Short answer
A 16x24 garage with a 6/12 pitch requires 4.3 roofing squares (429 square feet of sloped surface). To cover it, buy 15 shingle bundles, 2 underlayment rolls, and 80 linear feet of drip edge.
How this calculator works
The calculator takes three inputs — house length, house width, and a pitch factor — and converts your building's flat footprint into actual sloped roof area.
The core formula:
(length × width × pitch_factor) ÷ 100 = roofing squares
For this garage: (24 × 16 × 1.118) ÷ 100 = 4.29 squares.
Why a pitch factor instead of rise/run?
Rise-over-run notation (like 6/12) is useful on a job site, but the math requires a multiplier derived from the Pythagorean theorem. For every 12 inches of horizontal run, a 6/12 roof rises 6 inches — the actual rafter length over that run is √(6² + 12²) ÷ 12 = 1.118. That number is the pitch factor. Multiply the flat footprint by 1.118 and you get the true surface area a roofer will walk and a shingle will cover.
Common pitch factors for reference:
| Pitch | Factor |
|---|---|
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 12/12 | 1.414 |
What the secondary outputs mean
Shingle bundles (15): Most architectural shingles come three bundles to a square, covering 33.3 sqft per bundle. The calculator multiplies the raw square count by 3, then by 1.10 for a 10% waste factor, and rounds up. At 4.3 squares, that math lands at ceil(4.29 × 3 × 1.10) = 15 bundles. The waste covers starter strip, ridge cap cuts, and the inevitable snapped shingles.
Underlayment rolls (2): GAF FeltBuster and similar synthetic underlayments come in rolls that cover 10 squares (1,000 sqft) each — but that format is listed differently by manufacturer. The calculator uses a 4-square-per-roll standard roll size for conservative planning. ceil(4.29 ÷ 4) = 2 rolls. Better to have a partial roll left over than to make a second lumber-yard trip mid-job.
Drip edge (80 linear ft): Drip edge runs the entire perimeter of the roof deck before the underlayment goes down on the eaves, and on top of underlayment at the rakes. The perimeter of a 16×24 building is 2 × (24 + 16) = 80 ft. Drip edge comes in 10-foot sections, so order 8 pieces plus one extra if there are any tricky cuts at corners.
Ridge cap squares (1): Ridge cap shingles cover the peak where two roof planes meet. A rule of thumb is 10% of total roof area. ceil(4.29 × 0.10) = 1 square. Some roofers cut three-tab shingles into thirds for ridge cap; others buy pre-bent ridge cap bundles. Either way, budget 1 square of material.
What the calculator does not include
The calculator gives you field material quantities. It does not account for:
- Roof decking (OSB or plywood): If you're re-decking, measure the flat footprint — 384 sqft for 16×24 — and divide by the sheet size (32 sqft for a 4×8 sheet), giving you 12 sheets plus waste.
- Nails and fasteners: A roofing coil nail gun uses roughly 2.5 lbs of 1¼" nails per square.
- Flashing: Any pipe penetrations, chimneys, or wall intersections need step flashing or pipe boots, which are counted separately.
- Labor: Square counts are for materials only.
Recommended materials
For a 16×24 garage at 4.3 squares, you don't need commercial quantities — a single trip to a roofing supplier or big-box store covers it. Architectural shingles outperform three-tab on durability and wind resistance without a significant price premium at this scale. Synthetic underlayment is worth the upgrade over felt: it's lighter, tears less in wind while the deck is exposed, and most manufacturers require it for full warranty coverage on their shingles.
- GAF Timberline HDZ architectural shingles (33.3 sqft per bundle) — industry-standard laminated shingle, Class 4 impact resistance optional, 15 bundles covers this garage with waste
- GAF FeltBuster synthetic underlayment (10 sq roll) — lighter than 30-lb felt, won't wrinkle when wet, staple or cap-nail at 6" along laps
- Amerimax Home Products 10 ft white drip edge — aluminum, bends cleanly at corners, 8 pieces covers the 80 ft perimeter
FAQ
How many roofing squares does a 16x24 garage with a 6/12 pitch need? A 16x24 garage with a 6/12 pitch requires 4.3 roofing squares, or about 429 square feet of roof surface. This accounts for the slope multiplier of 1.118 applied to the footprint.
How many shingle bundles do I need for a 16x24 garage? You need 15 bundles of standard three-tab or architectural shingles. That figure includes a 10% waste factor for cuts, ridge lines, and starter courses.
What is a roofing square? One roofing square equals 100 square feet of roof surface. It's the standard unit roofers and suppliers use to quote materials and labor.
What is the pitch factor for a 6/12 roof? The pitch factor for a 6/12 roof is 1.118. You multiply the flat footprint area by this number to get the actual sloped surface area you'll be covering.
How many underlayment rolls do I need for a 16x24 garage? You need 2 rolls of underlayment. Each roll covers 4 squares (400 sqft), and the garage roof is 4.3 squares, so 2 rolls gives you full coverage with minimal waste.
How much drip edge do I need? The perimeter of a 16x24 structure is 80 linear feet, so you need 80 linear feet of drip edge. Buy in 10-foot sections, so that's 8 pieces.
How many ridge cap squares does a 16x24 garage need? Plan on 1 square (100 sqft) of ridge cap material. Ridge cap typically runs about 10% of total roof area, and the calculator rounds up to the nearest whole square.
Does the 16x24 measurement refer to the garage footprint or the roof surface? It refers to the building footprint — the outside dimensions of the structure at ground level. The pitch factor converts that flat area into the actual sloped roof surface.
Should I add extra shingles beyond the 10% waste factor? For a simple gable roof on a small garage, 10% is usually enough. If the roof has hips, valleys, skylights, or dormers, bump the waste factor to 15%.
Can I reroof a 16x24 garage in one day? Yes. At 4.3 squares, an experienced roofer or handy homeowner can strip and reshingle a simple gable garage roof in a single day. Allow extra time if you're working alone.
How does pitch affect the number of squares I need? A steeper pitch means more actual roof surface even though the building footprint stays the same. A flat roof (1.0 pitch factor) on a 16x24 building would be 3.84 squares; the 6/12 pitch adds about half a square.
What if I have a hip roof instead of a gable on my garage? The pitch factor and square footage calculation stay the same, but hip roofs have more cuts and waste. Use a 15% waste factor for shingle bundles instead of 10%, which would push your bundle count to 16.