The footprint is the easy part. Converting it to actual roof area is one multiplication that decides whether your order is right or short by up to a quarter.
Walk the outside of the building and measure the plan area the roof covers. Not the walls, the roof, which means including the eave overhang on every side. Overhangs of 12 to 24 inches are typical, and on a 40 by 30 building a forgotten 18 inch overhang loses you well over 100 square feet.
Split the shape into rectangles, calculate each, add. For anything other than a plain rectangle, sketch it first and mark which piece is which. The splitting and subtracting methods are covered here.
This is the step that gets skipped, and it is the whole reason roof estimates come up short.
A sloped roof covers more surface than the ground beneath it. The steeper it is, the greater the difference. The pitch factor is the multiplier that converts one to the other, and it is calculated as the square root of the rise squared plus 144, all divided by 12.
In practice nobody calculates it. You look it up.
| Pitch | Multiplier | Extra area vs footprint |
|---|---|---|
| 3/12 | 1.031 | 3% |
| 4/12 | 1.054 | 5% |
| 5/12 | 1.083 | 8% |
| 6/12 | 1.118 | 12% |
| 7/12 | 1.158 | 16% |
| 8/12 | 1.202 | 20% |
| 9/12 | 1.250 | 25% |
| 10/12 | 1.302 | 30% |
| 12/12 | 1.414 | 41% |
Look at the spread. A shallow 3/12 roof adds only three percent, which is inside most waste allowances and is why people get away with ignoring it on low roofs. A 12/12 adds 41 percent, which no waste allowance will absorb.
Ordering from the footprint rather than the sloped area produces a shortfall of roughly 5 to 25 percent depending on pitch. That is the difference between finishing the job and stopping two days in to wait for a delivery from a different batch.
A simple gable roof over a building 50 ft by 40 ft, with an 18 inch overhang all round, at 8/12 pitch.
Had you ordered from the 2,279 footprint you would have bought 23 squares for a 27.4 square roof, and been four and a half squares short.
It is the unit everything in roofing is priced and sold in. Divide your final area by 100 and round up. Nobody sells a partial square.
Calculate each plane separately, then add. Do not average pitches.
A house with an 8/12 main roof and a 3/12 porch has two different multipliers, and the porch may also be below the minimum pitch for the material going on the main roof. Averaging them to 5.5/12 gives a number that is wrong for both planes and hides a specification problem.
Hip roofs are worth care. The planes are triangles and trapezoids rather than rectangles, and it is easy to count a triangular section as though it were the full rectangle. Sketch it from above, mark each plane, and work through them one at a time.
What you now have is the surface area of the roof. It is what you need for underlayment, and it is the starting point for shingles.
It is not the shingle order. Shingles need additional material for the starter course along every eave, ridge cap along every ridge and hip, and cut waste at every valley. Those are linear measurements, not area, and they do not appear anywhere in the calculation above. Ordering shingles is covered separately, and metal roofing works on panel counts rather than area.
We cover laser measures and nothing else, and the front page is where the recommendations live.