The basic order: frame, sheathing, underlayment, roofing material

A shed roof is built in layers from the inside out. You start by building the frame — the rafters and support beams that hold everything up. Then you nail plywood or OSB sheathing to the top of that frame. After that comes underlayment, which is a water-resistant barrier. Finally, you attach the actual roofing material: shingles, metal panels, or whatever you chose. The whole process takes a few days to a week for a small shed, depending on the size and your experience.

The pitch — how steep the roof is — matters before you start. A steeper pitch sheds water faster and lasts longer, but it is harder to build and uses more materials. Most shed roofs are pitched between 4-in-12 and 8-in-12, meaning the roof rises 4 to 8 inches for every 12 inches it runs horizontally. A flatter pitch like 2-in-12 is easier to build but needs better underlayment and more frequent maintenance.

Key Takeaways

  • Rafters are the angled beams that form the roof shape; they sit on top of the shed walls and are spaced 16 or 24 inches apart depending on snow load and rafter size.
  • Plywood or OSB sheathing goes on top of the rafters and must be nailed every 6 inches along the edges and every 12 inches in the middle to prevent sagging.
  • Underlayment is a water barrier that goes under the roofing material and overlaps like shingles do, with the upper rows covering the lower rows.
  • Roofing material — whether asphalt shingles, metal, or wood — is fastened on top and must be secured according to the manufacturer's instructions for your climate.
  • Flashing at the edges and any roof penetrations (like vents) prevents water from running under the roofing material and into the shed.

Calculating rafter size and spacing

Rafter size depends on three things: the span (the distance the rafter has to cross), the pitch, and the load it needs to carry. Load includes the weight of the roofing material, underlayment, and snow or ice in your region. A 12-foot span with a 4-in-12 pitch in a light-snow area might use 2×6 rafters spaced 24 inches apart. The same span in a heavy-snow area might need 2×8 rafters spaced 16 inches apart.

You can find rafter sizing tables in the International Residential Code (IRC), which most building departments follow. Your local building department can tell you what load your area requires. If you are unsure, err on the side of larger rafters and closer spacing — the extra cost is small compared to a roof that sags or fails.

Rafters are typically cut with a bird's mouth notch where they sit on the top plate of the shed wall. This notch lets the rafter sit flat and prevents it from rolling. The tail of the rafter extends past the wall to create an overhang, usually 12 to 24 inches, which protects the walls from rain.

Framing and installing the rafters

Start by marking the rafter spacing on the top plates of the shed walls — either 16 or 24 inches apart, depending on your design. Cut your first rafter as a template, checking the bird's mouth notch and the angle cuts at the peak. Use this template to cut all the others so they are identical.

Install rafters in pairs, one on each side of the peak, and nail them to the top plate with 16-penny nails — at least three nails per connection. If the shed is wide, add a collar tie — a horizontal board connecting the two rafters about one-third of the way up — to prevent them from spreading apart under load. For a shed wider than 12 feet, you may also need a ridge beam at the peak to support the rafter ends instead of having them meet point-to-point.

Once all rafters are in place, add any bracing or ties your design calls for. Then install the fascia board along the rafter tails and the soffit underneath if you want a finished look. These are not structural but protect the rafter ends from weather.

Sheathing the roof with plywood or OSB

Sheathing is the layer that ties all the rafters together and provides a nailing surface for the roofing material. Use 1/2-inch plywood or OSB for most residential sheds. Start at the bottom edge of the roof and work your way up, staggering the seams so they do not all line up in one spot.

Nail the sheathing every 6 inches along the edges where it meets a rafter and every 12 inches in the field (the middle of the sheet). Use 8-penny nails for 1/2-inch sheathing. Leave a 1/8-inch gap between sheets to allow for expansion in humid weather. If you are using plywood, orient it so the grain runs perpendicular to the rafters for maximum strength.

Once sheathing is complete, inspect it for any soft spots or damage. If you find any, replace those sections before moving on to underlayment.

