A 16 × 10 ft shed with a 3/12 single-slope roof has a high wall 2 ft 6 in taller than the low wall. With 12 in overhangs at both eaves and both rakes, each rafter is 12 ft 4⅜ in long, cut from a 14 ft board, and fifteen 2x6 rafters at 16 in on center cover 223 square feet of roof. The calculator above works out any single-slope roof for a shed, porch, lean-to or addition, and checks that the rafters can carry your snow.
How the calculator works
Enter the roof length and the span between the low and the high wall, the pitch and the overhangs. Then the rafter size and spacing, the lumber, the snow load and the covering. The result is the rafter length and count, the height difference of the walls, the roof area and the sheathing or battens.
Rise and rafter length
The high wall stands taller than the low wall by the span times the pitch ÷ 12: 10 ft at 3/12 is 2.5 ft. A shed roof overhangs at both eaves, so each rafter covers the span plus both overhangs, all horizontal, times the slope factor √(1 + (pitch/12)²): (10 + 1 + 1) × 1.031 = 12.37 ft. The two triangular side walls under the slope add span × rise of siding, 25 ft² here.
Count and area
Rafters are the roof length, rakes included, divided by the spacing, plus one: 18 ft at 16 in is 14 spaces and 15 rafters. The roof area is the sloped rafter length times the roof length: 12.37 × 18 = 223 ft², 2.4 squares with 10 % for laps and cuts.
Is the rafter big enough?
Shed rafters span the whole building, not half of it, so they need to be bigger than gable rafters on the same footprint. The calculator checks them by the AWC span method: a 2x6 Douglas fir-larch No. 2 at 16 in spans 12 ft 1 in under 30 psf of snow and a 10 psf metal roof, enough for the 10 ft span. At 24 in spacing it reaches only 9 ft 10 in and the calculator flags it.
Pitch and covering
Low pitches are the reason shed roofs leak. Lapped metal panels need at least 3/12 without sealant (IRC R905.10.2), asphalt shingles 2/12 with double underlayment below 4/12, roll roofing 1/12; standing-seam metal and membranes go lower. Metal on a shed is often fastened to spaced battens, counted here in linear feet; shingles need solid OSB or plywood.
Common mistakes
- Counting the overhang once. A single slope has an overhang at the low eave and at the high eave.
- Sizing shed rafters like gable rafters. The span is the whole width.
- Too flat for the covering. Check the minimum pitch before buying the roofing.
For a two-slope roof, use the Rafter Length Calculator; for spans of any rafter, the Rafter Span Calculator.