Roofing Calculator
Shingle bundles for a simple roof. Use footprint dimensions including overhangs and a pitch of at least 2/12; complex roofs need separate sections.
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Re-shingling a roof is doable for a fit, careful DIYer on a simple roof — but it is genuinely dangerous work and unforgiving of leaks. The difference between a 25-year roof and a callback is the underlayment, flashing, and nailing, not the shingles themselves.
DifficultyAdvanced
TimeA long weekend for a small, simple roof with a crew
SkillsComfortable working at height with fall protection; steep or complex roofs are a job for pros
PermitsRe-roofing almost always requires a permit and an inspection, and codes limit how many shingle layers are allowed.
Tools you'll need
- Roofing nailer or hatchet
- Pry bar / tear-off shovel
- Chalk line and utility knife with hook blades
- Roofing harness and rope (fall protection)
- Sturdy ladder and roof jacks
Step by step
- Tear off and inspect Strip old shingles down to the deck and inspect for soft or rotten sheathing — replace any bad decking before going further.
- Drip edge, then underlayment Install drip edge at the eaves, then ice-and-water shield at the eaves and in valleys, with felt or synthetic underlayment over the field.
- Lay a starter course Run a starter strip along the eaves so the first shingles seal and resist wind uplift.
- Shingle up with the right offset Work up the roof keeping the manufacturer's offset and exposure, snapping chalk lines to stay straight. Nail in the nail zone — not too high, not overdriven.
- Flash and cap Flash all penetrations, walls, and valleys (new flashing, not reused), then finish with ridge-cap shingles.
Common mistakes
Nailing too high or overdriving nails
Nails must hit the shingle's nail line and sit flush, not cut through — high or blown nails cause blow-offs and leaks.
Skipping ice-and-water shield
Eaves and valleys need a self-adhering membrane to stop ice dams and valley leaks.
Reusing old flashing
Flashing is where most roofs leak. Replace step, valley, and penetration flashing with the new roof.
Working without fall protection
Use a harness, rope, and roof jacks. No roof is worth a fall.
Safety
- Wear a roofing harness and tie off — falls are the real danger here.
- Stay off wet, frosty, or hot-soft roofs; watch overhead power lines with ladders.
Frequently asked questions
How many shingle bundles do I need?
Enter your roof footprint and pitch above — the calculator returns squares and bundles (assuming 3 per square) with an editable waste allowance, 10% by default. Standard felt is excluded below 4/12 pitch because a specified low-slope system is needed. Check product coverage and complex roof sections separately.
Can I shingle over the old roof?
Sometimes one layover is allowed, but a full tear-off lets you fix the deck and flashing and lasts longer. Check local code for layer limits.
Should I DIY my roof?
Only a simple, low-slope roof with proper fall protection. Steep, tall, or complex roofs — and anything you're unsure about — are worth hiring out.