Shed / Lean-To Roof Calculator
A shed roof (also called a lean-to or mono-pitch roof) has a single slope running in one direction. It is the simplest roof to calculate and build, making it popular for garages, workshops, and home additions. Enter your shed's footprint and pitch to get your material quantities.
How to use this calculator
- Enter your roof's length and width — measure the outer footprint of the building, not the slope length.
- Select your roof pitch (rise:12). If unsure, see the pitch reference table below or count the rise in inches for every 12 inches of horizontal run.
- Choose your shingle type and adjust the waste factor (add more for complex rooflines).
- Read your squares, bundles, and underlayment rolls from the result panel — use these numbers to get supplier quotes.
How we calculate this
Shed roof area = footprint length × width × pitch multiplier. Because there is only one slope, there are no valleys or hip corners, so a lower 10% waste factor is usually sufficient. Ridge cap is minimal (the high wall typically uses flashing rather than ridge cap shingles). Underlayment is one roll per 400 sq ft of roof area.
Pitch multiplier reference
The pitch multiplier converts your flat footprint to actual sloped roof area. A steeper roof means more material.
| Pitch | Multiplier | Description |
|---|---|---|
| 1:12 | 1.0035 | Almost flat — commercial membrane |
| 2:12 | 1.0138 | Low slope — common on additions |
| 3:12 | 1.0308 | Low pitch — walkable |
| 4:12 | 1.0541 | Moderate — ranch homes |
| 5:12 | 1.0833 | Standard |
| 6:12 | 1.118 | Standard — most common in US |
| 7:12 | 1.1577 | Moderate-steep |
| 8:12 | 1.2019 | Steep — requires toe boards |
| 9:12 | 1.25 | Very steep |
| 10:12 | 1.3017 | Very steep — safety harness required |
| 12:12 | 1.4142 | 45° — high-end residential |
Example material estimates — Shed / Lean-To Roof at 4:12 pitch
Pre-calculated for common house footprints. Click any row to pre-fill the calculator above.
| Footprint | Roof Area | Squares | Bundles (arch.) | Underlayment | Ridge Cap |
|---|---|---|---|---|---|
| 800 sq ft (33 × 24 ft) | 843 sq ft | 9.3 | 28 bundles | 3 rolls | 1 bundles |
| 1,000 sq ft (37 × 27 ft) | 1,054 sq ft | 11.6 | 35 bundles | 3 rolls | 2 bundles |
| 1,200 sq ft (40 × 30 ft) | 1,265 sq ft | 13.9 | 42 bundles | 4 rolls | 2 bundles |
| 1,500 sq ft (44 × 34 ft) | 1,581 sq ft | 17.4 | 53 bundles | 4 rolls | 2 bundles |
| 1,800 sq ft (49 × 37 ft) | 1,897 sq ft | 20.9 | 63 bundles | 5 rolls | 2 bundles |
| 2,000 sq ft (51 × 39 ft) | 2,108 sq ft | 23.2 | 70 bundles | 6 rolls | 2 bundles |
| 2,400 sq ft (57 × 42 ft) | 2,530 sq ft | 27.8 | 84 bundles | 7 rolls | 2 bundles |
| 2,500 sq ft (58 × 43 ft) | 2,635 sq ft | 29 | 87 bundles | 7 rolls | 2 bundles |
| 3,000 sq ft (64 × 47 ft) | 3,162 sq ft | 34.8 | 105 bundles | 8 rolls | 2 bundles |
Includes 10% waste. Architectural shingles, 3 bundles/square.
What this calculator doesn't include
This calculator does not account for the high-wall flashing at the top of the shed roof where it meets an existing wall. That is typically metal step flashing, not shingles. If your shed roof has a dormer or skylight, add 5–10% to the waste factor.
Frequently asked questions
Most building codes require a minimum of 2:12 for asphalt shingles and 1:12 for metal or membrane roofing. A 3:12 to 4:12 pitch is the most common choice for sheds and garages — it sheds water well and is still safe to walk on without safety equipment.
A 12 × 16 ft shed with a 4:12 pitch has a roof area of 12 × 16 × 1.054 = 202 sq ft = 2.02 squares. With 10% waste that is 2.22 squares, or 7 bundles of standard 3-tab or architectural shingles.
Yes. Building codes require at least one layer of roofing felt or synthetic underlayment under asphalt shingles on all roof types, including shed roofs. In cold climates, an ice-and-water shield is required at the eave for the first 24 inches past the interior wall line.
Yes — metal roofing is a popular choice for sheds because it is durable and low-maintenance. Metal can be installed on slopes as low as 1:12. The material calc is different (metal comes in panels or sheets, not bundles) but the roof area calculation is the same.
They are the same thing. 'Lean-to' typically refers to a shed roof attached to an existing structure (the roof 'leans' against a wall). 'Shed roof' or 'mono-pitch roof' refers to the same single-slope geometry whether it is attached or freestanding.