What You Need Before You Start

A shed roof is the simplest roof style to build — it slopes in one direction instead of two, which means fewer cuts, fewer joints, and fewer places for water to pool. Before you cut any wood, you need a clear plan: the roof pitch (how steep it is), the span (how far the rafters must reach), and the materials you're using. Most shed roofs use a 4-in-12 pitch (4 inches of rise for every 12 inches of horizontal run), which sheds water without requiring special fasteners or underlayment.

Gather your materials on the ground first. You'll need rafters (usually 2x6 or 2x8 lumber, depending on span and snow load), a ledger board to attach to the shed wall, a rim board for the opposite end, collar ties to brace the rafters, roofing felt, shingles or metal panels, nails, and flashing for any penetrations. Check your local building code — some jurisdictions require permits for sheds over a certain size, and codes specify rafter spacing (usually 16 or 24 inches on center) and fastening methods.

Key Takeaways

  • A shed roof slopes one direction and requires calculating rafter length based on your roof pitch and the distance the rafters must span.
  • The ledger board attaches to the existing shed wall and carries one end of the rafters; the rim board at the opposite end carries the other.
  • Rafters are spaced 16 or 24 inches apart (on center) and must be sized for your local snow load and wind conditions.
  • Once rafters are in place and braced with collar ties, you add sheathing, felt, and shingles or metal roofing in that order.
  • Flashing at the ledger board and around any roof penetrations prevents water from running behind the shingles and into the shed.

Calculating Rafter Length and Angle

The rafter length depends on two things: how far it must span horizontally and how steep the pitch is. If your shed is 10 feet wide and you want a 4-in-12 pitch, the rafter rises 4 inches for every 12 inches of run. Use the Pythagorean theorem or a rafter calculator (search "rafter length calculator" and enter your span and pitch). For a 10-foot span at 4-in-12, the rafter is roughly 10 feet 5 inches long, measured along the slope.

The angle at which you cut the rafter's top end (the plumb cut where it meets the peak) and bottom end (the bird's mouth where it sits on the wall) depends on pitch. A 4-in-12 pitch creates a plumb cut of about 18 degrees. You can mark this angle on your first rafter using a speed square set to 4-in-12, then use that rafter as a template for the rest. Precision here saves time: a rafter cut wrong by half an inch will throw off your entire roof line.

Attaching the Ledger Board and Setting the First Rafter

The ledger board is a horizontal board (usually 2x8 or 2x10) that bolts to the existing shed wall and carries one end of every rafter. Mark its position on the wall at the height where you want the roof to start. If the shed wall is wood, use lag bolts or through-bolts spaced 16 inches apart. If it's concrete or block, use concrete anchors. The ledger must be level — use a 4-foot level and shim as needed.

Once the ledger is find, cut your first rafter and test-fit it. The top end (plumb cut) should rest against the rim board at the opposite end of the shed. The bottom end (bird's mouth) should sit on the ledger with an overhang of 12 to 18 inches for the gutter. Adjust the rafter until it's level along its length, then nail or bolt it in place. This first rafter sets the pitch for all the others.

Installing Remaining Rafters and Collar Ties

Once the first rafter is set, measure 16 or 24 inches (on center, depending on your code) along the ledger and mark the next rafter location. Cut the second rafter using the first as a template, fit it, and nail it down. Repeat across the entire span. As you go, check that each rafter is plumb (vertical when viewed from the end) and that the spacing is consistent. Uneven spacing will show up later when you're nailing sheathing.

Collar ties are horizontal boards (usually 2x4) that connect pairs of rafters near the top and prevent them from spreading under load. Install one collar tie for every two or three rafter pairs, nailed with at least three 16-penny nails on each end. The collar tie should sit roughly in the upper third of the rafter span. These ties are not optional — they carry tension and keep the roof from sagging.

Adding Sheathing, Felt, and Roofing

Once all rafters and collar ties are in place, nail plywood or OSB sheathing across the rafters. Use 7/16-inch or 1/2-inch material and nail it with 8-penny nails spaced 6 inches apart at the edges and 12 inches in the field. Stagger the seams so they don't line up across multiple rows. Leave a 1/8-inch gap between sheets to allow for expansion.

Nail roofing felt (usually 15-pound) over the sheathing, starting at the bottom and overlapping each row by 2 inches as you work up. The felt protects the sheathing from moisture while you're working and provides a slip-resistant surface for walking. Then install your roofing — asphalt shingles, metal panels, or standing seam — according to the manufacturer's instructions. Shingles typically start at the bottom edge and overlap upward; metal roofing is fastened through the ribs or flanges.

Flashing and Water Management

Flashing is metal (usually aluminum or galvanized steel) that directs water away from joints and penetrations. At the ledger board, install flashing that sits under the roofing felt and over the ledger, sloping downward and outward. This prevents water from running behind the shingles and into the shed wall. If your roof has a chimney, vent pipe, or skylight, install flashing around each one before the shingles go on — the shingles overlap the flashing, not the other way around.

At the gutter line (the bottom edge of the roof), install drip edge flashing that extends into the gutter and keeps water from running under the sheathing. The drip edge goes under the felt at the top and over the shingles at the bottom. These details matter: water finds every gap, and flashing is what stops it.

Common Mistakes and How to Avoid Them

The most common error is undersizing the rafters. A rafter that sags under snow load will eventually crack, and the roof will leak. Use a rafter calculator that accounts for your local snow load (your building department can tell you what it is) and don't guess. A second mistake is skipping collar ties or installing them too low — they must be in the upper third of the span to work.

A third mistake is poor flashing at the ledger. Water that gets behind the ledger will rot the shed wall from the inside, and by the time you notice, the damage is done. Install flashing before the shingles, and make sure it slopes downward and outward. Finally, don't rush the sheathing. Uneven spacing or loose nails will show up as ripples in the shingles and will eventually lead to leaks.

Frequently Asked Questions

What pitch should my shed roof be?

A 4-in-12 pitch is standard and sheds water well without requiring special materials. Steeper pitches (6-in-12 or higher) shed water faster and shed snow better in heavy-snow areas, but they require taller walls and longer rafters. Flatter pitches (2-in-12 or 3-in-12) are cheaper but need better underlayment and more frequent maintenance.

Can I use 2x4 rafters instead of 2x6?

Only if your span is very short (under 8 feet) and your snow load is light. A rafter calculator will tell you the minimum size for your conditions. Undersized rafters will sag, and a sagging roof leaks. It's not worth saving the cost of one or two boards.

Do I need a rim board at the far end of the roof?

Yes. The rim board (usually 2x8 or 2x10) carries the top end of the rafters and ties them together. Without it, the rafters would have nothing to rest against and would collapse. The rim board also provides a nailing surface for the sheathing.

What if my shed wall is not straight?

Shim the ledger board so it's level and plumb, even if the wall behind it is not. The roof will follow the ledger, not the wall. Once the ledger is set, the rafters will be straight regardless of what the wall looks like.

How much overhang should the roof have?

Twelve to 18 inches is standard. The overhang protects the walls from rain and provides a place for the gutter. Longer overhangs look better but require longer rafters and more material. Shorter overhangs (under 12 inches) don't protect the walls as well and can make gutters hard to clean.