Calculate roof area in squares, shingles needed, underlayment and ridge cap — supporting gable, hip, flat and mansard roof styles.
Enter your figures and click Calculate to see your results.
Enter the base length and width, select roof pitch and waste factor — results include squares, shingles and underlayment.
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A roofing square equals 100 square feet of roof area. Roofing materials (shingles, underlayment, tiles) are typically sold by the square. If your calculator shows 24 squares, you need enough shingles for 2,400 sq ft of roof area. Always add a waste factor of 10–15% for a simple gable roof and 15–20% for complex hip or multi-faceted roofs.
Multiply the flat (plan) area by the pitch multiplier. Pitch multipliers: 3:12 = 1.031, 4:12 = 1.054, 5:12 = 1.083, 6:12 = 1.118, 8:12 = 1.202, 10:12 = 1.302, 12:12 = 1.414. For a 40 × 30 ft house with 6:12 pitch: Plan area = 1,200 sq ft × 1.118 = 1,342 sq ft actual roof area = 13.42 squares.
Three-tab asphalt shingles: 3 bundles per square (80 shingles). Architectural (dimensional) shingles: 3 bundles per square, though some manufacturers vary. For 24 squares, you need 72 bundles, plus waste (for 15% waste: 72 × 1.15 = 83 bundles). Always check the manufacturer specification — bundle coverage varies slightly by product line.
Roof pitch is the slope expressed as rise over 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every 12 inches of horizontal distance — that is a 26.6° angle. Low-slope roofs are under 4:12, conventional slopes are 4:12 to 9:12, steep slopes are above 9:12. Pitch affects both material quantity and installation difficulty and cost.
Low-slope roofs (under 3:12) require special waterproofing systems (modified bitumen, TPO, EPDM, built-up roofing) because water drains slowly and can pond. Steep-slope roofs (over 4:12) use shingles, tiles, or metal panels — gravity provides drainage. Most residential roofs are 4:12 to 8:12. Installation methods, safety requirements and costs differ significantly between slope categories.
Three-tab asphalt shingles last 15–20 years. Architectural (dimensional) shingles last 25–30 years. Premium impact-resistant or Class IV shingles can last 30–50 years. Lifespan depends on climate (UV, freeze-thaw cycles, hail), ventilation (poor attic ventilation cuts lifespan significantly), installation quality and maintenance (clearing debris, fixing flashing early).
For slopes 4:12 and above: one layer of #15 or #30 felt paper (standard) or synthetic underlayment (more tear-resistant). For slopes 2:12 to 4:12: two layers of #15 felt with offset laps. For low-slope: self-adhering ice and water shield. In snow climates, ice and water shield is required in vulnerable zones: eaves, valleys, rakes and penetrations.
Drip edge is a metal flashing installed at the eaves and rakes of a roof. It directs water away from the fascia and into the gutter, prevents water infiltration at the roof edge and provides a clean finished look. Most building codes and shingle manufacturer warranties require drip edge. It is relatively inexpensive and a critical part of a properly installed roof.
In the US, a full roof replacement typically costs $5,000–$15,000 for an average home (1,500–2,500 sq ft of roof area) with architectural shingles. Premium materials (metal, tile, slate) can double or triple this cost. Factors include: roof area, pitch and complexity, material choice, local labour rates, removal and disposal of old materials, and any needed structural repairs.
Repair when damage is localised (a few missing shingles, isolated leaks, minor flashing issues) and the roof is under 15 years old. Replace when there is widespread granule loss, cracked or curling shingles across large areas, multiple leaks, sagging deck, moss/algae compromise, or the roof exceeds 20 years old. A professional inspection will identify the tipping point between repair and full replacement.
