A roofing quote is only as good as the measurement under it. Get the area wrong and either the price is wrong or the crew runs short of material halfway through the job.
You don't need to measure your own roof, and you shouldn't climb up to try. But knowing how the numbers are built lets you read a quote properly and ask the right question when two contractors give you different quantities for the same house.
Roof area is not floor area
Your house's square footage is living space, floor by floor. The roof covers something different: the footprint of the top of the house, plus the overhangs, plus any garage, porch or bump-out roof, and then adjusted for slope.
That's why the numbers rarely line up. A two-storey house can carry far less roof than its living space suggests, because the second floor sits under the same roof as the first. A wide bungalow with deep overhangs, or a house with a steep, cut-up roof, can carry more.
Pitch: the multiplier
Pitch (or slope) is written as rise over run: a 6:12 roof rises 6 inches for every 12 inches across. A steeper roof covers the same footprint with more surface, the way a ramp is longer than the floor under it.
To turn a flat footprint into real roof area, you multiply by a slope factor. IKO publishes these factors. The table below shows common ones.
| Pitch | Factor | 1,000 sq ft of footprint becomes |
|---|---|---|
| 4:12 | 1.054 | about 1,054 sq ft |
| 5:12 | 1.083 | about 1,083 sq ft |
| 6:12 | 1.118 | about 1,118 sq ft |
| 7:12 | 1.158 | about 1,158 sq ft |
| 8:12 | 1.202 | about 1,202 sq ft |
| 9:12 | 1.250 | about 1,250 sq ft |
| 10:12 | 1.302 | about 1,302 sq ft |
| 12:12 | 1.414 | about 1,414 sq ft |
Squares and bundles
Roofers count area in squares. IKO defines a square simply as 100 square feet of roof area, so a 2,300 sq ft roof is 23 squares.
Shingles are sold in bundles. For laminated shingles such as IKO Cambridge and Dynasty, IKO says three bundles cover one square. Other products can differ, so the bundle count comes from the product being installed, not a rule of thumb.
The edges are measured too
Area decides how many shingles and how much underlayment a roof needs. But a lot of the material goes along the lines of the roof, and those are measured in linear feet:
| Line | What it is | Material measured along it |
|---|---|---|
| Eave | The bottom, horizontal edge where the eavestrough hangs | Drip edge, starter strip, eave protection membrane |
| Rake | The sloped edge at the end of a gable | Drip edge, starter strip |
| Ridge | The horizontal peak where two slopes meet at the top | Ridge cap shingles, ridge vent |
| Hip | The sloped outside corner where two slopes meet | Hip cap shingles |
| Valley | The inside corner where two slopes meet and carry water together | Valley membrane and metal |
| Wall and chimney joints | Where the roof meets something vertical | Step flashing, headwall flashing, counterflashing |
Waste: the part nobody installs
Shingles get cut at every rake, hip, valley and wall, and the offcuts can't always be used elsewhere. So the material order is always more than the measured area. IKO puts typical waste at about 10 to 15 percent as a general range, and notes that it's hard to predict because it depends on the roof's complexity and size.
A simple gable roof with long, straight runs wastes less. A hip roof with valleys, dormers and short facets wastes more.
Why two quotes list different numbers
If one quote says 24 squares and another says 27 for the same house, that alone doesn't mean anyone measured wrong. Common, honest reasons:
- Measured area versus ordered area. One quote shows the roof's measured area. The other shows the area with waste already added.
- Rounding. Some round every facet up, or round the total up to the next full square or bundle.
- What's included. A detached garage, a shed, a porch roof or a low-slope section may be in one quote and not the other.
- Overhangs and pitch. Measuring the footprint without overhangs, or using one pitch for the whole roof, moves the number.
Don't climb up to check
You can get a rough sense of your roof from the ground: count the facets, note which slopes look steep, and look for valleys and dormers. Leave the tape measure on the roof to someone with fall protection. Contractors measure in different ways, including on the roof itself, from plans and from aerial imagery, and it's reasonable to ask which method a quote used.
How many square feet is a roofing square?
100 square feet of roof area. A roof with 2,300 square feet of surface is 23 squares.
How do you calculate roof area from pitch?
Multiply the footprint area, including overhangs, by the slope factor for that pitch. For example, IKO's factor for a 6:12 roof is 1.118, so 1,000 square feet of footprint is about 1,118 square feet of roof. Measure each section with a different pitch separately.
How many bundles of shingles are in a square?
For laminated shingles like IKO Cambridge and Dynasty, three bundles cover one square. Some specialty products need more, so check the product's coverage.
How much waste should a roofing quote include?
IKO gives about 10 to 15 percent as a general range, depending on how complex the roof is. A simple gable wastes less than a hip roof with valleys and dormers.
Why is my roof bigger than my house's square footage?
Sometimes it isn't. A two-storey house often has less roof than living space. Where it is bigger, it's usually overhangs, attached garages or porches, and pitch: a steep roof has more surface than the footprint under it.
Where the facts on this page come from. If a source has changed since we last checked, tell us and we'll fix the page.