Exterior Envelope Sheet

Footprint is not roof area

This is the mistake that costs money on a roof order. The house measures 32 by 44 on the ground, so the roof is 1,408 square feet — except it is not, because the roof is sloped and the ground is not. A 6/12 pitch is 12 percent more surface than the ground it covers. At 8/12 it is 20 percent. At 12/12 it is 41 percent.

Multiply the footprint by the pitch factor and you get the true sloped area. The sheet lists the factors; the spreadsheet looks them up for you once you type the pitch.

Gables are a triangle, and they are always forgotten

A gable end is half its base times its height. A house 30 feet wide with a gable rising 6 feet has 90 square feet of wall above the eave line on that end, and another 90 on the other one. That is most of two squares of siding sitting in a shape nobody measured because it is above head height.

Deduct the big openings only

A door is a hole in the wall, so subtracting it looks like the careful thing to do. In practice cutting siding or sheathing around an opening makes cut-offs that mostly cannot be used anywhere else, so subtracting the full opening leaves you short. Ignore anything under about 20 square feet, take half between 20 and 50, and take the whole thing only for something genuinely large.

Downspouts: count it twice and take the larger

There are two rules and they disagree, which is why the sheet asks for both. One downspout per 35 feet of gutter run keeps the water moving along the trough. One per 600 to 800 square feet of roof handles the volume arriving in it. A long gutter under a small roof is limited by the first; a short gutter under a big steep roof is limited by the second. Take whichever number is higher.

Pitch matters here too. A steep roof throws water at the gutter faster than a shallow one of the same footprint, so the same gutter overshoots in a downpour.