What You Need Before You Start

A shed roof is the simplest roof style to build — it slopes in one direction only, with no peak or valleys. Before you cut any lumber, you need to decide on pitch (how steep the slope is), measure your shed's width and length, and gather the materials. Most shed roofs use a pitch between 4:12 and 8:12, meaning the roof rises 4 to 8 inches for every 12 inches it runs horizontally. Steeper pitches shed water faster; shallower pitches use less material but may pool water in heavy rain.

Measure your shed's exterior wall dimensions and add 1 to 2 feet of overhang on all sides — this protects the walls from rain. Write down the total length and width you need to cover. You will also need to know the height difference between your front and back walls; this is what creates the slope. If your shed is 10 feet wide and you want a 6:12 pitch, your back wall needs to be 5 inches higher than your front wall (10 feet ÷ 2 = 5 feet, and 5 feet × 6 inches per 12 feet = 2.5 inches, rounded up).

Key Takeaways

  • A shed roof slopes one direction only and requires your front and back walls to be at different heights — calculate this height difference before you build the walls.
  • Rafters are the angled boards that form the roof frame; you can cut them all to the same length if you calculate the rise and run correctly using the Pythagorean theorem or a rafter calculator.
  • Rafters typically sit 16 or 24 inches apart and attach to a top plate on the walls with toe nails or metal brackets; spacing closer together makes the roof stronger but uses more lumber.
  • Roof sheathing (plywood or boards) nails to the rafters, then roofing felt and shingles or metal panels go on top in that order.
  • A shed roof needs at least one vent opening near the peak to prevent moisture buildup inside the structure.

Calculating and Cutting Rafters

Rafters are the angled boards that form the skeleton of your roof. To cut them correctly, you need the rise (vertical distance) and run (horizontal distance) of your slope. The run is always half your shed's width if the roof slopes from front to back. If your shed is 10 feet wide, the run is 5 feet. If your pitch is 6:12, the rise is 2.5 feet (5 feet × 6 ÷ 12). Use the Pythagorean theorem to find the rafter length: rise² + run² = rafter length². In this example, 2.5² + 5² = 6.25 + 25 = 31.25, so the square root is 5.59 feet, or about 5 feet 7 inches.

Many builders use a speed square (a triangular measuring tool) or a rafter calculator app to avoid math. Mark your first rafter with the rise and run, then use it as a template to cut all the others. Each rafter needs a notch where it sits on the top plate of the wall — this notch is called a bird's mouth and prevents the rafter from sliding down. The notch depth is usually 1.5 inches (the width of a 2×6 or 2×8 board). Cut the bird's mouth on the lower end of each rafter before you install them.

Space rafters 16 inches or 24 inches apart, measured from the center of one rafter to the center of the next. Sixteen-inch spacing is stronger and handles snow load better; 24-inch spacing uses less lumber and is common for small sheds in mild climates. Measure and mark the top plates of both walls before you start nailing to keep spacing consistent.

Installing Rafters to the Walls

Nail or bolt each rafter to the top plate using at least two 16-penny nails driven through the bird's mouth into the plate, or use metal rafter ties (L-shaped brackets) bolted on both sides. Rafter ties are stronger and prevent the roof from spreading the walls outward under load. Start at one end and work your way across, checking that each rafter is plumb (vertical) with a level before you nail the next one. If a rafter is twisted or bowed, do not force it — set it aside and use a straighter board.

Once all rafters are in place, add a collar tie — a horizontal board nailed across pairs of rafters about one-third of the way up from the base. Collar ties prevent rafter sag and are required by most building codes. Space them every 4 feet along the length of the roof. You may also add a ridge board at the high end where rafters meet, though a shed roof does not require one since both ends of the rafter rest on different walls.

Sheathing the Roof Frame

Sheathing is the layer of plywood or boards that covers the rafters and provides a nailing surface for roofing material. Use 1/2-inch plywood rated for roof use (marked "Roof" on the stamp) or 1×6 or 1×8 boards laid perpendicular to the rafters. Plywood is faster to install; boards allow better air circulation but require more labor. Start at the lower edge and work toward the peak, overlapping each sheet by one rafter space so the seams do not line up. Nail sheathing every 6 inches along rafters and every 12 inches in the field (middle of the sheet).

