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Roof retrofit for non-cathedral, non-low slope roof

Agedbyoldhouses | Posted in Green Building Techniques on

I’ve read several items about adding exterior insulation to existing roofs but haven’t found the detail that I’m looking for.

We have just purchased what amounts to a one story ranch home with a walk-out basement in Central WI built in 1985. The basement is block except for the walk-out wall. The walls are 2×6 stud construction with fiberglass and poly. The gable roof is framed with standard (not energy heel) trusses 2′ o.c at 4/12 pitch. There is a also a small intersecting gable addition with that roof framed on top of the original gable roof.

We are going to be replacing all windows and siding, soffits etc. as well as shingles. Our intent is to use at least 2″ of foam on the exterior of the walls and something similar to interior of basement walls (there is 1″ of foam on exterior block now).

Now the question: For the roof, which is currently vented and has a few inches of cellulose blown in I’d like to go to unvented and add rooftop foam. If I do that I’d also like to extend the gable overhangs from the current 11″ and raise the soffits on the sidewalls.

Does anyone have framing details figured out for that on a roof where 8″ of foam (minimum) is going to be added?

Thanks,

Scott

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Scott,
    Q. "Does anyone have framing details figured out for that?"

    A. I'm not sure what you mean by "framing details." Your roof framing (as you describe it) consists of roof trusses. If you are worried that the framing is inadequate, you need to consult an engineer.

    In most cases, a house built in 1985 that has a truss-framed roof should be able to handle the weight of 8 inches of foam, an extra layer of sheathing, and new roofing. But it never hurts to check.

  2. GBA Editor
    Martin Holladay | | #2

    Scott,
    I imagine that you have read all of the relevant articles on this topic on GBA, but in case you haven't, here is a link to an article you might want to read: How to Install Rigid Foam On Top of Roof Sheathing.

  3. Agedbyoldhouses | | #3

    Thank you Martin for the quick response. I will be consulting with a structural engineer on the trusses to make sure the planned roof assembly still checks with truss design. I have read the articles and looked through the details library and there is a lot of great information there about the insulation and sealing methods and perhaps I've overlooked something that is already there. I should have been more specific in my request about details. I read an article awhile back by Joseph Lstiburek about retrofitting an older house with roof-over foam and he talked about adding box beams in the process to recreate historic detail. I was wondering if anyone has done something similar but for the purpose of extending gable overhangs for better weather protection (especially with the added wall foam).

    In addition I'm trying to minimize the height of the roof edge profile along the sides without a lot of excess trim to simply break up the profile and was looking for additional details others may have come up with to essentially move the soffit up the sidewall. Maybe 'm grasping at straws. All of my experience is with restoration of balloon framed buildings from the 1800's so trusses and aluminum soffits are not something i have any experience with.

  4. GBA Editor
    Martin Holladay | | #4

    Scott,
    The best resource for the details your are talking about is probably this article by Joe Lstiburek: Over-roofing—Don't Do Stupid Things.

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