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lee12 08-16-2009 09:25 AM

flashing and styrofoam sheathing
Hi everyone,

I have built a large dormer (20' wide, 12' high) and planned on installing 1/2 OSB then 1/2 Tuff-R foam sheathing then housewrap and T1-11 siding. I think there is some debate about whether the housewrap is needed if the foam seams are taped - so feel free to comment on that - but my question has to do with the flashing around the gable wall and sidewalls of the dormer. The plan was to use water and ice barrier and metal flashing but I am having trouble determining if these should be attached to the OSB and covered by the foam - or if the flashing should be attached to the foam itself. The OSB is already installed and I have attached an 18" strip of water and ice to that. I have begun to install the foam with the idea that I could install another strip of water and ice (if needed) on top of the foam and then the metal flashing. I'm not really concerned if this is overkill - but I do want to know if it presents any problems. This is my first real experience with roofing and flashing so I would greatly appreciate any advice from the more experienced. Thanks.

Tom Struble 08-16-2009 11:12 AM

this is just my opinion but i like to think of the foam as a sheathing extention,if it were my my house i would install the windows and flashing on top of the foam then install housewrap

lee12 08-16-2009 11:16 AM

thanks - that was my original thinking as well. when I read the instructions for the ice and water shield though and saw that it did not list foam sheathing as one of the suitable substrates to attach to I started to wonder. I appreciate the input.

tinner666 08-17-2009 02:43 PM

Just put the step flashing to the osb. Don't add any more I&W.

lee12 08-17-2009 04:22 PM

I had assumed that the metal flashing - step flashing along the sides and solid (apron?) along the front - would be over the foam. This is because I believe that T1-11, fiber-cement and other panel siding products are supposed to be installed 1-2" above the roof line when vertical surfaces (such as the dormer walls) meet the roof. If the step flashing is behind the foam I would have to cut the foam 1-2" short as well so as not to see it. Since the whole point of the foam is to add some extra insulation it seems wrong to have a 1-2" gap in it. Here is what I think I am going to do: 1) OSB 2)I&W 3) Foam 4)Metal flashing 5)Housewrap 6)Siding. Does anyone see anything wrong with doing it this way?

lee12 08-18-2009 06:13 PM

I spoke with Dow (Super Tuff-R foam insulation) today and they said that the styrofoam WILL deteriorate with exposure to moisture so the flashing should be attached to the foam. Also spoke with GAF (WeatherWatch Ice and Water shield) - they said that the I&W should be attached to the OSB. Hmmmm.... Should I assume it doesn't matter? Is this just a matter of which manufacturer guarantee I want to void by improper installation? Or should I just continue with what I have - OSB then I&W then foam then metal flashing.

Tom Struble 08-18-2009 11:19 PM

thats how i would do it
your rigt not to have the flashing behind the foam imo

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