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Leak where window bottom meets trim

rdesigns | Posted in Construction Techniques on June 11, 2010 10:13am

In the process of removing old lap siding, I see evidence of repeated wetting of the OSB underneath. No structural damage yet.

It has been leaking in at the horizontal joint where the bottom of the vinly window meets  the horiozontal trim. The joint was tight, and it appears that the thin line of caulk opened enough to make a capillary space. Once water makes it past there, it creeps along to behind the trim and then down past the window’s flange and past any stick-on flashing until it wets the OSB.

It would be worse, except that the climate here in SE Idaho is dry–10″ of rain/year. For that reason, the code does not require house wrap or felt under the siding, and even if wrap of some kind  were installed and detailed right, a leak in the same place would wet the back side of the new siding. (LP Smartside)

If the bottom of the vinly frame (Millguard casement) had a drip edge, that would keep the water on the outside, but that’s not one of my options now.

My current best solution is to space the new trim piece away from the vinyl about 3/16″ to allow for a wider caulking bead, but I hate to rely solely on caulking for this.

Any better ideas?

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  1. DanH | Jun 11, 2010 10:20am | #1

    Well, the caulking gap should be 3/16" anyway, so that's a good start.

    You might try gluing on a drip edge, or forming one from caulk.  All you need is a drop of 1/8" or so from the horizontal surface of the vinyl in order to keep the water from running horizontally.

    Or you can just use this technique: http://news.bbc.co.uk/2/hi/science/nature/4955398.stm

  2. davidmeiland | Jun 11, 2010 10:40am | #2

    Better ideas?

    Use housewrap or felt, even though the code doesn't require it. You are using the OSB as the drainage plane and as you see it means the OSB gets wet.

    Some sort of sloped and projecting sill would be an improvement. I'm assuming you have a flat piece of 1x nailed to the wall under the window as the apron, and that there's no slope on the top of that piece. If it's raining and the wind isn't blowing, water running off the window will run down to the slope, out away from the face of the wall, and drip down, instead of capillating back into the wall directly under the window. When the wind blows, it will still go back into the wall. 

    I suppose you could install a drip cap flashing UNDER the sill trim. Water that gets behind the trim runs down the flange, down the peel-n-stick, and gets kicked out by the flashing, which has a leg out directly under the trim.

    Really, felt or wrap is your only good option, and yes, the back of the siding will get wet sometimes, unless you install it over a rainscreen detail of some sort.

  3. DanH | Jun 20, 2010 10:18am | #3

    See if you can find a piece of vinyl (like an edge trimmed from vinyl siding) and glue it on with silicone caulk.  You don't need much -- just a bump.

  4. User avater
    Matt | Jun 21, 2010 07:41am | #4

    First, I don't like caulk gaps larger than 1/8"

    after that, it starts getting to the point where you need backer rod.  Using the most flexable (and paintable) caulk you can find is the next thing.  I like one called OSI Quad, partially because of it's readily available from blowes.  I think it is polyeurethane based, and it is fairly hard to work with.  Another one I use is S/W Sher Max wiich is much easier to work with as it is a acrilic latex based, and I have found it to be superior to other acrylic latex caulks re flexability and resistance to cracking.

    Here is my main point though.  We use a flaskhing detail that works like this:  The window opening is prepaired by applying 12" wide black poly sheeting to the bottom, so that basically about 10" of poly is hanging out on the outside. The poly could be wider - say like 18" depending on how your siding courses out and how wide (if any) your window trim is. Now, install window with caulk, roofing nails through the flange, and flashing tape per normal.  Then when installing the siding, the first peice of siding below the window that is in the area of the black poly is installed under the poly.  Then the next piece of lap siding which goes directly below the window (or could be below the window trim), is installed on top of the black poly.   So the black poly is now hanging out of the siding - usually an inch or more comming out between the laps in the siding.  Cut off the excess with a razor knife.  Now any water that does make it behind the siding around the window area is channeled out via the black plastic.  Hope that make ssense.  I thought I had a pic but can't find it right now.

  5. rdesigns | Jun 21, 2010 10:02am | #5

    Matt, wish I could use your method,

    it would be really effective, but since this is a re-siding job, all the windows are already installed with their flanges adhesive-caulked to the OSB. I'm afraid I would destroy the flanges in the process.

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