How to use the roofing calculator
- Pick the roof type. Gable roofs have two slopes and gable ends. Hip roofs slope on all four sides. Shed roofs have one slope.
- Enter the footprint wall to wall: length along the ridge and width across it. Use
40'or40' 6. - Enter the pitch, like
6or6/12. Not sure? Use the roof pitch calculator. - Enter the overhangs, measured level from the wall. On a hip roof the eave overhang is used on all four sides.
- Add waste. About 10% for a plain gable and 15% for a hip or a cut-up roof with valleys.
The math
Roof area is the footprint, including overhangs, times the slope factor for the pitch. The slope factor is the rafter length per foot of run: √(1 + (pitch ÷ 12)²). For 6/12 it's 1.118. When every plane has the same pitch, this works for gable, hip and shed roofs alike, because each plane's area is its plan area times the same factor.
Example: a 40' × 30' house with a 6/12 gable roof and 12" overhangs. With overhangs the plan is 42' × 32' = 1,344 sq ft. Times 1.118 that's 1,503 sq ft, or 15.03 squares. Add 10% and you need 52 sheets of 4×8 sheathing and 50 bundles of 3-bundle-per-square shingles.
Roofing terms
- Square: 100 sq ft of roof. Most architectural and 3-tab shingles come 3 bundles per square. Some heavy designer shingles are 4 or 5.
- Sheathing: one 4×8 sheet covers 32 sq ft. Use the thickness and span rating your code and shingle maker call for, commonly 7/16" OSB or 1/2" plywood on rafters 24" O.C.
- Drip edge: metal edge along the eaves and rakes, sold in 10' pieces. Add a little for laps.
- Starter strip: runs along the eaves. Many installers also run it up the rakes.
- Ridge cap: covers the ridge and, on a hip roof, the hips. Check the bundle label for linear feet per bundle.
What it doesn't include
Valleys, dormers, skylights, chimneys and wall flashing all need their own material. Ice-and-water membrane along the eaves and in valleys is required in many cold climates. Check your local code and the shingle maker's installation instructions, since following them is often a warranty condition.
FAQ
How do I calculate roof squares?
Multiply the footprint, including overhangs, by the slope factor for your pitch, then divide by 100. A 42' × 32' plan at 6/12 (slope factor 1.118) is 1,503 sq ft, or about 15 squares before waste.
How many bundles of shingles are in a square?
Three for most 3-tab and architectural shingles. Some heavy or designer shingles take 4 or 5 bundles per square, so check the wrapper.
How many sheets of plywood do I need for my roof?
Divide the roof area by 32 (one 4×8 sheet) and add about 10% for waste. A 1,503 sq ft roof needs 52 sheets with 10% waste.
Does a hip roof have more area than a gable roof?
Not with the same footprint, pitch and overhangs. Both equal plan area times the slope factor. A hip roof does have more waste from cutting and more ridge cap, so plan on about 15% waste instead of 10%.