Planning a shingle roof
Work in this order. First find the pitch: measure it from the attic on a rafter with the roof pitch calculator rather than climbing up. Then turn the footprint into roof area and squares (100 sq ft of roof each) with the roofing calculator, and only then count bundles, underlayment and the extras.
Common buying mistakes
- Ordering from the footprint. A sloped roof is bigger than the ground it covers: a 30 × 40 ft footprint under a 6/12 roof holds about 1,342 sq ft of roof, 12% more.
- Too little waste. Allow about 10% on a simple gable and 15% on hip roofs or roofs with valleys and dormers.
- Forgetting the separate products. Starter strip, hip and ridge caps, drip edge and ice-and-water shield aren’t in the bundle count. In cold climates the code asks for an ice barrier from the eaves to at least 24 in inside the exterior wall.
- Shingles on a low roof. Asphalt shingles need a pitch of at least 2/12, and between 2/12 and 4/12 the code requires a double layer of underlayment.
- Roofing over two layers. If the roof already has two layers, both must come off before a new one goes on, so price the tear-off and a dumpster.
- Mixing production runs. Buy all bundles, plus a few spares, from the same lot so the color matches.
Roof work is one of the most dangerous DIY jobs. OSHA requires fall protection for roofers working 6 ft or more above the ground, and the same harness and anchors protect you. Check your local permit rules before you start, and hire a licensed roofer for steep or high roofs.