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 to frame than a gable or hip roof, uses less material, and sheds water reliably if you get the slope right. Most homeowners can build one with basic carpentry skills, a level, a circular saw, and a helper.

The core steps are: set your slope (usually 4 to 12 inches of rise per 12 inches of run for a shed), cut and install the rafters, add collar ties if needed for stability, sheath the frame with plywood, and finish with roofing material. The whole process takes a weekend for a small shed, longer for a large one.

Key Takeaways

  • A shed roof slopes one direction only and requires less material and labor than a gable roof, making it ideal for small structures.
  • Rafter spacing, length, and size depend on your roof slope, the span you are covering, and your local snow and wind loads — check your building code or ask your inspector before you cut.
  • The two walls supporting the roof must be at different heights; the difference between them determines your slope.
  • Sheathing plywood and adding roofing material must happen quickly after framing to protect the structure from rain.

Determine your roof slope and rafter size

Your roof slope 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 (6:12, 8:12) sheds water and snow faster but requires taller walls and longer rafters. A gentler slope (3:12, 4:12) uses less material but may not drain as quickly in heavy rain.

Check your local building code or ask your building inspector what slope is required for your area and what snow load your roof must support. This determines the size and spacing of your rafters. A small shed in a low-snow area might use 2×4 rafters spaced 24 inches apart; a larger structure or a snowy climate might need 2×6 or 2×8 rafters spaced 16 inches apart. Do not guess — undersized rafters can sag or fail under snow load.

Use a rafter calculator (available free online from lumber suppliers) or ask your supplier to size the rafters for you. You will need to know: the horizontal distance the roof must span, your slope, and your local snow load. Write down the rafter size and spacing before you buy materials.

Set up the two walls at different heights

A shed roof needs a high wall and a low wall. The difference in height between them creates your slope. If you are building a new shed, you can frame the walls to different heights from the start. If you are adding a roof to an existing structure, you may need to add a beam or posts to the low side to reach the correct height.

Calculate the height difference using your slope. If your roof must span 12 feet horizontally and you want a 4:12 slope, the high wall should be 4 feet higher than the low wall (12 feet ÷ 12 × 4 = 4 feet). Measure from the top of the wall plates on both sides and use a long level or a string line to confirm the difference is correct.

Make sure both walls are plumb (perfectly vertical) and that the wall plates are level along their length. A wall that leans or a plate that is not level will throw off your entire roof frame.

Cut and install the rafters

Rafters run from the high wall plate to the low wall plate. Each rafter is cut with a bird's mouth notch where it sits on the wall plate, and often with a tail that overhangs the wall to protect the siding and create a drip edge.

To cut the first rafter, use a framing square or a speed square to mark the angle at the high end (the plumb cut) and the angle at the low end (the seat cut, which includes the bird's mouth). Cut one rafter, test-fit it on the walls, and adjust if needed. Use this as a template to cut all the others — this saves time and ensures they are identical.

Nail or bolt the rafters to the wall plates with at least two fasteners per connection. Space them according to your code (usually 16 or 24 inches on center). Nail a temporary brace from the ridge of the high wall to the ground to keep the high wall from leaning outward under the weight of the rafters.

Add collar ties and bracing if required

A collar tie is a horizontal board that connects two rafters and prevents them from spreading apart. On a shed roof, you typically need collar ties only if the span is very wide (over 16 feet) or if your code requires them. Check your local code before deciding to skip them.

If you do install collar ties, place them in the upper third of the rafter span and space them every 4 feet along the length of the roof. Nail them with at least three nails on each end. Collar ties do not carry weight; they only prevent the rafters from pushing the walls outward.

If your shed is tall or in a windy area, your inspector may also require diagonal bracing from the rafters down to the wall frame. This is especially common if the structure is open-sided or if the walls are not very strong.

Sheath the roof frame with plywood

Once all rafters are in place and braced, cover the frame with plywood sheathing. Use 1/2-inch or 5/8-inch exterior-grade plywood, depending on your rafter spacing and local code. Plywood comes in 4-by-8 sheets; lay them perpendicular to the rafters so that the edges fall on rafter centers.

Start at the low end of the roof and work toward the high end. Stagger the seams so that the joints in one row do not line up with the joints in the row below. Nail the plywood every 6 inches along the edges and every 12 inches in the field (the middle of the sheet). Use 8-penny nails for 1/2-inch plywood or 10-penny nails for 5/8-inch plywood.

Leave a 1/8-inch gap between sheets to allow for expansion in wet weather. Do not leave gaps larger than this, or water will seep through. Once sheathing is complete, the frame is weather-protected and you can move on to roofing material at your own pace.

Install roofing material and flashing

The roofing material you choose depends on your budget, climate, and how long you want the roof to last. Asphalt shingles are the cheapest and fastest to install; metal roofing lasts longer but costs more; wood shakes or slate are expensive and require more skill. For a shed, asphalt shingles are common and adequate.

Before you lay shingles, install underlayment (a water-resistant paper or synthetic sheet) over the plywood, and install metal flashing where the roof meets the walls. Flashing is critical — it directs water away from the seams and prevents leaks. At the high wall, the flashing should tuck under the siding; at the low wall, it should extend over the siding to shed water away.

Lay shingles from the low end of the roof toward the high end, working in rows. Each shingle overlaps the one below it. Follow the manufacturer's instructions for nailing — usually four nails per shingle, placed 5/8 inch below the adhesive strip. Install a drip edge at the low end and sides to keep water from running under the shingles.

Frequently Asked Questions

What is the minimum slope for a shed roof?

Most codes require at least 2:12 (2 inches of rise per 12 inches of run) to shed water reliably. In heavy snow areas, 4:12 or steeper is safer. A slope less than 2:12 risks ponding — water collecting on the roof instead of running off — which can lead to leaks and rot.

Can I use 2x4 rafters for a large shed?

It depends on the span, slope, and snow load in your area. A 2×4 might work for a 10-foot span in a mild climate, but a 16-foot span or a snowy area usually requires 2×6 or larger. Ask your building inspector or use a rafter calculator with your local snow load data before you buy materials.

Do I need a permit to build a shed roof?

Most jurisdictions require a permit for any structure over a certain size (often 100 to 200 square feet) or for any roof work on an existing building. Check with your local building department before you start. A permit usually costs $50 to $200 and requires an inspection, but it protects you legally and ensures the roof is safe.

How do I prevent the high wall from leaning outward?

Install a temporary diagonal brace from the top of the high wall to the ground while you are installing rafters. Once the plywood sheathing is on, the sheathing itself braces the wall. If the wall is very tall or the roof is very wide, your code may require permanent diagonal bracing or collar ties.

What should I do if my walls are not perfectly level?

Small differences (under 1/4 inch) are normal and will not cause problems. If the difference is larger, shim the wall plates with shingles or metal shims until they are level. A wall plate that is not level will cause rafters to sit unevenly and can lead to leaks where the roof meets the wall.