MyWoodCreationsThe Weekend Builder's Journal
Tool

Shed Roof Pitch & Rafter Calculator

Enter your shed's width (span), desired roof pitch, and overhang. This calculator will instantly give you the exact rafter length and the precise angles you need to cut your birdsmouths and ridge connections.

By Mark S.

Roof Dimensions

The total width of your shed from outside wall to outside wall.

Horizontal distance the roof extends past the wall.

Cut Measurements

Total Rafter Length

7' 5"

Actual length: 88.5"

18.4°Plumb Cut (Ridge)
71.6°Seat Cut (Birdsmouth)
6' 0"Total Run
2' 0"Total Rise

Tired of doing roof math? Our premium plans include exact cutting templates for all rafters.

Get Ready-to-Use Shed Plans

How to Use the Rafter Calculator

Framing a roof is often the most intimidating part of building a shed for DIYers. One bad cut, and your entire roofline will sag or your ridge board won't sit level. This calculator takes the complex trigonometry out of roof building.

Understanding the Inputs

  • Shed Width (Span): This is the total outside-to-outside dimension of your shed's frame (excluding siding). If you are building a 10x12 shed with the gable on the 10' side, your span is 10 feet.
  • Roof Pitch: The angle of your roof, expressed as inches of vertical rise for every 12 inches of horizontal run. A 4/12 pitch (common for sheds) means the roof rises 4 inches for every foot it travels horizontally.
  • Overhang: How far the eaves extend past your walls. A 12-inch overhang is standard to keep rain away from your siding and foundation.
Measuring the cuts: The "Plumb Cut" angle is what you'll set your miter saw to for the top cut where the rafter meets the ridge board. The "Seat Cut" is the horizontal part of the birdsmouth notch that sits flat on your wall top plates.

Why Pitch Matters

Choosing the right pitch isn't just about aesthetics. In snowy climates, a steeper pitch (like 8/12 or higher) is necessary to shed snow load. In windy areas, a lower pitch might be preferable to reduce wind resistance.

If you aren't sure if your roof can handle the winter weather in your area, check out our Snow Load Calculator to ensure your framing is strong enough.

Skip the Math Entirely

While this tool helps with custom builds, making complex birdsmouth cuts can still be frustrating for beginners. The easiest way to guarantee a perfect roof is to use professional shed plans that include full-scale rafter templates. You just trace the template onto your lumber and cut—no math required!