1. The Critical Difference: Symmetrical Gable vs. Shed Roof
On a conventional symmetrical gable roof, two rafters rise from opposite walls and meet at a central ridge board. As a result, the horizontal run of each rafter is exactly half of the total building span:
On a shed roof (also known as a lean-to, skillion, or mono-pitch roof), the rafters slope continuously across the entire width of the structure in a single plane. Therefore:
2. Calculating Wall Height Differences
To achieve your desired pitch on a shed roof, the high supporting wall (or ledger board attached to an existing structure) must be taller than the low wall plate by an exact vertical dimension:
Example: You are building a 12-foot wide shed addition with a 3:12 pitch:
3. Low-Slope Waterproofing on Shed Roofs
Because shed roofs span the entire structure, builders often choose lower pitches (such as 2:12 or 3:12) to avoid excessively tall exterior high walls. However, low slopes require special attention to moisture protection:
- Slopes Under 2:12: Standard asphalt shingles cannot be installed. You must use continuous membrane roofing (such as self-adhered modified bitumen or single-ply EPDM/TPO).
- Slopes 2:12 to 4:12: If installing architectural shingles, building codes (IRC R905.1.1) mandate a double layer of underlayment or a continuous self-adhering ice-and-water membrane across the entire roof plane.
- High Ledger Flashing: If the shed roof leans against an existing house wall, ensure the metal headwall flashing extends at least 4 inches up behind the house drainage plane and at least 4 inches over the upper roofing courses.