Roofing Calculators

Roofing is sold in a unit nobody else uses, ordered against a measurement nobody can reach, and governed by a chapter most estimates read only once.

The calculators

• Roof Pitch CalculatorTrigonometry, plus the IRC R905 thresholds the same pitch trips
• Roofing CalculatorSlope factor computed, not tabulated. IRC R905 requirements by pitch

The square. One hundred square feet, and it exists because a roofer works in bundles that cover a third of one. It is the only trade on this site with its own areal unit, and it means a roof is described in a number that is neither the footprint nor the area but a third thing. Getting from a footprint to squares is two multiplications and both of them get skipped.

The measurement nobody can reach. Every other quantity here can be taken with a tape from the ground or a ladder. Roof pitch cannot, on a roof you have not climbed, and neither can the length of a valley. Which is why the calculators here take a footprint and a pitch and derive the rest, rather than asking for slope lengths you would have to be standing on the roof to know.

Tear-off, overlay, and the weight question

A second layer of shingles over a first is permitted in some circumstances and it changes two things a quantity calculator will not tell you: the dead load on the structure, and the fastener length, because the nail now has to reach through the old layer as well and still make 3/4 in of sheathing. The rafter and truss pages carry the load side of that.

What a quantity page cannot decide

Ventilation ratios, flashing details at walls and chimneys, valley construction, and whether the deck under the old roof is sound at all. The last one is the commonest reason a roofing job costs more than the estimate, and it is invisible until the tear-off.

Where the numbers here come from

Geometry for the areas, computed rather than tabulated, and IRC Chapter 9 for everything the code attaches to pitch. That chapter was read from a city government publication that prints the model text in full, because the ICC's own site returns HTTP 403 to any automated request. Where a requirement is triggered by a local table, these pages ask you for the answer instead of assuming one.