Roofing calculator
A 40 ft by 30 ft house with a 1 ft overhang on every side and a 6/12 pitch (the roof gains 6 inches in height for every 12 inches it runs horizontally) needs about 50 bundles of shingles. That number starts with the footprint plus overhang, which comes to 1344 sq ft, then scales up for pitch to 1502.6 sq ft of roof surface.
Roofing
Tap or type into any yellow number on the drawing to change it.
Ridge caps and cut waste add 10 percent extra, and the bundle count already covers that.
Shingles are sold by the roofing square, a unit of 100 sq ft of roof surface, and three bundles cover one square. The calculator starts with the house footprint plus overhang on every side, then scales that flat area up to the actual sloped roof using a pitch multiplier. You find the multiplier by squaring the pitch divided by 12, adding 1 to that result, and taking the square root. A 6/12 roof, for example, has a multiplier of about 1.118. After calculating the roof area, the tool adds a 10% waste allowance to the bundle count to cover ridge caps and cuts. It also estimates felt underlayment, with one roll covering four squares.
Depending on the unit system you select, the measurements for house length, width, and overhang appear in either feet or meters.
What is a roofing square?
A roofing square is a unit of roof area equal to 100 sq ft. It is the way shingles are counted and sold. Three bundles of shingles cover one square.
For the same footprint, why do steeper roofs require more shingles?
A steeper roof has a larger sloped surface than a flatter one over the same footprint. The pitch multiplier accounts for this, so the calculator scales the flat footprint area up by a factor you get when you square the pitch divided by 12, add 1, and take the square root. The multiplier grows as pitch rises, which means more shingles for the same house footprint.
For a roof, how many rolls of felt should you buy?
The calculator estimates felt underlayment separately from shingles. One roll covers four roofing squares, so you divide the total roof area in squares by four to get the number of rolls. For the 50-bundle example, the roof area is about 15 squares, which comes to roughly four rolls.