A lean-to roof, also referred to as a shed roof, slopes in one direction, with rafters that break against another structure at their high end and overhang support posts or an exterior wall in their low end. When a shed roof ties to an existing roof, the new roof might be a simple lean-to that covers a deck or deck, or it might be the shed roof of a substantial addition. In any case, the structure that the shed roof is leaning on is a existing roof, typically a sloping roof with a ridge and a steeper slope than that of the shed roof.

Attaching Near the Eaves

The up-slope end of a simple shed roof which rests close to the eaves of an existing roof can ordinarily be supported from the rafters of the roof. You will need to eliminate the existing shingles and cut back the existing eaves to create room for the shed roof, then nail a two-by-six ledger board on the existing roof; the ends of the shed rafters will rest on this ledger. The other end of the shed rafters will break on a horizontal beam supported by posts. After the shed roof framing is complete, finish the lean-to, along with any part of the existing roof where you have removed shingles, with new shingles to blend the lean-to to the roof.

Attaching Below the Ridge

In the event of a lean-to roof within an improvement, the existing rafters might need to be cut off to accommodate the addition, and also the shed roof will need support in which it ties to the framing of the existing roof. The rafters of a shed roof which connects a gable roof below its ridge rest against a header which extends horizontally between the roof’s rafters. The gable rafters on both sides of the shed roof are doubled, and short cripple rafters run between the header and the ridge over it; the shed rafters attach into the header contrary to the cripple rafters. Wherever vertical walls below the shed roof intersect the current roof, installation of proper flashing is necessary to stop escapes.

Attaching in the Ridge

A shed roof on a full-height addition may tie into a sloping roof by resting the upper end of the shed rafters from the roof’s ridge. If the ridge is structural — that is, if it’s designed to support the weight of the roof in its own — it may be able to support the burden of the shed roof, also, preventing the roof from exerting too much outward pressure on the front wall of the addition. The lower ends of the shed rafters are supported by this front wall, which then rests on the joists beneath the roof.

Utilizing Collar Ties

If the gable roof’s ridge is not structural or is not otherwise capable to adequately carry the load of the shed roof, then you can use horizontal collar ties to tie the lower ends of the shed rafters into the gable rafters on the other side of the gable roof. These collar ties resist the force exerted by the weight of the roof and stop the front wall of the addition from bowing outward. They also function as ceiling joists at the inside of the addition and at the space under the roof.

Consulting a Professional

The forces working on any roof structure are complicated. At any time you consider making modifications to your home’s roof program, you need to consult a structural engineer to ensure that any alterations won’t undermine the stability of the roof and also that the size and design of new framing members will be adequate. In the event of a shed roof addition, it’s important to realize that the existing structure can safely support the additional weight of the roof.