The job
Modelled, not recorded. Every figure here is the answer the Roofing Shingle Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The building is a detached two-car garage whose roof measures 24 ft (7.32 m) by 24 ft (7.32 m) on plan, taken to the drip edge so the overhangs are in. It is a plain gable, two equal slopes meeting at one ridge, with no valleys, no dormers and nothing through the roof but perhaps a vent. That is the simplest roof there is to estimate, which is why it is a good one to check a method on.
The pitch is 4/12 — 18.4° (low slope): four inches of rise for every twelve of run, the shallow slope common on garages and outbuildings, and the one the calculator itself labels low slope. At this pitch the roof surface is only a little larger than the plan, so most of what follows is about counting rather than geometry.
The allowance is set to 10 per cent, below the calculator's default, because a straight gable with square ends produces very little offcut: each course is cut once at each rake. A hip roof, where every course is cut at an angle, is the case the variant below runs at a larger figure.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Roof Footprint Length | 24 ft (7.32 m) | Along the ridge, from rake edge to rake edge, including the overhang at each gable end. |
| Roof Footprint Width | 24 ft (7.32 m) | Across the building from eave to eave at the drip edge, which on a garage is often a deeper overhang than the walls suggest. |
| Roof Pitch | 4/12 — 18.4° (low slope) | Read with a level and a tape on a rafter, or from the truss drawings; a garage roof is often shallower than the house beside it. |
| Waste Factor (%) | 10 | A plain gable with no valleys loses little to cutting, so the allowance is set below the calculator's general default. |
The working, step by step
1. Roof footprint area
576 sq ft (53.51 m²)
The plan, length times width, is 576 sq ft. Because the measurements were taken at the drip edge this is the roof's shadow on the ground at noon, overhangs included, and it is the figure the slope is applied to.
2. Actual roof surface area
607.2 sq ft (56.41 m²)
The calculator multiplies the plan by the slope factor for the pitch, the hypotenuse of a twelve-unit run, which its second source gives. At this shallow pitch the roof comes to 607.2 sq ft, only a few per cent more than the plan; on a steep roof the same step adds a third or more.
3. Roof surface area
607.2 sq ft (56.41 m²)
The same sloped area restated as 607.2 sq ft, the unit a roofing square is defined in. This is the area the squares in the headline count, before the allowance is added.
4. Bundles needed
21 bundles
The allowance goes on, the area is divided into squares, and each square is bought as three bundles of standard shingles, which the calculator's first source gives as the norm for three-tab and architectural shingles. Rounded up to whole bundles, that is 21 bundles, the number on the delivery ticket.
Estimated roofing shingle needed
6.679 squares
62.05 m²
The headline, 6.679 squares, is the measure a roofer prices and plans in, and the bundle row is what is carried up the ladder. On a roof this size the difference between them is the round-up: a fraction of a bundle becomes a whole one. Ask the supplier how it handles unopened bundles before the order is placed: the count is a minimum, and running short with one course to go costs a second delivery.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated roofing shingle needed | 6.679 squares (62.05 m²) | The field area of the roof in roofing squares, with the allowance, for comparing quotes and planning the work. |
| Bundles needed | 21 bundles | Standard bundles of field shingles; starter strip, ridge caps and underlayment are separate products. |
The allowance, and what it does here
The 10 per cent is applied to the sloped area before it is turned into bundles, and then the bundles are rounded up, so on a small roof the round-up is part of the margin too. It covers the cut at each rake, a course that has to be adjusted to land the ridge cleanly, and the odd damaged shingle. It does not cover the starter course along the eaves or the caps along the ridge, which are counted on their own.
The common mistake
Taking the bundle count as the whole roof
The calculator counts field shingles, the ones that cover the slope. A finished roof also needs a starter course along every eave and rake, ridge caps along the top, underlayment over the whole deck and drip edge around the perimeter, and none of them are in this number. On a garage the eave and rake are long compared with the area, so the starter and the drip edge are a bigger share of the job than on a house. Count them before the order goes in, or the last day is spent waiting for a second delivery.
Change the job, and see what moves
The same garage, at the common house pitch
Roof Pitch: 6/12 — 26.6° (common) → 7.084 squares (65.81 m²)
Build the same garage at 6/12 — 26.6° (common), the pitch the calculator calls common, and the roof grows to 7.084 squares, or 22 bundles. Steepening a roof adds surface without adding plan; on a building this small the change is worth one bundle, and the bigger the plan, the more bundles the same change of pitch adds.
The same garage, cut for a hip roof
Waste Factor (%): 15 → 6.982 squares (64.87 m²)
Raise the allowance to 15 per cent, the kind of figure a hip roof's angled cuts need, and the squares rise to 6.982 squares while the bundles stay at 21 bundles. The extra area is still inside the last bundle of the round-up, so on this roof the larger allowance costs nothing extra to order; that only stays true while the roof is small.
What to run next
- Roofing Underlayment Calculator
The deck is covered before the shingles go on, and at a low pitch the underlayment specification is worth checking against the shingle maker's instructions and your code.
- Roof Ridge Cap Calculator
The single ridge of a gable is finished with caps, counted by its length rather than by area.
- Gutter Calculator
A garage that sheds water onto a drive needs gutters along its eaves, measured from the same length.
Sources
The method and every constant behind these figures belong to the Roofing Shingle Calculator, version 1.0.1, last verified 2026-09-05. It rests on:
- 1 roofing square = 100 sq ft; standard 3-tab and architectural shingles use 3 bundles per square
- Roof surface area = footprint area x slope multiplier, where slope multiplier = sqrt(1 + (rise/12)²) for a given X/12 pitch
Frequently asked questions
- How many bundles of shingles for a 24 by 24 ft garage roof?
- At 4/12 — 18.4° (low slope) with a 10 per cent allowance, the calculator gives 6.679 squares, which is 21 bundles of standard shingles. Starter strip and ridge caps are extra.
- Can asphalt shingles go on a low-pitch roof like this?
- Shingle makers and the US residential code set a minimum slope for asphalt shingles and a stricter underlayment rule for the shallowest slopes they allow. The calculator counts the shingles either way; whether this roof needs the stricter underlayment is for the shingle maker's instructions and the local code to settle before the roof is felted.
- Why does a larger waste factor not change the bundle count here?
- Because the bundles are rounded up. At 15 per cent the area grows to 6.982 squares, but it still fits in 21 bundles. On a larger roof the same change would cross a bundle boundary.