ROOFING SHINGLES

Measure the building footprint length and width in feet — that's the outer dimensions of the area covered by the roof, NOT the actual roof surface (the actual surface is larger because of the slope). Then enter the pitch as inches of rise per 12 inches of horizontal run. A 6/12 pitch rises 6 inches for every 12 horizontal inches; a 12/12 pitch is 45°.

The calculator multiplies your footprint by the pitch factor to get true roof area, converts to roofing squares (1 square = 100 ft²), and returns bundle count assuming 3 bundles per square with a 10% waste factor for cuts, ridge caps, and starter rows. For a complex roof with multiple ridges, hips, valleys, or dormers, increase the waste factor by ordering 1–2 extra squares above the calculated number.

For a 40 × 30 ft footprint with a 6/12 pitch:

Pitch multiplier = √(6² + 12²) ÷ 12 = 13.42 ÷ 12 = 1.118. So a 1,200 ft² footprint becomes 1,341 ft² of actual roof surface.

In squares: 1,341 ÷ 100 = 13.41 squares. With 10% waste: 14.75 squares = ~45 bundles of standard 3-tab or architectural shingles.

At ~$35–50 per bundle for architectural shingles, materials run $1,575–$2,250. Add roofing felt (1 roll per 4 squares = 4 rolls), ice & water shield for the eaves and valleys, drip edge, and ridge cap shingles. Total materials for a typical re-roof of this size land around $2,500–$3,500. Pro re-roofs typically cost $7,000–$10,000 for this footprint installed.

Measuring the actual roof surface instead of the footprint. The calculator wants the building footprint (top-down view) and applies the pitch math itself. Measuring the slope directly and entering that as L × W double-counts the pitch.

Forgetting hips, valleys, and dormers. A simple gable roof matches the calculator output closely. Hip roofs need 5–10% more shingles; cut-up roofs with multiple dormers need 15–25% more.

Underordering ice & water shield. Code in cold climates requires it from the eave to 2 feet inside the heated wall line, plus all valleys. Underordering forces a second supply trip mid-tear-off.

Skipping the dump fee in the budget. Tearing off old shingles generates 1–2 tons of debris per square. Renting a 10-yard dumpster runs $400–$700 — easy to forget when costing the project.

3 bundles per square. 1 square = 100 ft² of roof surface (not footprint).

Pitch multipliers: 4/12 = 1.054 · 6/12 = 1.118 · 8/12 = 1.202 · 12/12 = 1.414.

10% waste for simple gable roofs; 15–20% for hip, complex, or cut-up roofs.

3-tab shingles last 15–20 years; architectural last 25–30; premium 40–50.

Materials cost is roughly ⅓ of total re-roof cost; labor is the other ⅔.

Tear-off generates ~1.5 tons of waste per square — plan dumpster size accordingly.