A shed roof is a single-slope roof that angles from a higher wall down to a lower eave. It is the simplest roof form, requiring only one set of rafters or trusses. It is widely used on sheds, additions, porches, and contemporary residential designs.

How steep should a shed roof be?

The minimum pitch depends on the roofing material. Asphalt shingles typically require at least 2:12 (some manufacturers specify 4:12). Metal panels can go as low as 1:12 with proper sealant at laps. Membrane (TPO/EPDM) works on nearly flat surfaces.

Is the waste factor lower for a shed roof than a gable?

Generally yes. A shed roof is a single rectangular plane with minimal cuts, so 10% waste is usually sufficient. A gable or hip roof has more edges and typically needs 12–15%.

How do I measure horizontal run vs. rafter length?

Horizontal run is the flat, level distance from the low eave to the high wall (measured on the ground or ceiling). Rafter length is the actual sloped distance along the roof surface, which is always longer than the horizontal run.

Can I use this calculator for a skillion roof?

Yes. A skillion roof is another name for a mono-pitch or shed roof. The calculation is identical: one slope, one direction.

Do I need underlayment on a shed roof?

Yes. Building codes require underlayment beneath asphalt shingles. Even for metal roofs, synthetic underlayment acts as a secondary water barrier and reduces condensation noise.

How do I convert shed roof area to material quantities?

Divide the adjusted area by 100 for roofing squares (shingles). For metal panels, divide the adjusted area by the panel coverage area (panel width × length). For underlayment, divide by the coverage per roll.

Should I account for the overhang?

Yes. Include the eave overhang in the horizontal run and any rake overhangs in the length. Typical eave overhangs are 6–12 inches, and rakes are 6–18 inches.

Shed Roof Area Calculator

Calculate the surface area of a single-slope shed roof from length, width, and pitch. Estimate roofing materials with waste factor included.

Quick Presets:

Building Length

Horizontal Run

Roof Pitch

Materials & Costs

Slope Factor

1.054

4:12 pitch

Rafter Length

10.54 ft

Measured slope

Roof Area (base)

126.5 sq ft

12' × 10.5'

With Waste/Overhang

139.1 sq ft

10% added

Roofing Squares

1.39

100 SF per square

Shingle Bundles

3 per square

Material Estimate

Underlayment

1 rolls

~$40-50 ea

Drip Edge

70 linear ft

~$0.50/ft

Total Material Cost

$393

Materials only

Roof Slope Visualization

Pitch Angle: 18.4°

Rise 4 inches per 12 ft run

Area Multiplier: 1.054×

Higher pitch = larger area

Building Dimensions	Roof Area (SF)	Squares	Bundles
8' × 6' @ 4:12	56	0.56	2
10' × 8' @ 4:12	93	0.93	3
12' × 10' @ 4:12	139	1.39	5
16' × 12' @ 6:12	236	2.36	8
20' × 16' @ 8:12	423	4.23	13

Planning notes, formulas, and examples

About the Shed Roof Area Calculator

A shed roof — also called a mono-pitch, single-slope, or lean-to roof — is the simplest roof design, with only one sloping plane. It is commonly used on sheds, additions, porches, carports, and modern home designs. Despite its simplicity, you still need to account for the slope when estimating roofing materials.

This shed roof area calculator takes the building length (along the high or low edge), the horizontal run (width from eave to the higher wall), and the roof pitch. It computes the rafter length using the slope factor and multiplies by the building length to give you the true sloped area.

Adding a waste factor accounts for cuts, starter strips, and overlap. The result tells you exactly how many square feet of shingles, metal panels, or membrane you need, plus how many roofing squares that equates to.

When This Page Helps

Even a simple shed roof has more surface area than a flat footprint. A 6:12 pitch increases the area by about 12%, and a 12:12 pitch increases it by 41%. This calculator ensures you order enough material the first time, saving trips and preventing project delays.

How to Use the Inputs

Measure the building length (the horizontal distance parallel to the eave edge).
Measure the horizontal run (the distance from the low eave to the high wall, measured horizontally).
Enter the roof pitch as rise per 12 inches of run.
Set a waste factor (10% is typical for a shed roof).
Review the rafter length, total roof area, and adjusted area.
Divide by 100 to get roofing squares for material ordering.

Formula used

Slope Factor = √(1 + (pitch/12)²)
Rafter Length = horizontal run × Slope Factor
Total Area = length × Rafter Length
Adjusted Area = Total Area × (1 + waste%/100)

Example Calculation

Result: 278.1 sq ft (adjusted)

Slope factor = √(1 + (4/12)²) = √1.1111 ≈ 1.0541. Rafter length = 12 × 1.0541 = 12.65 ft. Total area = 20 × 12.65 = 252.8 sq ft. With 10% waste: 252.8 × 1.10 = 278.1 sq ft.

Tips & Best Practices

Include the eave overhang in the horizontal run measurement for accurate material ordering.
Shed roofs on additions must tie into the existing wall with proper step flashing — order extra flashing.
A minimum pitch of 2:12 is recommended for asphalt shingles; lower pitches need membrane or metal.
For metal panel shed roofs, measure from eave to ridge to determine panel length — one continuous panel is ideal.
Add drip edge along all four edges of a shed roof: eave, two rakes, and the high edge.
On very long runs (> 30 ft), consider panel overlap joints and additional sealant.

Shed Roof Design Considerations

Shed roofs excel in simplicity and material efficiency. Because there is only one slope, there are no valleys, hips, or ridge intersections. This means less waste, simpler flashing details, and faster installation. However, the entire roof drains to one edge, so adequate gutter and downspout capacity is essential.

Minimum Pitch Requirements

Asphalt shingles need a minimum 2:12 pitch (some manufacturers require 4:12 for full warranty). Metal panels can go as low as 0.5:12 to 1:12 with sealed-seam profiles. TPO, EPDM, and modified bitumen membranes work on pitches below 2:12, including dead-flat applications.

Shed Roof Framing

Shed roof rafters span from a ledger board on the high wall to a bearing plate on the low wall. For spans over 12–14 feet, purlins or intermediate supports may be necessary. Engineered trusses are available for longer spans and can eliminate interior support walls.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

A shed roof is a single-slope roof that angles from a higher wall down to a lower eave. It is the simplest roof form, requiring only one set of rafters or trusses. It is widely used on sheds, additions, porches, and contemporary residential designs.