Quick answer
Step 1: Measure your home from the ground
Use a tape measure or a measuring wheel. Measure the length and width of your home's outside walls. For an L-shaped home, split it into rectangles and measure each one.
Step 2: Add the overhang
The roof usually sticks out past the walls, often about 1 foot. Add the overhang to both sides of each measurement.
Example: a 40 × 30 foot home with 1-foot overhangs becomes 42 × 32 feet. That's 1,344 square feet of "plan" area.
Step 3: Find your roof pitch
Pitch is how many inches the roof rises for every 12 inches across.
- From the attic: hold a level against a rafter. Measure 12 inches along the level, then measure straight up to the rafter. That number is your rise. A 6-inch rise means a 6/12 pitch.
- From the ground: some phone apps can estimate pitch from a photo.
- Not sure? 4/12 to 6/12 is common for many homes.
Step 4: Multiply by the pitch factor
| Pitch | Multiply by |
|---|---|
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 12/12 | 1.414 |
Example: 1,344 × 1.118 = about 1,503 square feet of roof.
Step 5: Convert to squares
Roofers price and order by the square, which is 100 square feet.
Example: 1,503 ÷ 100 = about 15 squares.
Step 6: Add waste if you're ordering shingles
Add about 10% for a gable roof or 15% for a hip roof to cover cuts. Most shingles come about 3 bundles per square.
Example: 15 squares + 10% = about 16.5 squares, or about 50 bundles.
Hip roofs and odd shapes
A gable roof has two sloped sides. A hip roof slopes on all four sides. The ground-and-pitch method works for both, since the plan area is the same. Hip roofs just need more waste for the extra cuts. For complex roofs with many sections, measure each rectangle separately and add them up.
Why your number won't match the roofer's exactly
Dormers, valleys, chimneys, skylights, and multiple roof levels all add area and waste. A good roofer will measure exactly. Use your number to check that the quotes you get make sense. Here's how to compare roofing estimates.
Safety first: you don't need to get on your roof to measure it. Falls from roofs are a serious risk. Measure from the ground and the attic, and leave the roof to the pros.
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