Roofing calculators
Shingles, metal panels, pitch and true sloped area.
What is in this category
Two calculators covering the two materials on most residential roofs: asphalt shingles and metal panels. They work completely differently, and it is worth knowing why before you measure anything.
A roof is bigger than the house it sits on
This is the number one roofing mistake, and it is pure geometry. A sloping surface is always larger than its footprint, and the steeper it gets the bigger the gap.
The fix is a pitch multiplier. Find the rise per 12 inches of run, then multiply the flat footprint by the figure below:
| Pitch | Multiplier | 1,500 sq ft footprint becomes |
|---|---|---|
| 3/12 | 1.031 | 1,546 sq ft |
| 4/12 | 1.054 | 1,581 sq ft |
| 6/12 | 1.118 | 1,677 sq ft |
| 8/12 | 1.202 | 1,803 sq ft |
| 12/12 | 1.414 | 2,121 sq ft |
Both calculators apply the multiplier for you. Measure the building on the ground — there is no need to get on the roof.
Shingles are counted by area. Panels are counted across the eave.
Shingles cover area, so the sum is squares — one square is 100 square feet — and then bundles, at three to the square for standard architectural shingles.
Metal is different. Panels run from ridge to eave in a single unbroken piece, so the count is the eave length divided by the panel's coverage width, and the second number you need is what length to order them. Coverage width is not sheet width: a panel sold as 39 inches typically covers 36 once it laps its neighbor. Order on the sheet width and you will be about 8% short.
What the panels and shingles sit on
Neither calculator's headline number is the whole order. Both roofs need underlayment. Shingles need a starter course along the eaves and ridge cap along the top, both of which are separate products from the field shingles. Metal needs ridge cap, closure strips, rake and eave trim, and sealant tape — every one a separate part number with its own lead time.
In snow country both want an ice and water membrane at the eaves, carried far enough up the slope to reach inside the exterior wall line. Order the trim with the roofing, not after.