canvas and keeps them separate underneath: each has its own pitch, its own material, its own sheet runs and its own name.

Where two roofs share an edge, that edge is counted once. Counting it twice is one of the standard ways a take-off comes out over, and it is very hard to spot by eye on a six-roof property.

Sheet runs are measured to the gutter that actually catches them

A sheet run is the distance from the ridge down to the gutter it drains into. On a simple rectangle that is obvious. On a roof with a wing, a step-down or a second gutter at a different height, the nearest gutter in a straight line is often not the one the sheet actually reaches — and a run measured to the wrong one comes out metres long.

RoofMap measures each face to the gutter that catches it, ignoring gutters the run would have to cross another roof line to reach. Each face carries its run length on the drawing, so you can see the number that is going to be ordered.

What it does not do

It does not fly a drone, and it does not read a roof off a photo by itself. You trace it. That takes a couple of minutes on a normal house, and it is the reason there is no per-roof fee — nobody is sitting in an office drawing your roof for you.

It also does not know what is under the iron. Purlin condition, rot, and whether the old fixings will come out are still a site visit.