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Community and Q&A

How to tie in existing shingled roof when adding exterior foam?

otter1492 | Posted in GBA Pro Help on

I am adding 4″ of rigid foam to my house and am looking for suggestions on how to flash where the adjacent wall ties into existing roof shingles. I am curious if there is a way to put the foam on top of the shingles and then flash. This would, of course, cover the existing step flashing and I am not sure if it is a good idea. My original thought was to remove the shingles close the wall and butt the foam straight to the roof underlayment and redo the connection on the outside of the foam with proper tape, peel and stick, and step flashing. Is there a better way? an easier way where I don’t have to do anything with the roof shingles?

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  1. GBA Editor
    Martin Holladay | | #1

    The right way to do it is to cut back the asphalt shingles, below the plane of the anticipated new siding. Then insert new pieces of step flashing, with one piece of step flashing per course of shingles. The upper legs of the step flashing pieces must be integrated with the WRB (water-resistve barrrier) of your new wall, so you need to know where your WRB layer is do to this properly.

  2. otter1492 | | #2

    Thanks for the response. To clarify, WRB will be on the outside and it will have a rainscreen. Would I want to use continuous counterflashing at this connection too? I'm thinking cosmetically, it always looks better. What's your opinion on that?

  3. dankolbert | | #3

    Your WRB should lap over the steps. Continuous counter-flashing is fine, if not necessarily useful or needed. Just make sure it's lapped right - as my first boss always said, "think like a drip."

  4. GBA Editor
    Martin Holladay | | #4

    Counterflashing is usually only necessary with a brick wall or chimney. If you have a flexible WRB like Tyvek, the Tyvek simply laps over the top of the step flashing, as Dan explained.

    Don't make the mistake of bringing your siding down too close to the roofing. If you do, the siding will wick water. A 2 or 3 inch gap between the roofing and the bottom of the siding is a good idea. That leaves some of your step flashing exposed, so make sure that your step flashing has an adequately high vertical leg.

  5. mfredericks | | #5

    Brian, see the best practices manual from Hammer and Hand link below. They have an illustrated guide for this detail. They don't shown continuous insulation but if your WRB is over the foam, then it shouldn't be too hard to wrap your head around how this detail can be applied to your situation.

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