Roofing calculators
Work out pitch, rafter lengths and what the roof needs, with the formula behind every number shown.
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.
Roofing calculator
Roof area, squares and shingle bundles from the house size and pitch, with underlayment, drip edge, starter, ridge cap and ice barrier.
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.
The numbers a roof job starts from
- Pitch. Rise in 12 of run. It sets the rafter factor, which turns every level measurement into a length along the slope, and it decides which roofing materials the code allows.
- Rafter length. Half the span, less half the ridge board, times the rafter factor, plus the tail.
- Roof area. The footprint including overhangs, times the same rafter factor. Roofing is sold by the square, which is 100 sq ft of roof.
Measure twice on an existing house. Old framing is rarely the pitch the drawings say, and a quarter inch per foot changes the area of a big roof by more than a bundle.