How the roof is worked out
- The roof's edge. The house's length and width plus the overhang on every side. A 50 by 30 foot house with 12 inch overhangs has a roof edge of 52 by 32 feet.
- Area on the slope. The flat area inside the edge times the pitch's roof area factor. At 6/12 the factor is 1.118, so 52 × 32 = 1,664 sq ft of footprint is 1,860 sq ft of roof. The roof pitch calculator gives the factor for any pitch.
- Squares. A square is 100 sq ft of roof, so 1,860 sq ft is 18.6 squares.
- Waste. The pieces cut off at rakes, hips and valleys never cover roof. Add about 10% for a plain gable and about 15% for a hip roof.
- Bundles. Squares with waste times the bundles per square, rounded up: 20.46 × 3 = 62 bundles.
The edges and what goes on them
- Eaves are the level bottom edges. They take drip edge and starter strip, and where ice dams form, an ice barrier from the edge to at least 24 inches inside the wall line (IRC R905.1.2).
- Rakes are the sloped edges at a gable's ends. They take drip edge and, on most shingle lines, starter strip. Hip roofs have none.
- Ridge is the level line at the top, capped with ridge shingles or a ridge vent.
- Hips are the sloped outside corners of a hip roof, capped like the ridge. Their length comes from the hip and valley factor.
- Drip edge is required at the eaves and rakes of shingle roofs (IRC R905.2.8.5) and is sold in 10 foot pieces.
What the calculator assumes
- A rectangular house with every roof plane at one pitch, and the ridge along the longer side.
- Underlayment in synthetic rolls that cover 10 squares, doubled from 2/12 up to 4/12 as the code requires (IRC R905.2.2).
- Starter strip and ridge cap in linear feet. Coverage per bundle differs by product; the bundle wrapper gives the feet one bundle covers.
- An L-shaped or cut-up roof is measured as rectangles, each through the calculator, then added. Valleys, dormers and skylights add waste.
Common questions
How many bundles of shingles are in a square?
Three for most 3-tab and architectural asphalt shingles, each bundle covering about 33.3 sq ft. Some heavier designer lines come four or five bundles to a square. The wrapper says which.
How many squares is a 2,000 square foot house?
It depends on the footprint, the pitch and the overhang. A one-story 50 by 40 foot house is 2,000 sq ft; with 12 inch overhangs and a 6/12 gable roof it has 2,442 sq ft of roof, 24.4 squares, or 26.9 with 10% waste: 81 bundles. A two-story house of the same living area has about half the footprint, so about half the roof.
How much waste should a shingle order include?
About 10% for a simple gable, about 15% for a hip roof, and more for roofs cut up by valleys and dormers, often up to 20%. Starter and ridge cap are counted on their own, so they are not part of the waste.
How is a roof measured from the ground?
Measure the outside walls for the length and width, the overhang at an eave, and the pitch from a gable end or a ladder. Those four numbers are all the calculator needs. For an exact roof with every facet measured, order a roof report from the address.
Is roof area the same as the house's square footage?
No. Roof area is larger than the footprint because of the overhang and the slope. The 6/12 roof above covers 1.24 times the house's 1,500 sq ft footprint.