Leave a 1/4-inch gap between plywood sheets to allow for expansion in humid weather. If you are using boards, space them 1/4 inch apart for the same reason. At the peak end of the roof, cut the sheathing back 1 to 2 inches to create a vent opening — this allows air to flow into the attic space and prevents moisture and heat buildup. You can cover this opening with a soffit vent (a plastic or metal screen) to keep out insects and birds.

Adding Roofing Felt and Final Layers

Once sheathing is complete, roll out roofing felt (also called tar paper or underlayment) starting at the lower edge. Overlap each row by 4 to 6 inches and nail or staple it every 12 inches. Felt protects the sheathing from moisture and provides a slip-resistant surface for roofers. It also acts as a secondary water barrier if shingles are damaged. Overlap the felt at the peak and at the edges, folding it over the fascia board (the board that runs along the edge of the roof).

Install roofing shingles, metal panels, or other roofing material according to the manufacturer's instructions. Asphalt shingles are nailed with four nails per shingle, starting at the lower edge and working upward. Metal panels bolt or screw to the rafters through the sheathing. Whichever material you choose, make sure water drains off the lower edge into gutters or away from the shed foundation. Seal any penetrations (vent pipes, flashing) with roofing cement to prevent leaks.

Ventilation and Moisture Control

A shed roof traps heat and moisture inside the structure if there is no way for air to move. At minimum, cut a vent opening near the peak (as described above) and another near the base of the opposite wall to create cross-ventilation. You can also install a ridge vent (a strip of plastic or metal with slots) along the high edge, or a gable vent (a louvered opening) at the peak end. These vents should total at least 1 square foot of opening per 150 square feet of attic space.

If your shed will store items that generate moisture (tools, firewood, plants), consider adding a small fan or increasing vent size. Moisture trapped inside rots wood, rusts metal, and damages stored goods. Vents are cheap insurance and take only an hour to install.

Common Mistakes and How to Avoid Them

The most common error is miscalculating the height difference between front and back walls. Double-check your math or use a rafter calculator before you build the walls — fixing this mistake after the fact means rebuilding walls. Another frequent problem is spacing rafters unevenly. Use a tape measure and mark both walls before you start nailing; do not eyeball it.

Undersizing rafters for the span and snow load is also common. A 12-foot-wide shed in a snowy climate needs larger rafters than a 6-foot-wide shed in a dry climate. Check your local building code or consult a lumber supplier for the correct rafter size and spacing for your location. Forgetting ventilation is straightforward but costly — add vents during construction, not after the roof is finished and leaking.

Frequently Asked Questions

What size lumber do I need for rafters?

Rafter size depends on the span (distance between walls), spacing, pitch, and snow load in your area. A 10-foot span with 24-inch spacing in a mild climate typically uses 2×6 lumber; a 12-foot span or snowy area needs 2×8 or 2×10. Check your local building code or ask a lumber supplier for the correct size.

Can I use a shed roof on a large building?

Yes, but as the building gets wider, rafters must get larger and spacing must get closer to handle the load. A 20-foot-wide shed needs much heavier rafters than a 10-foot shed. For buildings over 16 feet wide, consider a gable or truss roof instead, which distributes weight more efficiently.

Do I need a building permit for a shed roof?

Most jurisdictions require a permit for any permanent structure, including sheds. Permit requirements vary by location and shed size. Check with your local building department before you start; they will tell you what inspections are needed and what code requirements explore to your roof.

What is the minimum pitch for a shed roof?

Most building codes require at least a 2:12 pitch (2 inches of rise per 12 inches of run) to shed water. Flatter roofs pool water and leak. If your shed is very small or in a dry climate, you may get away with 2:12, but 4:12 or steeper is safer and lasts longer.

How do I prevent ice dams on a shed roof?

Ice dams form when heat escapes through the roof and melts snow at the edge, which then refreezes. Improve ventilation to keep the roof cold, add insulation below the rafters to reduce heat loss, and install metal flashing or ice and water shield (a sticky membrane) along the lower edge to direct meltwater into gutters instead of under shingles.