About

Shed roofs, also known as skillion or mono-pitch roofs, require a different calculation approach than standard gable roofs. The critical dimension is the differential between the high wall and the low wall over a specific horizontal run. This difference dictates the pitch, which fundamentally affects water drainage and snow load capacity.

Builders often encounter two scenarios: they either have a desired pitch and need to know how much taller to frame the front wall, or they have existing wall heights and need to verify if the resulting pitch meets roofing material warranties. This tool handles the conversion of mixed units (feet and inches) to prevent accumulation errors in framing.

Formulas

The relationship between wall heights and roof pitch is linear.

Wall Differential (Rise):

Rise = H_high − H_low

Pitch Calculation:

Pitch = RiseRun × 12

Rafter Length:

L = √Rise² + Run² + Overhangs

Accurate Run measurement must be horizontal distance, not along the slope.

Reference Data

Material	Min Pitch Ratio	Min Angle (°)	Notes
Built-Up Roof (BUR)	0.25:12	1.19°	Requires positive drainage
Modified Bitumen	0.25:12	1.19°	Torch-down or self-adhered
Standing Seam Metal	1:12	4.76°	Check manufacturer specs
Corrugated Metal	3:12	14.04°	Exposed fasteners require slope
Asphalt Shingles	2:12	9.46°	Low slope application required (2:12-4:12)
Asphalt Shingles (Std)	4:12	18.43°	Standard application
Clay/Concrete Tile	4:12	18.43°	Heavy weight considerations

Frequently Asked Questions

Ideally, a shed roof should have at least a 1:12 pitch to ensure water does not pool. However, for asphalt shingles, the minimum is typically 2:12. Flat roofs (less than 2:12) require specialized membrane roofing (EPDM, TPO, or torch-down).

If you know your desired pitch (e.g., 4:12) and the width of the shed (e.g., 10 feet), the calculation is: Run (10) × Pitch Ratio (4/12) = 3.33 feet rise. Add this rise to the height of your low wall to get the high wall height.

This calculator provides the "Rafter Span" (hypotenuse of the triangle formed by walls). You must manually add the length of your desired eaves/overhangs to both ends to get the total lumber length required.

Yes. For a lean-to attached to an existing structure, the "High Wall" is the connection point on the existing house, and the "Low Wall" is the outer wall of the addition.