Shower window sill treatment [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-24-2017, 11:27 AM
Hello all,

First time posting here. I have what I think is a relatively common issue in older homes that have had a bathroom updated with a modern shower. MANY houses in my area had a double-hung wooden window in the bathroom that became part of the shower area when a shower was added. When I bought my house I removed the old rotten window and put in glass block, flush with the existing tile surface. This solution worked for us for 20 years.

I'm now remodeling the bathroom. I've gutted it, and torn out the glass block and replaced it with a much nicer, clad, obscure picture window option. I'm still using the existing wood window frame. I have cement board installed up to the frame, and I need to figure out the best solution to waterproofing the sill/recessed wood area and the exposed edge.

I found a FineHomebuilding article that describes a solution for this exact thing. I will try to attach the image. But, basically it describes wrapping the sill edge with a "waterproofing membrane", which I took to mean something like Kerdi-Band. I emailed Schluter about this and of course they won't touch it because they say none of the Kerdi products should be applied over wood, nor should I ever use modified thinset.

Any thoughts/comments/suggestion on this?

PS - My original question for Schluter was related to modified vs unmodified thinset. They basically won't even help me with that question. My take is that I can use modified as indicated in this image because the substrate is wood and cement board.

Thanks for your help!


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08-24-2017, 11:37 AM
Welcome, Chris. :)

Be better to have a photo of what you're working with. And Fine Homebuilding pretty frequently gets it wrong when it comes to their ceramic tile articles and your photo is no exception. You want the waterproofing layer sloped to drain, not just the tile surface as shown in their drawing.

1. Of what material is the frame of your new window?

2. Can you install a CBU over your wood sill and jambs for that window?

3. What waterproofing material are you using for the other walls of your shower?

Let's start with that.

08-24-2017, 11:49 AM
Thanks, CX.

I don't have a pic handy, but the scenario is exactly what is show in the image I posted. So, picture a vinyl window set into a wooden jamb, with cement board installed right up to the jamb.

1. Of what material is the frame of your new window?

* The actual window is an Anderson vinyl-like product - 100 series. The original jamb is 1920's wood.

2. Can you install a CBU over your wood sill and jambs for that window?

* I could in theory put strips of cement board on the sill surface, but not the edge of the jamb itself. At least not without changing the thickness of the wall.

3. What waterproofing material are you using for the other walls of your shower?

* I have 6mil vapor barrier all the way around the tub, siliconed over the front of the tub flange, then cement board.

08-24-2017, 12:36 PM
Can you give a few pictures Chris? Our concern is this wooden window jam you speak of. Exposed wood has no place in a shower. When it comes to windows in showers vinyl is the best product and your waterproofing should mate up and seal against that vinyl frame. The tile should then meet the vinyl with silicone at the tile to window grout line. There should be no wood visible anywhere.

08-24-2017, 01:13 PM
Yep, I understand exactly what you're saying. I don't want any exposed wood, which is why I'm trying to implement what is in the picture I uploaded. I want to cover the wood jamb/sill with Kerdi, wrapping it over the seam to the cement board, and then tile over it and silicone to the window. Exactly as shown.

I'll try to capture some pics. I'm not at home at the moment. :)

08-24-2017, 01:18 PM
Here is a pic I had on my phone from before the cement board went up. Pretend the cement board is covering the vapor barrier. It runs right up to the jamb, and the surface is flush with the edge of the jamb board. So the edge of the wood is flush with the surface and needs to be covered. Again, it's exactly as shown in the graphic I uploaded earlier. That's why I was so latched onto that solution - it's perfect for my situation.

08-24-2017, 01:21 PM
PS - as I mentioned previously, the window itself is a vinyl product. The wood is the jamb, and that's what I want to cover with membrane and tile and silicone to the vinyl window.

08-24-2017, 01:34 PM
While it won't be correct, that might be your best bet at this point, Chris. But you must first do something to make your waterproof membrane slope to drain on that sill. Your window sill is in a very, very bad location for a shower installation and it's gonna see a whole gaggle of water, which you want pointed to the tub. Doesn't take much slope for that short span, but you gotta have some.

How deep is the exposed sill? Can't really tell in your photo.

I would recommend you bond your Kerdi Band or similar to your window sash and to your sill and jambs with Kerdi Fix or similar pookey. Still not per any manufacturer's requirements, but better than the alternatives.

My opinion; worth price charged.

08-24-2017, 01:44 PM
The exposed sill is about 2 inches.

My plan was to bond the Kerdi to the sill and over the edge using modified thinset.

Are you suggesting to also wrap it up onto the window a little and then silicone it down to the vinyl? In my mind the extra complication of that might make it harder to ensure a good seal, as opposed to using silicone directly where the window meets the jamb and membrane. Just thinking through the implementation of that and worried I might mess it up! :D

Thanks to everyone listening!

08-24-2017, 04:37 PM
You don't even want to think about bonding anything to sawn boards with thinset mortar.

I'm not suggesting silicone for anything. I'm recommending Kerdi Fix or a similar product, such as maybe some SikaFlex from your local home center. But I'd be more comfortable with Kerdi Fix. Good pookey, that, and I know it will bond to both your wood sill and your vinyl window.
In my mind the extra complication of that might make it harder to ensure a good seal...Yep, might be a little more difficult, but I bet nobody promised you this would be easy. If someone did, she is not your friend. :)

My opinion; worth price charged.

08-25-2017, 09:50 AM
If I understand what you're looking for - this might work: PVC molding
I used this to DIY myself a coupl eof permanent, all-vinyl windows & frames in the shower.

Instead of tiling the exposed wood and trying to somehow waterproof it (that is going to be a prime wet spot), you may be able to cover it with something like this. Use a waterproof / polyurethane adhesive and of course silicone the heck out of it.

This one is 1/4 thick by 1 3/4 wide, shown at the bottom (plus i used another style around the side)
There may be other sizes you can rip to size

Its white and paintable and cuts like butter.