What a roofing square is
A roofing square is 100 square feet of actual roof surface area. A bundle of architectural shingles typically covers about 33 ft² (3 bundles per square). Underlayment, drip edge, and ice-and-water shield are similarly priced by the square. Shingle manufacturers publish coverage per bundle on the wrapper; verify your specific product before ordering.
Plan area vs roof surface
The plan area is the building's footprint when viewed from directly overhead. A 40 × 30 ft house has a 1,200 ft² plan area. The roof, because it's pitched, has more surface area than the footprint.
The pitch multiplier converts plan area to surface area. It's the secant of the slope angle, computed as:Pitch multiplier = sqrt(1 + (rise / 12)²)For a 6/12 pitch: sqrt(1 + (6/12)²) = sqrt(1.25) ≈ 1.118. Multiply your plan area by 1.118 to get the surface area on a 6/12 roof.
Memorize the common pitches:
- 0/12 (flat) — multiplier 1.000
- 2/12 — 1.014
- 4/12 — 1.054
- 6/12 — 1.118 (standard residential)
- 8/12 — 1.202
- 10/12 — 1.302
- 12/12 (45°) — 1.414
- 16/12 — 1.667
Converting to squares
Once you have the roof surface area, divide by 100 to get squares. A 1,200 ft² footprint at 6/12 pitch:1,200 × 1.118 = 1,342 ft² ÷ 100 = 13.42 squaresAdd waste, then round up. At 12% waste: 13.42 × 1.12 ≈ 15.03; round up to 15.5 or 16 squares for ordering.
Hip-and-valley waste
Hips and valleys are the diagonal lines where two roof planes meet. They're where shingles get cut at angles, generating more waste than a simple gable roof.
- Simple gable — 10% waste. Two planes meeting at a ridge, no diagonal cuts beyond rake edges.
- Hip roof — 12-15%. Four planes meeting at hips; every hip is a diagonal cut on shingles.
- Hip-and-valley with dormers — 15-20%. Multiple intersections, valleys, and complex geometry.
- Heavily cut roofs — 18-22%. Multiple dormers, turrets, skylights, integrated chimneys.
Steeper pitches consume more material per square foot of footprint but waste at roughly the same percentage as low-slope roofs. Closed-cut valleys waste less than woven valleys; metal valleys with shingle cut-back waste the most.
Ridge cap, hip cap, and starter
Beyond field shingles, a complete roof order includes:
- Ridge cap — bagged hip-and-ridge shingles. Coverage runs about 20-25 linear feet per bundle. Measure ridge + all hips in linear feet.
- Starter strip — at eaves and rakes. About 100 LF per bundle. Measure the perimeter of eaves and rakes.
- Ice and water shield — at eaves, valleys, and penetrations. Priced by the square. In cold climates, code usually requires 24 inches inside the warm wall line at eaves.
- Underlayment — synthetic underlayment is now standard. Priced by the square; rolls cover 4-10 squares each.
- Drip edge — perimeter linear footage.
Verifying on the roof plan
The roof plan in the construction drawings shows the roof from above with each plane labeled. The footprint is what you measure. Pitch is called out per plane in standard rise-over-12 notation (e.g. “6:12” or “6/12”). For complex roofs, the architect often labels the squares directly — verify against your own calculation.
When a satellite imagery service or aerial measurement report is used (EagleView, Hover, GAF QuickMeasure), the report gives you per-plane surface area, valley LF, hip LF, ridge LF, and perimeter. Skip the manual pitch math and use the report figures directly — but verify the report total squares against your own rough sanity check.