Lean-to roof pitch calculator: two wall heights
A lean-to roof pitch is set by how much taller the high wall is than the low wall over the width between them. Measure both walls to the top of the plate, measure the width outside to outside, and the calculator gives the pitch, angle and rafter length. Working the other way, enter the pitch you want and the low wall, and it tells you how tall to build the high wall.
| Height difference | 8 ft wide | 10 ft wide | 12 ft wide |
|---|---|---|---|
| 12″ | 1.5/12 | 1.2/12 | 1/12 |
| 18″ | 2.25/12 | 1.8/12 | 1.5/12 |
| 24″ | 3/12 | 2.4/12 | 2/12 |
| 30″ | 3.75/12 | 3/12 | 2.5/12 |
| 36″ | 4.5/12 | 3.6/12 | 3/12 |
Minimum pitch for a shed roof
The roofing sets the floor. Under IRC section R905: asphalt shingles need 2/12 (with double underlayment up to 4/12), mineral-surfaced roll roofing 1/12, lapped metal panels 3/12 without sealant or 1/2/12 with it, and standing-seam metal or a membrane 1/4/12[1].
- A shed roof below 3/12 should also have its supporting beam designed rather than taken from a plain ridge detail[2].
- Low slopes collect snow and leaves; a little extra pitch costs only a taller high wall.
Rafter length and cuts
A shed rafter bears on both walls, so it gets a birdsmouth at each end and a tail at each overhang. Its length is the full level distance (width plus both overhangs) times the rafter factor. A lean-to attached to a house hangs from a ledger instead of sitting on a high wall; measure the height of the ledger's top edge as the high wall. For two-sided roofs use the rafter calculator, and for panel counts the metal roof calculator. The roof pitch calculator converts any pitch to degrees.
Frequently asked questions
What is a good pitch for a shed roof?
At least 2/12 for asphalt shingles; 3/12 or 4/12 drains better and avoids double underlayment, which is required from 2/12 to 4/12.
How do I calculate the pitch of a lean-to roof?
Subtract the low wall height from the high wall height, divide by the width between the walls and multiply by 12.
How much higher should the back wall of a shed be?
Width × pitch ÷ 12. A 10 ft wide shed at 3/12 needs a back wall 30 in taller than the front.
Can a shed roof be flat?
It still needs some slope: membranes and standing-seam metal are allowed down to 1/4 in per foot under the IRC.