Re-Roof a House: Calculators in Order

Roofing is sold by the square, which is 100 sq ft of roof surface, and the surface is bigger than the footprint because the roof is sloped. So the job starts with the pitch: it turns the footprint into roof area, and the area turns into squares, bundles, underlayment and nails.

Measure the pitch from the attic or with a level on the roof, and enter it as rise over a 12 in run. A 6 in 12 pitch multiplies the footprint by about 1.12; a steep 12 in 12 roof by about 1.41. Add the overhang and the difference between a simple gable and a hip roof, and you have the area. Three bundles of architectural shingles usually cover one square, and starter strips, ridge caps and drip edge are separate lines that people forget.

Check what is under the old roof before you order. The model code allows at most two layers of asphalt shingles, so a second layer over one existing layer is possible, but a third means a tear-off. Roofs with a pitch from 2 in 12 up to 4 in 12 need double underlayment. Damaged sheathing is replaced sheet by sheet, so keep a few extra panels on the order.

Ventilation decides how long the new shingles last: the common rule is 1 sq ft of net free vent area for every 150 sq ft of attic floor, or 1 for 300 with balanced intake and exhaust. Gutters change with the roof area they drain. End with a cost range for your state, and add the lists to your project list to hand one order to the supplier.