Window Flashing and T1-11 Siding
delson
| Posted in General Questions on
what is the best way to install and flash a window into T1-11 siding?
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Delson,
Is this a new build or a window retrofit? That may change answers. Though I've never done it on T1-11 myself, I've read that many install windows directly to the framing before installing the siding, along with the WRB and flash it that way.
If it's a retrofit, I'm not sure of the steps involved to make it reliably water tight in a way that doesn't depend on some type of sealant, unless you say use an oscillating tool to cut a narrow groove in above the window so that you can slip the flashing in behind the sheathing.
New build, shed/workshop, no wrb. If installing before/behind the T1-11, how do I protect the top edge of the T1-11 and in between the window flange and the T1-11, below/at the bottom of the window?
Normally you would seal the edges, with paint and primer, if it were siding. I'm assuming you're going to finish the T1-11 in some way, right? As well as trimming out the window? Leaving it bare is a no go. You would apply the finish to the cut edges of the T1-11 at the top, just above the z flashing, leaving a gap of around 3/8" between the two. To flash the bottom, you'd need to a wide piece of tape up to the front face of the bottom window flange, with the backing still attached to the part that's not stuck to the window. Then when the siding is installed over the window, bring that out to the front, stick it to the front face of the T1-11, and trim over top of it. These kind of difficulties are what always made me reconsider T1-11. as a structural panel and finish.
And if installing over the T1-11, how to best flash over the top flange, considering the grooves and that there's no continuous overlapping water repelling going up.
I'm thinking if a horizontal cut in the T1-11, just above the window flange to insert a z flashing would work..
If your window is tucked up pretty far beneath the eaves, It's unlikely that it's going to experience significant wetting, fortunately. It's the 1st story windows on a mult-story house, or house without overhangs that tend to get into trouble.
"To flash the bottom, you'd need to a wide piece of tape up to the front face of the bottom window flange, with the backing still attached to the part that's not stuck to the window. Then when the siding is installed over the window, bring that out to the front, stick it to the front face of the T1-11"
That is exactly what I was planning to do, however i realized that this would only prevent water from getting in at the bottom, which I'm not really concerned about, it is any water getting in above, and ending up behind the T1-11 below the window that I'm trying to prevent.
since I'm sloping the roof only one way, like a lean-to shed, only one side will be protected by the eave.
So I was wondering how to do this with comparable protection to most other installations?
Am I the first one to try and get this right...
Naw, there's plenty of others out there who have discussed this before. Try googling it with a few different terms, I see a lot of hits on my initial attempts. I think that's where the recommendation for Tyvek on the studs comes into play, though I'm not sure where it goes from there, since it would have to drain towards the bottom somewhere.
If I were being honest, I don't think it's a good product for walls that have windows, even for outbuildings. It always rots towards the bottom after a short few years, and It's just a lot of fuss for what it is, aesthetically.
Delson, the first house I helped build, 30-ish years ago, had T1-11 siding and we installed the flanged windows directly on the framing and cut the T1-11 somewhat loosely around the window, and filled the gap with caulking. I don't know that there is really a better way to do it. It's not a good way, because using a single material for sheathing, siding and WRB is not a good way to build. But if you're committed to doing so, just use a high-quality sealant at the gap and keep an eye out for leaks.
Delson,
As Kyle and Michael have said: it really depends on how long you want this to last and how important it is the walls stay dry.
The spectrum runs from:
- Installing the window on the surface of the siding, caulking the flanges and covering them with trim.
to:
- Installing a sill-pan on the framing. Installing the window in the RO, wrapping the framing in a WRB, sliding in a head-flashing, taping the window, installing furring to form a rain-screen cavity, and putting up the T1-11.
With as many variations in between as you can think up. Unfortunately there is no "right" way to install T1-11.
What I've done is put a piece of trim that's thicker than the siding above the opening, then a drip cap on the trim and then 18" of WRB above the drip cap just stapled to the framing. Then WRB below the window until the bottom of the siding or the next horizontal break. Basically like a normal window with sheathing/WRB/siding, except only for the width of the window and imaginary sheathing.
Right now T1-11 is $45 a sheet, or $1.41 per square foot. OSB in 7/16 is $18, or $.56 a square foot, and Tyvek is $0.15 a square foot. For an extra $0.71, or $2.11 instead of $1.41, you could put OSB and Tyvek underneath the T1-11 and detail it like a normal wall, and save all the trouble. I realize it's 50% more but it's short money if you want the building to last.
In an exercise of "what if?" Suppose you ripped down 3 studs by 7/16" and rabbeted the two on the ourside, installed one sheet in every window slot with regular 7/16, flashed and all, and then T1-11 over that? Lot of work, but it would solve the flashing problem cheaper than sheathing the whole wall?
Possibly. Or just put a piece of 2x3 instead of the rabbet. Or instead of ripping, just inset the studs by 7/16", depending on how your interior was finished. Or if not every wall has windows or doors, just doing an OSB layer on the walls that have an opening. But you get the idea.
Great feedback, thank you all.
At this point, having all the materials and started the build, what I will likely do is make about a 70 degree (pointing down) cut through the T1-11 just above the window flange, spanning the final width of the side flashing tape, taping flashing behind the T1-11 through the slit, taped over the window flashing and side tape.
I can't picture exactly what you're describing, but you want the flashing over the window to be able to weep any water that gets behind the siding. So you want the flashing to be open at the bottom. Think of how horizontal joints in the t1-11 are done, with z-flashing. Basically if there was a leak anywhere in the wall above it would run down the back of the siding and then get redirected to the front, and not down the inside of the window. You want the flashing to have enough of a lip on the outside that wind-driven rain can't get between it and the top of the window, and enough of a vertical on the inner side that if wind-driven rain gets in there it flows right back out.