A shed roof is the simplest roof frame you can build
A shed roof (also called a lean-to roof) slopes in one direction only, from a higher wall to a lower wall. It has no peak, no valleys, and no hips — just a single plane. This makes it faster and cheaper to frame than a gable or hip roof, and it's the right choice for sheds, garages, additions, and small outbuildings where headroom on one side doesn't matter.
The basic steps are: build the walls to different heights, install rafters that span from the high wall to the low wall, add sheathing on top, then install underlayment and roofing material. The whole process takes a few days for a small shed, and you can do it with basic carpentry tools and a helper.
Key Takeaways
- A shed roof needs two walls at different heights — the difference in height determines the roof slope, which should be at least 4 inches of rise per 12 inches of run for water drainage.
- Rafters are cut at angles on both ends and fastened to the top plates of both walls, spaced 16 or 24 inches apart depending on snow load and rafter size.
- Sheathing (plywood or boards) goes on top of the rafters, then underlayment, then roofing material — each layer protects the one below it from water.
- The roof overhang at the low end should be at least 12 inches to keep water off the wall, and you'll need a fascia board and soffit to finish the edge.
- Local building codes set minimum slope, rafter spacing, and fastening requirements — check with your building department before you start.
Determine the roof slope and rafter length
The slope of a shed roof is expressed as rise over run — for example, 4:12 means the roof rises 4 inches for every 12 inches it travels horizontally. A steeper slope sheds water faster and looks more finished; a shallow slope is cheaper and easier to frame but requires more underlayment and careful flashing.
Measure the horizontal distance from the high wall to the low wall (this is called the span). Decide on your slope — 4:12 is a common minimum, 6:12 is moderate, and 8:12 or steeper is steep. Use a rafter calculator or a framing square to find the rafter length. For example, a 12-foot span with a 4:12 slope needs rafters about 12 feet 8 inches long (measured along the sloped edge). Add 12 to 18 inches for the overhang at the low end.
Write down the rafter length and the height difference between the two walls. You'll need both when you cut the first rafter and use it as a pattern for the rest.
Cut and install the rafters
Rafters need angled cuts at both ends. The high end (called the bird's mouth or seat cut) sits on top of the high wall's top plate. The low end is cut at an angle to sit on the low wall's top plate. A framing square or speed square makes these cuts easier — mark the angle on the rafter board, then cut with a circular saw or miter saw.
Cut your first rafter and test-fit it between the two walls. It should sit flat on both top plates with no gaps. Once it fits, use it as a pattern to cut the rest. Space the rafters 16 inches apart (center to center) if you're using standard lumber and expect moderate snow load, or 24 inches apart if your area has light snow and your rafter size is large enough. Check your local building code for the spacing required in your area.
Fasten each rafter to the top plate with at least three 16-penny nails driven through the rafter into the plate, or use hurricane ties (metal brackets) if your code requires them. In high-wind areas, ties are often mandatory. Make sure the rafters are straight and level as you go — sight along each one before you nail it permanently.
Add collar ties and brace the frame
Collar ties are horizontal boards that connect pairs of rafters and prevent them from spreading apart under the weight of snow or wind. Install them in the upper third of the rafter span, spaced every 4 feet along the roof. Nail them with at least two 16-penny nails at each end.
If your shed is tall or your rafters are long, you may also need a beam or post running down the center of the building to support the rafters from below. Check your local code or consult a framing guide for your specific span and slope — undersized rafters or inadequate bracing can sag or fail under snow load.
Install sheathing and underlayment
Sheathing is the layer of plywood or boards that covers the rafters and gives the roof its shape. Use 1/2-inch exterior plywood (rated for roof use) or 1-by boards spaced 1/4 inch apart for ventilation. Fasten the sheathing with 8-penny nails or 2-inch screws, spaced 6 inches apart along the rafters and 12 inches apart in the field (the middle of each sheet).
Once the sheathing is down, install underlayment — a water-resistant layer that protects the sheathing if water gets under the roofing material. Asphalt-saturated felt (15-pound or 30-pound) is common and inexpensive. Synthetic underlayment lasts longer and is easier to walk on. Staple or nail it down, overlapping each course by at least 2 inches, and work from the low end of the roof upward so water runs over each layer.
Install roofing material and flashing
The most common roofing for sheds is asphalt shingles — they're affordable, durable for 15 to 25 years, and straightforward to install. Start at the low end of the roof and work upward. Each shingle overlaps the one below it, and nails go through the nailing strip (usually marked on the shingle). Use roofing nails (shorter and wider-headed than framing nails) and drive them straight so the heads don't tear the shingle.
At the high end where the roof meets the wall, install flashing — a bent metal or rubber strip that directs water down the roof instead of behind the wall. The flashing should tuck under the siding on the wall and over the shingles on the roof. Seal the seams with roofing cement or caulk rated for roofing.
At the low end, install a drip edge (a metal strip that hangs over the fascia board) and a gutter if water runoff is a concern. The drip edge keeps water from running back under the shingles and into the wall.
Finish the edges with fascia and soffit
The fascia is a board that covers the rafter ends at the low side of the roof and gives the shed a finished look. It also provides a surface to attach gutters. Use 1-by or 2-by lumber (cedar or treated pine resists rot) and fasten it with 16-penny nails driven through the board into the rafter ends.
The soffit is the underside of the overhang — the space between the fascia and the wall. You can leave it open (which allows air to flow under the roof), close it with plywood, or install vented soffit panels if you want to ventilate the rafter space. Vented soffits are useful if you're worried about moisture buildup, but they're not required for a straightforward shed roof.
Frequently Asked Questions
What slope should a shed roof have?
A minimum of 4:12 (4 inches of rise per 12 inches of run) is standard for most areas and roofing materials. Steeper slopes like 6:12 or 8:12 shed water faster and look better, but they cost more to frame and require longer rafters. Check your local building code — some areas have different minimums based on climate or wind.
Can I use 2x4s for rafters on a 12-foot span?
It depends on snow load, rafter spacing, and roof slope. In light-snow areas with 24-inch spacing, 2x4s might work for a 12-foot span. In heavy-snow areas or with 16-inch spacing, you'll need 2x6s or larger. Use a rafter sizing table or consult your building department — undersized rafters can sag or fail.
Do I need a permit to build a shed roof?
Most areas require a permit for any permanent structure, including sheds. The permit process includes a plan review and an inspection after framing is complete. Permits cost money and take time, but they may support the roof meets code and protect you if there's a problem later. Check with your local building department before you start.
What's the difference between underlayment and roofing felt?
Roofing felt is one type of underlayment — it's asphalt-saturated paper that's been used for decades. Synthetic underlayment is newer, lasts longer, and doesn't tear as easily when you walk on it. Both do the same job: they protect the sheathing from water. Synthetic costs more upfront but may save money over the life of the roof.
How much overhang should the roof have at the low end?
At least 12 inches is standard — it keeps rain off the wall and protects the foundation. If you're in a high-wind area, a smaller overhang (6 to 8 inches) is sometimes preferred because it reduces wind uplift. Check your local code and consider your climate when you decide.