How to measure a roof (without climbing it)
Measure the building's footprint from the ground — length (eave to eave) and width — and add 2 feet to each dimension for overhangs if you measured wall-to-wall. Then multiply by the pitch factor: the steeper the roof, the more surface area hiding in the same footprint.
| Pitch | Factor | Notes |
|---|---|---|
| 4/12 | 1.054 | Walkable, minimum for shingles |
| 6/12 | 1.118 | Most common residential |
| 8/12 | 1.202 | Steep — expect labor premium |
| 10/12 | 1.302 | Very steep, safety equipment required |
| 12/12 | 1.414 | 45° — specialty labor pricing |
Find your pitch from the attic: mark 12 inches horizontally on a rafter, measure the vertical rise. Six inches of rise = 6/12. Complex roofs (hips, valleys, dormers) add 15%+ waste instead of 10% — every valley and hip eats shingles in cuts.
What the numbers mean
- Square = 100 sq ft of roof. The industry's unit for everything.
- Bundles: 3 bundles of architectural shingles = 1 square (4 for some 3-tab). Always buy by the bundle count with waste included — dye lots vary.
- Underlayment: synthetic underlayment covers ~1,000 sq ft per roll; traditional #15 felt ~400. Ice-and-water shield goes in valleys and along eaves in cold climates.
- Drip edge: metal flashing along all eaves and rakes. Skipped on cheap jobs, required by most codes now — don't skip it.
- Ridge cap: pre-cut cap shingles for hips and ridges, roughly 35 linear feet per bundle.
Tear-off vs. overlay
Most codes allow a maximum of two shingle layers, but that permission isn't a recommendation. An overlay hides rotted decking, adds ~250 lbs per square of dead weight, and voids many manufacturer warranties. It also runs hotter, shortening the new shingles' life.
A full tear-off lets the roofer inspect and replace bad decking, install proper underlayment and ice shield, and gives you the full warranty. It costs $100–$150 more per square in labor — money well spent on a 25-year roof.
What affects roofing cost
Architectural shingle replacement typically runs $450–$700 per square installed — $9,000–$14,000 for a 20-square roof. The spread comes from tear-off layers, pitch (steep roofs cost 15–30% more in labor), stories (two-story adds staging cost), and complexity (valleys, dormers, skylights each add flashing labor).
Material upgrades: architectural shingles are the baseline; impact-resistant (Class 4) shingles cost ~20% more but can earn insurance discounts; metal roofing runs 2–3× shingles but lasts 40–70 years. Always get 3 itemized quotes specifying shingle brand, underlayment type, and warranty terms.
Common roofing mistakes to avoid
- Overlaying over two layers. Most codes allow only one overlay; a third layer traps heat, voids warranties, and hides deck rot. Tear off and inspect the deck.
- Ignoring ventilation. New shingles over a poorly vented attic bake from below and fail early. Make sure intake (soffit) and exhaust (ridge) vents are balanced when you re-roof.
- Nails driven wrong. High nailing and overdriven nails are the top two causes of wind blow-offs — shingles need 4–6 nails in the nailing strip, flush, not sunk.
- Hiring on price alone. A bid 30% under the others usually means skipped underlayment, reused flashing, or no permit. Compare line items, not just totals.
Roofing FAQs
How many bundles of shingles do I need?
Roof area ÷ 100 = squares; squares × 3 = bundles; add 10% waste (15%+ for complex roofs). A 2,000 sq ft roof needs about 66 bundles.
What is a roofing square?
100 square feet of roof area. Three bundles of architectural shingles cover one square.
How do I find my roof pitch?
From the attic, measure 12" horizontally along a rafter and read the vertical rise. Multiply footprint area by the pitch factor (e.g., 1.118 for 6/12).
Can I shingle over the old roof?
Codes usually allow two layers max, but overlays hide deck damage, add weight, and shorten shingle life. A tear-off is the better long-term value.
How long does a shingle roof last?
Architectural shingles: 22–30 years with proper ventilation. 3-tab: 15–20 years. Poor attic ventilation is the #1 killer of shingle life.