Expert: Daniel Humphrey - 3/13/2008

Question
We would like to cover our back patio with a lean to roof. We have a  basic cape cod style home and the lean to would be coming off the back roof. My question is do we need to attach a ledger board to the existing roof or can we lean the rafters directly to the existing roof?

Answer
Renee,

The proper way to extend a roof is to sister-up the new rafters next to the existing ones. This means you have to remove the existing edge trim and fascia to plug the new rafters in alongside the existing rafters, allowing the new ones to bear down on the wall plate in approximately the same place as the older ones and use the same hurricane ties. If no hurricane ties are present, you can nail the new rafters to the old ones and add a hurricane tie to hold both rafters to the wall.

If the pitch is changed (lessened), the portion of roof decking over the eaves needs to be slotted or removed to allow the new rafters to insert.

You can, of course, place the new rafters right on top of the existing ones if the shed roof pitch is less than the main roof. But you need to make sure to attach the new rafters to the wall with proper hold downs for wind lift. A plywood gusset that links old and new rafters is the standard attachment and eliminates the need for any kind of ledger.

If you don't want to disturb the roof that much and decide to place the rafters right on top of the existing roof, decking and all, a ledger board can help distribute weight evenly along the tops of the old rafters, but you should still put in hurricane tiedown straps alongside the new rafters, inserted through the decking,  and nailed down to the wall itself.

Daniel