How to Measure a Roof for Shingles
The number that trips people up on a re-roof: a roof is bigger than the house it sits on. A steep roof can carry 20% or more surface than the floor under it, and that gap is shingles you either ordered or didn’t. Measure it right and you buy once.
Here’s how to measure a roof without climbing up with a tape every time, turn that into roofing squares, and build the full materials order, not just the shingles.
Footprint is not roof area
Start with the footprint: the area the roof covers seen straight down from above. For a simple gable, that’s the length of the house times its width, including the eave and rake overhangs. A 40 by 30 foot footprint is 1,200 square feet. That’s the flat measurement, and it is not what you order shingles against, because the roof tilts away from flat.
Measure the footprint without a ladder
You have three good options:
- From the ground: measure the building’s length and width and add the overhangs (usually 1 to 2 feet per side). Good enough for an estimate.
- In the attic: measure the building’s interior and add the wall thickness and overhangs.
- Satellite tools: a measured aerial image gives footprint dimensions you can verify against the ground.
Break complex roofs into rectangles, figure each plane’s footprint, and total them. Don’t forget dormers and porch roofs.
Find the pitch, then the multiplier
Pitch is how many inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof climbs 6 inches per foot. You can measure it from inside the attic by holding a level against a rafter, or off a gable end with a level and a tape.
Every pitch has a multiplier, the number that converts flat footprint into actual sloped area. It’s the rafter length over the run.
| Roof pitch | Approx. angle | Area multiplier |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
So a 1,200 square foot footprint on a 6/12 roof is really 1,200 × 1.118, about 1,342 square feet of roof surface.
Get your pitch and multiplier
Open the full roof pitch calculator
Convert to roofing squares
Roofers order in squares. One square is 100 square feet of roof. Take your true area and divide by 100. The 1,342 square foot roof above is 13.4 squares.
Then add waste for cuts. Figure 10% for a simple gable and 15% or more for hips, valleys, and lots of penetrations, since every cut leaves an offcut you often can’t reuse. At 10%, 13.4 squares becomes about 15 squares to buy.
Bundles, and the rest of the order
Architectural (laminate) shingles run three bundles to the square. Fifteen squares is about 45 bundles. Three-tab shingles are also usually three bundles per square; heavier designer shingles can be four.
Shingles are only part of the job. A complete order also includes:
| Material | How to figure it |
|---|---|
| Underlayment | Same square count as the shingles |
| Starter strip | Linear feet of all eaves and rakes |
| Ridge cap | Linear feet of ridges and hips |
| Drip edge | Full perimeter, in 10-foot sticks |
| Roofing nails | About 320 per square (4 per shingle) |
| Ice and water shield | Eaves and valleys, per local code |
A full house, worked out
Say a simple gable home with a 40 by 28 foot footprint (1,120 sq ft) and an 8/12 pitch. True area is 1,120 × 1.202 = about 1,346 sq ft, or 13.5 squares. Add 10% waste and you’re buying 15 squares, roughly 45 bundles of architectural shingles, 15 squares of underlayment, and drip edge for the 136-foot perimeter. Measure ridges and rakes for the cap and starter.
Frequently asked questions
How many bundles of shingles do I need per square?
Three bundles per square for most architectural and three-tab shingles. Premium designer lines can be four. Always confirm on the wrapper.
How do I measure roof pitch from the ground?
Hold a level horizontally against the gable end, measure 12 inches along it, then measure straight down from the 12-inch mark to the roofline. That drop is your rise over a 12-inch run.
How much waste should I add for shingles?
10% for a straightforward gable, 15% or more for roofs with hips, valleys, and many cuts.
What is a roofing square?
100 square feet of roof area. It’s the standard unit roofers and suppliers price in.