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Replacing a roof after foam board wall upgrade

user-953356 | Posted in Green Building Techniques on

I am about to embark on residing a house and adding 2″ rigid foam board on the exterior with furring and cedar shingles on top. The roof of the house is in decent shape and not ready to be replaced (when it is ready, would like to consider adding SIPs on top to boost its thermal capacity.)

What flashing/finish detail would you recommend at wall/roof or wall/decking intersection? Being a retrofit, and keeping the original windows, the drainage plane is being maintained between the sheathing and foam. Any eloquent way of maintaining access to the shingles or decking without leaving a larger than normal clearance between the foam and the deck?

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  1. GBA Editor
    Martin Holladay | | #1

    I don't understand your question. Does your house have a soffit? If so, the best way to proceed is to disassemble the soffit so that you can see the underside of your roof sheathing. Your wall foam should extend up to the underside of the roof sheathing (unless you want to maintain a ventilation channel at that location).

    If your house has no soffit, then your wall foam would extend all the way up to the underside of your roof sheathing. Do your best to maintain the continuity of your thermal barrier and your air barrier.

  2. user-953356 | | #2

    Thanks for the reply. Sorry for the confusion. Plenty of soffit. That end is not the problem. I am concerned with the bottom end of the wall on a second story that intersects with the roof below, where there is step flashing and we usually leave 3/4" gap between the bottom cedar shingle course and the roofing. If I use 3/4" foam, the additional wall thickness can be managed when the roof is replaced in the future. If we go with 2" foam, then the existing step flashing and roofing material gets buried deep in the new wall.

  3. GBA Editor
    Martin Holladay | | #3

    The roof will have step flashing, of course, and that step flashing will need to be removed when you re-roof and add roof foam.

    As you are now working on the wall above the roof, install a wide belt of counterflashing that laps over the top of your step flashing. This counterflashing will be visible for a few years, until you re-roof. Make the counterflashing wide enough so that you can keep the cedar wall shingles to the height where you want them to be after the re-roofing job.

    When you re-roof, trim the bottom of the counterflashing with a sharp blade or metal snips, and bend the counterflashing up so that you can lap the counterflashing over the new step flashing.

    By the way, I prefer to see a much wider gap than 3/4 inch between the roofing and the bottom of the siding in this location. I like high step flashing, and I like to keep the siding at least 4 inches above the roofing. Otherwise, the bottom of the siding rots prematurely due to splashback and snow.

    Then again, I live in snow country... and perhaps you don't.

  4. user-953356 | | #4

    Thanks Martin, that makes sense. I guess the short of it is that regardless of adding rigid foam or not to future roof deck, we need to leave a more than customary gap between siding and roof deck for installing future step flashing and roofing.

  5. [email protected] | | #5


    I am currently in the process of doing what you are describing here. Did everything work out ok for you? Do you have any pictures of what the flashing looks like? I am trying to picture what is being described here and I can't visualize it.



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