How Many Shingles for a 20x20 Garage Roof?
Roofing squares needed
4.6 squares
A 20 by 20 foot garage at a 4/12 pitch with ten percent waste.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Building length20 ft
- Building width20 ft
- Roof pitch, rise per 12 in4 /12
- Waste allowance10 %
- Ridge length, 0 if unknown20 ft
Roofing squares needed4.6 squares
| Footprint area | 400 sq ft |
|---|---|
| Slope multiplier at 4/12 | 1.05 x |
| Actual roof surface | 422 sq ft |
| Squares before waste | 4.2 |
| Squares with 10% waste | 4.6order to this number |
The material list this produces
| Shingles, bundles (3 per square) | 14 |
|---|---|
| Synthetic underlayment, rolls (10 sq each) | 1 |
| Starter strip, linear ft | 96 |
| Ridge cap, linear ft | 20 |
| Drip edge, linear ft | 80 |
| Roofing nails, lb | 10 |
Roof pitch compared
Pitch is the number most people guess at, and it is worth more shingles than any other input. Same house, five pitches.
| Roof pitch | Roofing squares needed |
|---|---|
| 4/12 | 4.6 squares |
| 6/12 | 4.9 squares |
| 8/12 | 5.3 squares |
| 10/12 | 5.7 squares |
| 12/12 | 6.2 squares |
What else matters here
Four hundred square feet of footprint at 4/12 is 422 square feet of roof, 4.6 squares with waste, fourteen bundles.
Every hole through a roof is a future leak that has been postponed, and on a garage the usual holes are a plumbing vent, a bath fan outlet or an exhaust flue. Each gets a boot: a flange that sits under the shingles above and over the ones below, with a collar sealing around the pipe.
The standard boot has a rubber or neoprene collar and that collar is the shortest lived component on the roof. It hardens and cracks in ultraviolet light, often at ten to fifteen years against a shingle rated for thirty, which is why so many roofs leak at a vent pipe long before they need replacing. Lead boots and boots with a metal storm collar cost a few dollars more and outlast the roof, which is the right trade every time.
What this calculator assumes
- Footprint times the slope multiplier gives the actual roof surface. A 6/12 roof is 11.8 percent larger than the ground it covers, which is the number most people forget.
- Three bundles per square is standard for both three tab and architectural shingles. Check your brand, a few premium lines run four.
- Synthetic underlayment is figured at 10 squares per roll, starter and ridge cap at one linear foot per foot.
Questions people ask
How many bundles of shingles for a 20x20 garage?
About 14 bundles at a 4/12 pitch with ten percent waste, which is 4.6 squares.
Why do roofs leak around vent pipes?
The rubber collar on a standard pipe boot hardens and cracks under sunlight, often in ten to fifteen years, which is well short of the life of the shingles around it.
What is the best pipe boot for a roof?
Lead, or one with a metal storm collar rather than a bare rubber collar. The extra cost is small and the boot then lasts as long as the roof.
The roofing calculator takes any dimensions you like and hands back the same list.