Roofing calculator
Enter the size of the house and the roof pitch. You get the true roof area, the squares and bundles to order with waste, and every other item on the material list, each with the quantity it is based on.
Inputs
Materials
Result
| Item | Order |
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Roof area multiplier by pitch
Multiply the footprint, overhangs included, by the factor for your pitch to get the true roof area. Squares are per 1,000 sq ft of footprint.
| Pitch | Multiplier | Roof per 1,000 sq ft | Squares |
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How roof area is worked out
Measure the house on the ground, not the roof. Add the overhangs to get the footprint out to the drip edge, then multiply by the rafter factor for the pitch. This holds for any roof where every plane has the same pitch, gable, hip or something more cut up, because each plane is its plan area tipped up by the same angle.
The default house, 40 by 28 ft with 12 in overhangs, has a 42 by 30 ft footprint: 1,260 sq ft. At 6/12 the factor is 1.118, so the roof is 1,408.7 sq ft, or 14.09 squares. With 10% waste that is 15.5 squares, and at three bundles a square, 47 bundles.
The rest of the list
- Underlayment. Rolls are sold by gross area. A synthetic 10-square roll covers about 9 squares once laps are taken out; No. 15 felt covers 4 and No. 30 covers 2. Between 2/12 and 4/12 the IRC requires two layers under shingles, so the quantity doubles.
- Drip edge. Every eave and rake, in 10 ft lengths lapped 2 in.
- Starter strip. Along the eaves and up the rakes. Bundle coverage varies from about 100 to 120 ft by brand; the count here assumes 100.
- Ridge cap. Every ridge and hip. Bundles cover roughly 20 to 33 ft; the count here assumes 30, so check the wrapper.
- Ice barrier. Where the code official requires it, IRC R905.1.2 runs it from the eave edge to at least 24 in inside the exterior wall line. That distance is measured up the slope, so steep roofs with deep overhangs often need two courses or more.
- Nails. Four per shingle is the standard pattern and six is the high-wind pattern, at about 80 shingles a square.
Choosing a waste factor
Waste is the shingles cut off at rakes, hips and valleys, plus starter and the odd damaged piece. A plain gable with long runs wastes the least, about 10%. A hip roof cuts a triangle off every course at every hip, so 15% is normal. Add about 5% more for each feature that makes cuts: dormers, valleys, skylights and chimneys. On a complicated roof, 20% is not unusual.
Questions
How many bundles of shingles are in a square?
Three for most shingles, both 3-tab and standard laminates. Some heavy designer laminates come four or even five to the square. A square is 100 sq ft of roof, so a 25-square roof takes 75 bundles of a 3-bundle shingle before waste.
How do I calculate roofing squares?
Multiply the footprint, overhangs included, by the rafter factor for the pitch, divide by 100, then add waste. A 1,500 sq ft footprint at 6/12 is 1,500 x 1.118 = 1,677 sq ft, or 16.8 squares, and 18.5 squares with 10% waste.
How many squares is a 2,000 sq ft house?
It depends on the footprint, not the living area. A single-storey 2,000 sq ft house with 12 in overhangs has a footprint of about 2,200 sq ft; at 6/12 that is about 24.6 squares, or 27 with 10% waste. The same floor area on two storeys has half the footprint and roughly half the roof.
How much extra roofing should I order for waste?
10% for a simple gable, 15% for a hip roof, and about 5% more for each feature that adds cuts, such as valleys, dormers and skylights. Very cut-up roofs can reach 20%.
Does roof pitch change how many shingles I need?
Yes. A steeper roof has more surface over the same footprint. At 4/12 the roof is 5% bigger than its footprint, at 6/12 12%, at 8/12 20%, and at 12/12 41%.
Do I need two layers of underlayment?
Under asphalt shingles on a pitch from 2/12 up to 4/12, yes: the IRC requires two layers there. At 4/12 and steeper, one layer is enough. Below 2/12 shingles are not allowed at all.
Where is ice and water shield required?
Wherever the local code official says there is a history of ice forming at the eaves. IRC R905.1.2 then requires it from the lowest edge of the roof to at least 24 in inside the exterior wall line. Many roofers also run it up valleys and around chimneys and skylights.
More Roofing calculators
Roof sheathing calculator
Sheets of OSB or plywood roof decking from the house size and pitch, with H-clips where the panel needs them, nails and weight.
Attic ventilation calculator
Net free vent area for an attic by the IRC 1/150 and 1/300 rules, split between ridge and soffit vents.
Siding calculator
Squares of siding and the number of lap planks, Hardie or wood, for walls and gables, net of openings.
Roof pitch calculator
Pitch from rise and run or an angle, in degrees and percent, with rafter length, hip factor and the roofing each pitch allows.