Building a Lean-to Roof

Rafters

• Rafters are the main structural components forming the support for a lean-to roof. Composed of large boards running parallel from the high end to the low end of the roof, they attach to the wall of an existing structure on the high end and a plate, or horizontal support beam, on the lean-to building's wall at the low end. Standard 2-inch-by-12-inch Douglas fir lumber or its equivalent is suitable for use as rafters in a lean-to roof. Determine the length of the rafters, which form the hypotenuse of a right triangle when viewed from the side, using the Pythagorean theorem (the square of the hypotenuse is equal to the sums of the squares of the other two sides of a right triangle).

Purlins

• Purlins are smaller boards running lengthwise across the rafters at regular intervals. In addition to providing additional structural support, purlins act as an anchor for the roof covering, which is nailed to them. In older construction, the purlins may be attached to the rafters with complex wood joinery that, while sturdy, is also extremely labor-intensive. Today, purlins may be attached to the rafters with metal plates, or simply toe-nailed into place.

Roof Covering

• Depending on the type of lean-to being constructed, the final roof covering may be of corrugated metal, or plywood sheathing covered with tar paper and finished with whatever roofing material is on the original building to which the lean-to is attached. Asphalt shingles, clay tile and wooden shakes are all suitable materials. If noise from rain is a concern when using metal roofing, add stiff foam insulation under the sheet metal to deaden the sound.

Attachment to Existing Wall

• For long-term stability, the rafters of a lean-to must attach securely to the structural members of the wall of the original building. Ideally, the rafters tie into the plate at the top of this wall. Alternately, the ends of the rafters may attach to any horizontal structural beam in the wall by means of mortises cut through that beam. If such skilled joinery techniques are not feasible, the rafters may also be nailed into place using metal plates. A lean-to roof is simpler to build when the pitch is not as steep as the pitch of the existing building's roof.