Reference
How big is my roof?
You can measure a roof accurately without going on it. The footprint gives you the plan area, the pitch gives you a multiplier, and the two together give you the real surface — the number every roofing quote is built from.
Roof measurement from the ground
A roof is a sloped surface stretched over a flat area. Measure the flat area — the building footprint plus the overhangs, taken at ground level — then stretch it by the amount the slope demands. The only figure you need from the roof itself is its pitch, and that can be read from a gable end or from inside the attic against a rafter.
roof area = footprint × pitch multiplier
squares = roof area ÷ 100
ordered = squares × waste factor
Pitch is written as rise over run: the inches a roof climbs for every 12 inches it travels horizontally. Hold a level out from a rafter, mark 12 inches along it, and measure straight up to the underside of the roof — that measurement is the rise.
Roof pitch multipliers
The multiplier is the hypotenuse of a right triangle with a run of 12 and a rise equal to the pitch, divided by 12. Multiply the footprint by it and you have the sloped area.
| Pitch | Multiplier | Angle | Character |
|---|---|---|---|
| 1/12 | 1.003 | 4.8° | Low slope — shingles not permitted |
| 2/12 | 1.014 | 9.5° | Low slope — double underlayment required |
| 3/12 | 1.031 | 14.0° | Minimum for asphalt shingles |
| 4/12 | 1.054 | 18.4° | Walkable, very common |
| 5/12 | 1.083 | 22.6° | Walkable |
| 6/12 | 1.118 | 26.6° | The most common residential pitch |
| 7/12 | 1.158 | 30.3° | Roof jacks start to be needed |
| 8/12 | 1.202 | 33.7° | Steep — staging required |
| 9/12 | 1.250 | 36.9° | Steep |
| 10/12 | 1.302 | 39.8° | Steep — labor premium applies |
| 11/12 | 1.357 | 42.5° | Steep |
| 12/12 | 1.414 | 45.0° | Steep — often priced by the day |
The multiplier is why two houses with identical footprints can differ by 40% in roofing cost. A 1,500 square foot footprint is 1,581 square feet of roof at 4/12 and 2,121 at 12/12 — the same house, 34% more material and considerably more labor.
What is a roofing square?
A roofing square is 100 square feet of roof surface. It is the unit every roofer quotes in, every manufacturer packages to, and every estimate is priced by — so a quote given per square and a measurement taken in square feet differ by a factor of a hundred, which is worth checking before comparing two bids.
| Material | Per square | Note |
|---|---|---|
| Asphalt shingles | 3 bundles | Heavy architectural can be 4 or 5 |
| Roofing nails, 4-nail pattern | about 320 | Roughly 2.3 lb |
| Roofing nails, 6-nail pattern | about 480 | Required in high-wind zones |
| Synthetic underlayment | 1/10 of a roll | A roll covers 10 squares |
| #15 felt | 1/4 of a roll | A roll covers 4 squares |
| Starter strip | by measurement | Eaves and rakes, in linear feet |
| Ridge cap | by measurement | Ridges and hips, in linear feet |
| Drip edge | by measurement | Eaves and rakes, 10 ft lengths |
Waste, by what the roof actually looks like
Waste is not a safety margin, it is the material that gets cut off and thrown away. Every hip and every valley means shingles cut at an angle, and the offcut is rarely usable anywhere else.
| Roof | Add | Why |
|---|---|---|
| Simple gable, no penetrations | 10% | Rake cuts only |
| Gable with dormers or vents | 12% | Cuts around each penetration |
| Hip roof | 15% | Every hip is an angled cut on both sides |
| Multiple valleys and hips | 15–20% | Cuts on nearly every course |
| Steeper than 8/12 | add 2% | More breakage handling on the slope |
Worked example. A 40 × 30 ft house with 1 ft overhangs has a 42 × 32 ft footprint, or 1,344 square feet. On a 6/12 gable that is 1,344 × 1.118 = 1,503 square feet, or 15.0 squares. Add 10% for a simple gable and the order is 16.5 squares — 17 in practice, because shingles come by the bundle and three bundles make a square.
Measure your roof →