How big is my roof?

What it works out

A roof at a constant pitch has a surface area equal to the area it covers on the ground multiplied by a factor that depends only on the pitch. That is the whole of the mathematics, and it is why this asks for a footprint and a slope rather than for the length of a rafter you cannot reach.

From there: divide by a hundred for squares, which is the unit roofs are counted in, and add whatever allowance you want for waste. Every number is shown with the sum that produced it, so nothing has to be taken on trust.

It measures. It does not price

There is no cost per square on this page and there is not going to be one.

What a roof costs depends on what is underneath the shingles, how many layers are already up there, what the deck is like when it is exposed, what the code requires at your eaves, how exposed to wind the site is, how a truck and a dumpster get to the house, and what the season is doing. None of that is visible from a form. A number produced without it is not a price — it is a guess that somebody has to walk back later, in front of you.

A scope and a price go in writing before work starts, and that happens after somebody has seen the roof. This page exists to make that conversation start from real dimensions instead of from nothing.

A number produced without seeing the roof is not a price.

And it is not a survey

A measurement taken from the ground, or off a satellite photograph, cannot see a second layer of shingles, a deck that has gone soft, flashing that was never right around the chimney, a valley that has been patched twice, or the plane on the far side that is not visible from the driveway.

So treat what comes out of this as the size of the job, not as the state of it. Somebody still has to get up there.

Nothing you type here is sent anywhere

Every measurement stays in your browser, on this device. There is no account, nothing is stored, and nothing is transmitted — the page has no way to send it. These are the dimensions of somebody's house, which is a detail worth not collecting.

At the end there is a link that carries a short summary — the totals only, no address — into the guided intake, if you want the office to see it. You choose whether to follow it, and you can read the summary first.

How the arithmetic works

That is every step. If you want to check a result, the page shows each sum with its own numbers in it, so you can follow it with a calculator and get the same answer.

The footprint

Length times width of the area the roof covers, measured on the ground and including the overhang on every side. On a plain rectangle that is one sum.

The pitch factor

A roof described as 6 / 12 rises six inches for every twelve it runs. The sloped length over that run is the hypotenuse, so the factor is the square root of (rise squared plus 12 squared), divided by 12. At 6 / 12 that is 1.118.

The sloped area

Footprint multiplied by the factor. A 40 by 28 foot footprint is 1,120 square feet on the ground; at 6 / 12 the roof over it is 1,120 × 1.118, which is 1,252 square feet.

Squares

One square is 100 square feet. It is how roofs are counted, ordered and talked about. 1,252 square feet is 12.5 squares.

Waste

An allowance for cuts, starters, overlaps and the offcuts that cannot be used. It is a judgement, not a constant, which is why you choose it here rather than being given one.

Why the shape barely matters

A gable, a hip and a shed roof over the same rectangle at the same pitch all have the same surface area. That surprises people, and it is worth knowing: a hip roof is cut into four planes rather than two, but the four planes together cover exactly the same ground at exactly the same angle, so they come to the same number of square feet.

What the shape does change is the waste. A hip roof has four hips and often several valleys, so more of every course ends up as an offcut, and a cut-up roof with dormers and returns more so again. That is why the shape you pick here changes the note beside the waste field and nothing else in the sum.

A roof with two different pitches on it — a gambrel, or a mansard, or a porch at a shallower slope than the house — is two measurements, not one. Enter it as two sections and the page adds them together.

Measuring it without a ladder

1. Pace the footprint

   Walk the outside of the building and measure the ground it covers, adding the overhang at each end. A long tape and somebody to hold it beats an estimate; a pace is about two and a half feet if you have nothing else.

2. Break it into rectangles

   Almost every house is two or three rectangles once you stop trying to see it as one shape. An L is two. A house with a garage wing is two. A porch is a third.

3. Read the pitch from the gable end

   Stand back from the end of the house and hold a level out against the gable: the rise over twelve inches of level is the pitch. From the ground, comparing the slope against something you know — a 6 / 12 is close to a 26 degree angle — gets you near enough to be useful.

4. Say what you are not sure about

   If you had to guess the pitch or you could not see the back, say so when you call. A rough measurement honestly labelled is far more useful than a confident one that is wrong.