A 40×30 ft plan roof at 4/12 pitch needs about 14 squares of shingles after 10% waste — roughly 42 bundles at 3 bundles per square — not the 12 you get from dividing plan area by 100.
The short count shows up on the ridge. You ordered from the footprint, skipped pitch, and left waste at zero. The last course needs another square you do not have, so someone drives back while the crew waits.
The shingle-square formula is:
squares = ceil( L × W × √(1 + (rise/run)²) × 1.10 ÷ 100 )
Plan area is length × width. Multiply by the pitch factor (4/12 ≈ 1.054). Add about 10% waste, then divide by 100 — one square is 100 sq ft — and round up. Typical 3-tab packs are 3 bundles per square. Estimates only — not engineering or building-code advice.
| Plan L×W | Pitch | Roof sq ft | Buy squares | Bundles |
|---|---|---|---|---|
| 20×30 | 4/12 | 632 | 7 | 21 |
| 24×36 | 4/12 | 911 | 11 | 33 |
| 30×40 | 4/12 | 1,265 | 14 | 42 |
| 40×30 | 4/12 | 1,265 | 14 | 42 |
| 40×40 | 4/12 | 1,687 | 19 | 57 |
| 40×30 | 6/12 | 1,342 | 15 | 45 |
How many squares for a 40×30 roof?
Plan area is 40 × 30 = 1,200 sq ft. At 4/12 the pitch multiplier is √(1 + (4/12)²) ≈ 1.054, so roof area ≈ 1,265 sq ft. With 10% waste → about 1,391 sq ft. Divide by 100 and ceil → buy 14 squares (~42 bundles).
Order from plan area alone and you get 12 squares before waste — two squares short before the pitch even bites. At 6/12 the same footprint needs 15 squares after waste.
The formula, on a tape
Measure plan length and width in feet. Read pitch as rise over run (inches of rise per 12 in of run is common in the US).
- Pitch multiplier = √(1 + (rise ÷ run)²)
- Roof sq ft = plan length × plan width × multiplier
- With waste = roof sq ft × 1.10 (default 10%)
- Buy squares = ceil(with waste ÷ 100)
- Bundles (3-tab typical) = squares × 3
Example: 20×30 at 4/12. Plan 600 → roof ≈ 632 → with waste ≈ 696 → buy 7 squares (21 bundles).
In Roofing Takeoff Master you enter the plane once — size, pitch, waste — and get squares, bundles, felt, and drip edge on your phone. Free for a simple plane; Pro unlocks extras at $9.99. AU/UK/NZ can switch region in the same app for metal or tile counts.
Three mistakes that short the shingle order
Ordering from plan area. Plan sq ft is the floor under the roof, not the roof surface. A 40×30 plan is 1,200 sq ft; at 4/12 the roof is about 1,265 before waste. Hang shingles from roof math.
Zero waste. Starter, ridge, hips, valleys, and bad breaks eat stock. About 10% is the common pad for a simple rectangle. Complex roofs need more.
Ignoring pitch — or mixing bundles and squares. Steeper pitch means more surface. And one square is 100 sq ft of coverage, not one bundle. Most 3-tab packs ship three bundles per square; architectural packs often differ — check the label.
Pitch multipliers you will use
Common US rise/run factors: 4/12 ≈ 1.054, 6/12 ≈ 1.118, 8/12 ≈ 1.202, 12/12 ≈ 1.414. Multiply plan area by the factor, then add waste. Degrees work too (multiplier = 1 ÷ cos(angle)); the app accepts rise/run or degrees.
Worked example for a 40×30 garage
Plan 1,200 sq ft × 1.054 ≈ 1,265 roof sq ft. × 1.10 ≈ 1,391. Buy 14 squares (~42 bundles of 3-tab). Same plan at 6/12 → buy 15. For the count plus felt and drip edge on your phone, open Roofing Takeoff Master. Related: how many bags of concrete, how many deck boards, Sheet Takeoff Master review.
Estimates only — not engineering or building-code advice. Measure pitch. Add waste. Round up.
Open Roofing Takeoff MasterEstimate only — not engineering or building-code advice.