Skip to content
Matholio

Shed roof pitch: rise, rafter length and minimum slope

A shed roof, also called a lean-to or skillion roof, slopes one way across the whole building. That makes the math simpler than a gable roof, but one detail trips people up: the run is the full depth of the shed, not half of it. This guide shows how much higher the tall wall must be, how long the rafters are and how low the pitch can go.

Updated By Matholio Editorial Team2 min read

The short answer

Multiply the depth of the shed in feet by the pitch to get the rise in inches. An 8 ft deep shed with a 4/12 roof needs the high wall 32 in taller than the low wall, and each rafter is 8 × 1.054 = 8.43 ft long before overhangs. With asphalt shingles, the roof must be at least 2/12, with double underlayment below 4/12.

How much higher is the tall wall?

Pitch is inches of rise per foot of run. On a shed roof the run is the full horizontal distance from the low wall to the high wall, measured to the outside of the walls.

Rise (in) = shed depth (ft) × pitch

Pitch = rise (in) ÷ shed depth (ft)

Rise of a shed roof by shed depth and pitch
Shed depth 1/12 2/12 3/12 4/12
6 ft 6 in 12 in 18 in 24 in
8 ft 8 in 16 in 24 in 32 in
10 ft 10 in 20 in 30 in 40 in
12 ft 12 in 24 in 36 in 48 in

Rafter length

Multiply the run by the pitch factor to get the sloped length of a rafter from wall to wall, then add the overhang at each end measured along the slope.

Shed rafter length before overhangs
Pitch Angle Pitch factor 8 ft deep 10 ft deep
2/12 9.46° 1.014 8.11 ft 10.14 ft
3/12 14.04° 1.031 8.25 ft 10.31 ft
4/12 18.43° 1.054 8.43 ft 10.54 ft
6/12 26.57° 1.118 8.94 ft 11.18 ft
  1. Shed: 10 ft deep, roof at 3/12.
  2. Rise: 10 × 3 = 30 in, so the high wall is 30 in taller.
  3. Rafter between walls: 10 × 1.031 = 10.31 ft.
  4. Add 12 in overhangs at both ends: about 12.4 ft, so buy 14 ft lumber.

How low can a shed roof be?

The lowest allowed slope depends on the roofing. These are the minimums in the International Residential Code; the manufacturer’s instructions may ask for more.

Minimum roof slope by roofing material
Roofing Minimum slope Notes
Asphalt shingles 2/12 Double underlayment from 2/12 up to 4/12
Mineral-surfaced roll roofing 1/12 Common on low sheds
Lapped metal panels, no lap sealant 3/12 Lower with sealed laps
Standing seam metal 1/4 in per 12 Nearly flat roofs

In snowy areas a steeper pitch sheds snow better, and local rules may limit the height of a shed near a property line. Check with your building department before you build.

Calculators for this job

These calculators do the arithmetic from this guide. Each card shows the result for a typical project; open one to enter your own measurements.

Roof Pitch Calculator 6 in rise over 12 in 6/12 Open calculator Roof Angle to Pitch Calculator 26° roof angle 5.85/12 Open calculator Roofing Calculator 40 × 30 ft roof at 6/12 14.76 squares Open calculator

Frequently asked questions

What is a good pitch for a shed roof?

Many single-slope sheds use 2/12 to 4/12. At 4/12 or more, standard asphalt shingle installation applies; below that you need double underlayment or a different roofing.

How do I calculate the pitch of a lean-to roof?

Divide the difference in wall height in inches by the depth of the building in feet. A lean-to 10 ft deep with a 30 in difference is 30 ÷ 10 = 3/12.

What is the minimum pitch for a shed roof with shingles?

The IRC allows asphalt shingles down to 2/12, with double underlayment between 2/12 and 4/12.

Is a 1/12 pitch enough for a shed?

Only with roofing made for low slopes, such as roll roofing or standing seam metal. Shingles are not allowed below 2/12.

Sources

  1. International Code Council, 2021 International Residential Code, Chapter 9: Roof Assemblies
  2. This Old House, How to Determine Roof Pitch
Paint How much paint do I need for a room? Measure the walls, subtract openings and turn square feet into gallons and quarts, with a worked 12 × 12 ft example. Flooring How much flooring do I need? How to measure a floor, how much waste to add for plank and tile layouts, and how to round up to boxes. Roofing How to measure a roof and order shingles Measure the footprint from the ground, find the pitch and turn roof area into squares and bundles of shingles.