I'm in the early stages of planning an addition which will involve extending a flat roof out from a sloped roof. Are there reference sources/videos/etc out there discussing approaches to shortening and supporting the existing rafters to allow the flat roof to be raised to the height I'd like, as well as how to properly attach the flat roof to the sloped roof? Here's a rough mockup of the current rafter structure:

enter image description here

And an equally rough mockup of what I'd like to end up with:

enter image description here

  • 1
    Are you building with a permit? You might start at the permit desk to find out what they require... they could be anywhere on the continuum of "relaxed" to "you have to have an engineer's stamp". Commented Dec 31, 2019 at 4:55
  • How far up the existing roof will the flat shed roof rest on the existing rafters? That will increase the load on the existing rafters, and you need to make sure they can carry that without compromising their integrity. You're basically adding half the total load of the new roof to the rafters on the old roof.
    – SteveSh
    Commented Dec 31, 2019 at 12:54
  • I would consider what jack has to say, in my jurisdiction any modification of the structure requires a engineering stamp but adding the flat roof without modification is a simple permit review.
    – Ed Beal
    Commented Dec 31, 2019 at 17:13
  • Is there a specific purpose for cutting into the main roof? There really is no advantage to doing so, only disadvantages.... Also, is the addition the only new construction or is the main roof new too? The drawing you have suggests everything is new.
    – Jack
    Commented Jan 1, 2020 at 0:45

1 Answer 1


Typically what is done is the main roof is left intact and the shed roof rafters are set to a ledger that is set on top of the main rafters. That will mean a long angled cut on the shed roof rafter which, according to your drawing, by the time the angled cut it set to the ledgers the bottom corner so to speak of the shed roof rafters will be very close to the bearing wall below. The ledger may need to be done in two runs to cover the whole cut of the shed rafter. Or, with some layout, a single run of 2X12 set so the heel of the cut is at the lower edge of the 2X12 ledger. The rest of the rafter will set directly to the roof sheathing which can be applied before any of the ledgers are set. Just be sure that ventilation will flow from one roof to the other so heat does not build up with no place to go.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.