Building a garden shed, greenhouse, or lean-to? Determining the correct roof pitch is critical for proper water runoff and snow load handling. A roof that is too flat will leak; a roof that is too steep wastes lumber.
Use our free Roof Pitch Calculator below to instantly convert between Rise/Run, Angle degrees, and Pitch (x/12). It also provides the “Rafter Multiplier” you need to cut your common rafters to the exact length.
Roof Pitch Calculator
Find Slope, Angle & Rafter Length
How to Use This Calculator
We built this tool with two modes depending on what information you have:
- From Rise / Run: Use this if you know dimensions.
- Rise: The vertical height from the top plate of the wall to the ridge.
- Run: The horizontal distance from the outside of the wall to the center of the ridge (usually half the building width).
- From Angle: Use this if you are matching an existing roof and measured the angle with a digital protractor (e.g., 33.7°).
Why We Built This (The “Secret Sauce”)
Most calculators just give you the pitch. Ours gives you the Rafter Length Multiplier. This is the magic number for carpenters.
How to use the Multiplier:
Instead of using complex trigonometry, simply take your Run (in feet) and multiply it by our Multiplier result. This gives you the exact length of the common rafter from the ridge to the wall plate (excluding the overhang).
Example: A 10ft Run with a 6/12 pitch (Multiplier 1.118) needs a rafter length of 11.18 feet.
Educational Guide: Choosing the Right Pitch
For garden structures, pitch isn’t just aesthetic—it’s functional.
- Low Slope (1/12 to 3/12): Easy to walk on, but requires roll roofing or metal. Shingles will leak because water doesn’t drain fast enough.
- Medium Slope (4/12 to 9/12): The sweet spot for sheds. Standard asphalt shingles work perfectly. A 4/12 pitch is considered the minimum safe slope for standard shingles.
- Steep Slope (10/12+): Great for snow shedding in northern climates and provides extra storage space in the loft/attic, but harder to build.
Frequently Asked Questions (FAQ)
What is the standard pitch for a shed roof?
The most common pitch for a garden shed is 4/12 (18.4°). This is low enough to be easy to build but steep enough to use standard 3-tab shingles without leaking. For snowy areas, a 6/12 or 8/12 is better.
How do I calculate pitch from degrees?
The formula is Tan(Angle) * 12. For example, if your angle is 30°, the tangent is 0.577. Multiply that by 12, and you get 6.9. So, a 30° roof is roughly a 7/12 pitch.
Can I use shingles on a 2/12 pitch?
Generally, no. Standard installation voids the warranty on slopes below 2/12. Between 2/12 and 4/12, you must use a special “double layer” underlayment method. Below 2/12, you should use rolled roofing (mineral surface) or EPDM rubber.