Installing underlayment and flashing

Underlayment is a water-resistant barrier that sits between the sheathing and the roofing material. The most common type is asphalt-saturated felt, sold as 15-pound or 30-pound felt, or synthetic underlayment, which is more durable and easier to walk on. Synthetic underlayment is more expensive but lasts longer and does not degrade in sunlight the way felt does.

Roll out the underlayment starting at the bottom edge of the roof, overlapping each row by at least 2 inches so water runs down and off rather than under. Nail it down with roofing nails or staples every 12 to 18 inches. Do not pull it tight — it should lie flat but with a little slack so it does not tear in wind.

Install flashing at any edges where water might run under the roofing material. This includes the rake edges (the angled sides), the eaves (the bottom edge), and around any roof penetrations like vents or chimneys. Flashing is usually galvanized steel or aluminum and is nailed to the sheathing before the underlayment goes on at those points, so the underlayment overlaps the top of the flashing.

Attaching the roofing material

The roofing material goes on last and is what actually sheds the water. Asphalt shingles are the most common choice for sheds because they are inexpensive and straightforward to install. Metal roofing is more durable and lasts 40 to 70 years compared to 15 to 25 years for asphalt. Wood shakes and shingles are attractive but require more maintenance.

For asphalt shingles, start at the bottom edge and work your way up, overlapping each row so the tabs of the lower row are covered by the upper row. Nail each shingle with four nails placed in the nail strip — the line marked on the shingle. Use roofing nails, which have a large head and are designed not to tear through the shingle. In high-wind areas, use six nails per shingle instead of four.

For metal roofing, follow the manufacturer's instructions for fastening, as different profiles require different attachment methods. Some metal roofs use exposed fasteners, while others use concealed fasteners. Fasteners must be sealed with a rubber washer to prevent leaks around the nail hole.

Once the roofing material is on, install ridge caps at the peak to cover the joint where the two sides meet. Ridge caps are either shingles cut and bent to fit or metal caps designed for the roofing profile you chose.

Ventilation and drainage details

Proper ventilation under the roof keeps moisture from building up and damaging the rafters and sheathing. For a straightforward shed roof, soffit vents at the bottom and a ridge vent at the top create airflow. If you do not have a ridge vent, install roof vents — usually 4-inch or 6-inch round vents spaced evenly across the roof.

Gutters and downspouts are not required but are worth adding if the shed is near a foundation, a garden, or a neighbor's property. They direct water away from the base of the shed and prevent erosion and standing water. Gutters should slope slightly toward the downspout — about 1/4 inch per 10 feet — so water does not pool.

Frequently Asked Questions

Do I need a building permit for a shed roof?

Most jurisdictions require a permit for any structure over a certain size, usually 100 to 200 square feet. Even if your shed is smaller, a permit is often required if you are adding a roof to an existing structure or if the roof will be over 12 feet tall. Check with your local building department before you start.

What is the difference between felt and synthetic underlayment?

Felt is cheaper and has been used for decades, but it can tear easily and degrades in sunlight. Synthetic underlayment costs more but is tougher, does not degrade, and is easier to walk on. For a shed, synthetic is worth the extra cost if the roof will be exposed to sun for long periods or if you live in a windy area.

Can I use a single-layer roof without underlayment?

No. Underlayment is a water barrier that protects the sheathing and rafters if water gets under the roofing material. Without it, any leak will damage the structure. Building codes require underlayment in most jurisdictions.

How do I know if my rafters are strong enough?

Check the rafter sizing tables in the IRC or ask your building department what load your area requires. If you are unsure, use larger rafters and closer spacing — the extra material cost is small compared to the cost of repairing a failed roof.

What pitch should I use for a shed roof?

A 4-in-12 to 6-in-12 pitch is standard for sheds and balances ease of construction with good water drainage. A steeper pitch sheds water faster but is harder to build. A flatter pitch is easier but requires better underlayment and more maintenance in wet climates.