Shed and Lean-To Roof Calculator
Last reviewed How we calculate
A shed or lean-to roof has a single slope, running from a high wall down to a low one. It is the simplest roof to build and the one most often built too shallow. This works out its area with the overhangs, the slope length, and how much taller the high wall needs to be.
Single-slope shed or lean-to roof
Area, slope length and wall height difference for a single-slope shed or lean-to roof, with overhangs.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Roof area
- Squares to order, with waste
- Slope length, with overhangs
- Roof length, with end overhangs
- How much taller the high wall is Wall to wall, before overhangs.
- Roof angle
Below 2 in 12 this is a low-slope roof: it needs a membrane or a metal panel rated for the pitch, not shingles.
Between 2 in 12 and 4 in 12, shingles need a double layer of underlayment. Metal panels and roll roofing each have their own minimum pitch on the product sheet.
At 4 in 12 and steeper, shingles go on with ordinary underlayment laps.
A shed roof is simple, but its high edge needs finishing: trim where it ends in open air, or flashing where it meets a wall. On a lean-to against a house, the flashing at the house wall is where leaks usually start.
How the calculation works
The run is the width from wall to wall plus the overhang at each side. Multiply it by the pitch multiplier for the slope length, and by the building length plus the end overhangs for the area. The high wall has to be taller than the low one by the wall-to-wall width times the pitch over 12.
A worked example
A shed 10 ft wide and 12 ft long at 4 in 12, with a 12 in overhang at the low eave, 6 in at the high side and 6 in at each end, has a run of 11.5 ft and a slope length of 12.12 ft. With the end overhangs the roof is 13 ft long, so the area is about 157.6 sq ft. The high wall needs to be about 3.33 ft, or 40 in, taller than the low one.
Get the pitch right first
Shed roofs are often built as shallow as possible to keep the high wall low. Below 4 in 12, shingles need a double layer of underlayment; below 2 in 12 they cannot be used at all, and the roof needs a membrane or a metal panel rated for that pitch. On a lean-to built against a house, the flashing where the roof meets the house wall is where leaks usually start, so it deserves as much care as the roof itself.
Frequently asked questions
What pitch should a shed roof have?
It depends on the roof covering. At 4 in 12 and steeper, shingles go on normally. From 2 in 12 up to 4 in 12 they need a double layer of underlayment, and below 2 in 12 the roof needs a membrane or a metal panel rated for that pitch.
How much taller does the high wall need to be?
The wall-to-wall width times the pitch over 12. A 10 ft wide shed at 4 in 12 needs a high wall about 3.33 ft taller than the low one.
What is different about a lean-to against a house?
The top edge meets the house wall instead of ending in open air, so it needs proper flashing tucked behind the wall covering. That junction is where lean-to roofs most often leak.