How Many Shingles for a 16x20 Garage Roof?
Roofing squares needed
3.7 squares
A 16 by 20 foot garage at a 4/12 pitch with ten percent waste.
When the figures look right, the roofing material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
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 length16 ft
- Building width20 ft
- Roof pitch, rise per 12 in4 /12
- Waste allowance10 %
- Ridge length, 0 if unknown20 ft
Roofing squares needed3.7 squares
| Footprint area | 320 sq ft |
|---|---|
| Slope multiplier at 4/12 | 1.05 x |
| Actual roof surface | 337 sq ft |
| Squares before waste | 3.4 |
| Squares with 10% waste | 3.7order to this number |
The material list this produces
| Shingles, bundles (3 per square) | 12 |
|---|---|
| Synthetic underlayment, rolls (10 sq each) | 1 |
| Starter strip, linear ft | 84 |
| Ridge cap, linear ft | 20 |
| Drip edge, linear ft | 72 |
| Roofing nails, lb | 8 |
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 | 3.7 squares |
| 6/12 | 3.9 squares |
| 8/12 | 4.2 squares |
| 10/12 | 4.6 squares |
| 12/12 | 5 squares |
What else matters here
Three hundred and twenty square feet of footprint at 4/12 is 337 square feet of roof, 3.7 squares with waste, twelve bundles.
If this garage is attached, or has any wall rising out of the roof plane, the leak will start at that wall and not in the field of the shingles. The detail that stops it is step flashing: a separate L shaped piece of metal woven in with every single course of shingles, one per course, each lapping the one below.
Nothing else does that job. A continuous strip of flashing bent into the corner will leak, and a bead of caulk in the corner is not a flashing detail, it is a two year repair. Above the step flashing goes counter flashing, either let into a masonry joint or tucked under the siding and housewrap, so water coming down the wall lands on top of the metal rather than behind it.
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 16x20 garage?
About 12 bundles at a 4/12 pitch with ten percent waste, which is 3.7 squares.
What is step flashing?
Individual L shaped pieces of metal woven in one per shingle course where a roof meets a wall, each lapping the one below. It is the only correct way to flash that joint.
Can I use caulk where the roof meets a wall?
No. Caulk is a sealant, not a flashing, and it fails within a few years. The joint needs step flashing with counter flashing over it.
The roofing calculator takes any dimensions you like and hands back the same list.