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Stepped Foundation Metal Flashing Detail with Exterior Rigid Foam

I am replacing my existing siding and I am doing the following -stud - 5/8" plywood - 1-1/2" of Polyisol, 3/8", WRB, rain screen, fiber cement siding. The house is in the San Francisco bay area on a hill with a stepped foundation.

I plan on putting a galvanized metal flashing were the bottom of the plywood meets the foundation (the rigid foam board will be slightly above the metal flashing). I have read numerous articles on this in regards to a horizontal run, but have not found one with details for a stepped foundation. I am assuming that the metal flashing should also run vertically. Should the metal flashing extend beyond the bottom of the fiber cement siding, or just up to it, or just before it? Have any of you had experience with this detail? Any suggestions?


Randy Mason

Asked by Randy Mason
Posted Aug 30, 2014 1:39 PM ET
Edited Sep 2, 2014 10:32 AM ET


5 Answers

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Randy, The steps are best covered with trim rather than flashing. You could use wood or something like Anzec. What are you using for corners? The step detail should look the same.

Answered by Malcolm Taylor
Posted Aug 30, 2014 2:52 PM ET


Thanks for responding Malcom. I will probably use Anzek or a similar product for the corners of the house. So if I follow your approach - I would use metal flashing on the horizontal run and a trim piece on the vertical run. So the detail I need to worry about is any water that collects on the horizontal flashing can't flow to the top of the lower step trim piece. The easy way to do this would probably be to slightly tilt the horizontal metal flashing towards the next up step in the foundation. And I assume that I would leave no gap between the vertical trim piece and the foam board so that no debris or insects get in - in fact I am assuming I would want to make this as a tight as seal as possible?.

Answered by Randy Mason
Posted Aug 30, 2014 5:34 PM ET


Randy, this is not to your question, but normally the WRB goes below furring strips, not on top. Also if your furring is only 3/8" would you be nailing the siding clear through the polyiso? Better to use 1x furring (3/4" thick) and then shorter nails, with siding attached only to the furring. Thin furring (3/8") would not effectively hold siding nails.

As to the flashing, you could snip the first and third legs of the zee profile and then bend the flashing, so it follows the step at the foundation. Then just butt the siding up against the vertical leg, or leave a slight reveal.

Answered by TJ Elder
Posted Aug 30, 2014 5:36 PM ET


I guess this ties in with my response to your other thread. I wouldn't use horizontal flashing at the base, but rather perforated U flashing.
Whenever you do use similar flashing, such as at the heads of windows, the best practice is to include end dams to stop water getting around the edges.

Answered by Malcolm Taylor
Posted Aug 30, 2014 11:36 PM ET



My error in the original post - the WRB is going over the Rigid foam. I am using cor-a-vent sturdy strips for furring strips- so I will nail the siding through the rigid foam ( I can't use 1 x 4 furring strips due to exterior space limitations - a deck)

Answered by Randy Mason
Posted Sep 2, 2014 10:32 AM ET

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