Lean To Roof Pitch Calculator
Calculate high wall height, low wall height, run, rise, pitch x:12, roof angle, rafter length, overhang allowance, and roofing type minimum pitch.
Calculation Breakdown
| Roofing type | Common minimum pitch | Approx angle | Lean-to note |
|---|---|---|---|
| Standing seam metal | 0.25:12 to 1:12 | 1.2° to 4.8° | Use system-specific specs |
| Single-ply membrane | 0.25:12 | 1.2° | Best for very low slope |
| Polycarbonate panels | 1:12 | 4.8° | Good for covered walks |
| Asphalt shingles | 2:12 | 9.5° | Low-slope detailing needed |
| Corrugated metal panels | 3:12 | 14.0° | Often used on sheds |
| Wood shingles | 3:12 to 4:12 | 14.0° to 18.4° | Check exposure and underlay |
| Clay or concrete tile | 4:12 to 5:12 | 18.4° to 22.6° | Heavy roof framing required |
| Slate tile | 4:12+ | 18.4°+ | Use verified structural design |
| Pitch | Slope percent | Angle | Rise per 8 ft run |
|---|---|---|---|
| 1:12 | 8.3% | 4.8° | 8 in |
| 2:12 | 16.7% | 9.5° | 16 in |
| 3:12 | 25.0% | 14.0° | 24 in |
| 4:12 | 33.3% | 18.4° | 32 in |
| 5:12 | 41.7% | 22.6° | 40 in |
| 6:12 | 50.0% | 26.6° | 48 in |
| 8:12 | 66.7% | 33.7° | 64 in |
| 12:12 | 100.0% | 45.0° | 96 in |
| Lean-to preset | Typical run | Typical pitch | Common roofing |
|---|---|---|---|
| Garden shed roof | 5 ft to 8 ft | 3:12 to 5:12 | Metal or shingles |
| Porch cover | 6 ft to 12 ft | 2:12 to 4:12 | Metal or membrane |
| Carport roof | 10 ft to 16 ft | 3:12 to 4:12 | Metal panels |
| Greenhouse lean-to | 4 ft to 10 ft | 5:12 to 8:12 | Polycarbonate |
| Firewood cover | 3 ft to 6 ft | 4:12 to 6:12 | Corrugated metal |
| Workshop annex | 8 ft to 14 ft | 4:12 to 7:12 | Shingles or metal |
| Spec item | Formula | Use this for | Check |
|---|---|---|---|
| Rise | High wall - low wall | Wall layout | Positive drop |
| Pitch x:12 | Rise / run x 12 | Roofing selection | Meets minimum |
| Angle | atan(rise / run) | Cut layout | Degrees |
| Rafter | sqrt(run2 + rise2) | Stock length | Round up |
| Overhang | Horizontal / cos(angle) | Eave extension | Along slope |
| Allowance | Length x factor | Trim and layout | Do not reduce |
The math of roofing gets tricky. Looks collide with drainage. And most people start with a fuzzy idea in their head and end up at hardware store after doing some math they don’t really get. For instance, you want a shed that looks nice but sheds (hence its name). Roof pitch is where the tension between looking good and shedding water exist.
And the answer isn’t as much about paper steepness as it does: Will the rain come off the roof or will the wind blow it back under the shingles? That’s the balancing act, and it involves more than just a single number. A gable roof has a bit more geometry, but a lean-to is easier. But it still have a few pitfalls.
How to Choose the Right Roof Slope
The calculator will do the math for you (and convert the rise/run/pitch ratio to/from degrees, too) so you can avoid cracking open your trigonometry flash cards. More important than doing the math, though, is knowing what numbers to believe. A 1:12 pitch sounds reasonable. Heck, it’s even sloping! But guess what? Asphalt shingles hate such a gentle slope. Ideally they want 2:12 or better. You can also use a special underlayment system to keep them from slipping right off. Metals are more forgiving (down to quarter inch drop per foot…provided they’re sealed correctly).
Beyond materials cost, this also impact your leak risk in several years, after you’ve stashed all your good tools inside and repainted the walls. This note assumes that asphalt shingles is used as the roofing material. Compare horizontal run to rafter length.
This is one of those common carpenter errors where people get the two mixed up and end up short on lumber just when they need some most. Run is the flat distance from the wall to the edge of the roof. Because the rafter runs at an angle, it’s always longer then the run. For example, a 6-foot run with a modest rise may only be a little more than 6 feet as a rafter. Add an overhang for weather protection and/or looks, and it goes even longer. Remember: always measure distance to make the final cut along the slope, never the flat distance off the floor. It’s a little thing, but when you’re standing on ladders trying to piece things together in the rain, it matters.
Then there’s where planning meets reality: how to handle different heights between walls? What if you have an 8-foot-tall low wall and a 9-foot-tall high wall? Sounds like plenty of pitch. But that changes when you look at the width of your run, which is twelve feet. Oops, that means your pitch falls well below what most standard roofing materials can manage.
It is time to go with membrane systems intended for flat roofs. Or narrow the footprint. You could also raise the back wall. And each option come with tradeoffs. Walls might reach their limit for headers. Reducing the footprint cuts down on storage space. Changing material changes the entire look.
You’ll also find that wind load enters into the framing of such structures. Shallower pitches can result in water ponding where the ground underneath is not absolutely flat, and steep pitches catches much more wind uplift on the leeward side. Again, there are no right answers; just local compromises. In snow country, you need steeper angles so gravity can help out. In sunny, dry climates, you may be able to get away with flatter designs as fast evaporation will take care of any minor moisture issues. It’s all physics, though, so in the end, expect the unexpected and build your lean-to accordingly.
You should of spent more money on sealants and flashing and you can make anything work, but why fight the battle? Smart design will avoid it altogether, so check your local codes for minimums, take note of what’s being used well around your neighborhood and let the geometry determine the materials, not the other way around. Nobody will care how much or little it pitches if the water comes off quick and quiet. They’ll just care if it doesn’t. Pitch first, build second.
