Two layers of cement board?

[Brandon in VA]

I'm remodeling a bathroom and the tub installation instructions recommend adding 1/4 furring strips on the studs down to the tub lip prior to installing 1/2 inch cement board that overlaps the lip. If I add the furring strips the thickness of the wall wouldn't line up with the rest of the room. What I'd considered doing in lieu of the furring strips is to put a short section of 1/4" cement board (maybe 12" high) as my "furring" above the front and back of the tub, then layer on another 1/4" cement board on top of that which is slightly longer (maybe 13") that would overlap the lip. The wall above the 1/4" cement board sandwich would be 1/2" thickness as would the entire back wall, which I could fur out without affecting the continuity of the wall thickness since it runs corner to corner. I suspect two 1/4" boards are flimsier than one 1/2" so I would nail blockings in behind for additional stability for tiling. Are there any issues with using two pieces attached together for this application, and should I put anything in between the two sheets? If not, anyone have another recommendation?

 

[Jadnashua]

Is this tub also being used as a shower? If it is, drywall has no part in the walls, at least up to the height of the showerhead unless covered by an approved waterproofing membrane (the only two I'm aware of that have been certified for use like that are Kerdi and HydroBan sheet).

You could use KerdiBoard from www.schluter.com, and make a small rabbit joint to fit over the tub flange. Seal it to the flange, and the entire wall would be waterproof and is designed for tile application. KerdiBoard is an extruded polystyrene sheet, with special tileable sheets bonded to both sides.

 

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[Brandon in VA]

Is this tub also being used as a shower? If it is, drywall has no part in the walls, at least up to the height of the showerhead unless covered by an approved waterproofing membrane (the only two I'm aware of that have been certified for use like that are Kerdi and HydroBan sheet).

You could use KerdiBoard from www.schluter.com, and make a small rabbit joint to fit over the tub flange. Seal it to the flange, and the entire wall would be waterproof and is designed for tile application. KerdiBoard is an extruded polystyrene sheet, with special tileable sheets bonded to both sides.

The tub will be used as a shower, and I had no plans for drywall in the wet areas in and around the tub surround. KerdiBoard looks like a nice product but it is nearly 10x as expensive as cement board so I don't think it's a reasonable option for this project. I had thought about a rabbet on a 1/2" piece of cement board but was concerned it might crumble so didn't know if that would be a good option.

 

[Jadnashua]

You can't reliably put a rabbit joint on CBU. You need to understand that CBU is not waterproof, it's just not damaged by being wet and is a good interface to bond tile to. By the time you buy the special screws, then tape the seams, and the fact that cbu can wick moisture and is not waterproof, KerdiBoard doesn't look as expensive - plus, it's super easy to cut and is easy to carry. Your choice, your house, either can work.

Depending on the size of the tile, you don't need to run the cbu down over the flange...as long as more than half of the tile will be on the cbu, you can end the cbu slightly above the flange. If you go that route, you DO want to install a vapor barrier behind the cbu, and to run it down over the flange to direct any possible moisture back into the tub verses the wall cavity.

 

[Brandon in VA]

OK, got it. I do understand the CBU isn't waterproof. I was planning to waterproof the CBU with redgard and put a 4mil vapor barrier behind the CBU to address moisture. I do like the fact that the Kerdi-board is relatively easy to lug around and cut though, esp. since I'm doing this without help. If I went with KerdiBoard with a rabbet, is it safe to assume there is no need for a vapor barrier behind it and onto the flange as long as the joint between the Kerdi and the flange is properly sealed with silicone? I'll be tiling with 3 x 6" subway and the flange is 1 1/4" (I would tile to leave 1/4" for silicone sealant) so I may be pushing it if I leave the backing above the flange.

 

[Jadnashua]

Both external layers of KerdiBoard are waterproof, as is the extruded foam core, so no, you do not need or want any further waterproofing behind it. You do need to seal the seams with Kerdiband, and you should use their screws and washers, but you don't need nearly as many as you do with cement board (only about at 12" intervals, and you can use one screw between two sheets to anchor both on the edges because of the size of the washer). You seal any penetrations or seams with the Kerdiband. You can also use it between the tub and the bottom of the board. There are numerous videos on their website that show how to do this. They recommend KerdiFix, but silicon can be used to seal the bottom to the tub. KerdiFix is neat stuff, VERY strong, slightly flexible (not nearly as much as silicon), and while not an issue here, cures under water or onto wet surfaces. The make seals for the shower arm penetration and around the shower valve, and the tub spout...it is a well though out system that works. If you want to add a niche, they also come in numerous sizes premade, but you can make your own if you have extra board material and some Kerdifix. That would let you make them any size verses sticking to one of their stock ones.

www.johnbridge.com whole site is devoted to tiling things. This place is great for plumbing and other issues, but they are better for tiling. Spend some time on the Schluter website and view their KerdiBoard tub/shower series to get a good idea of what it is, and how to use it, then decide. You probably will only remodel once, amortize the costs over 20-30 years or more, and it's all in the noise. It will save you time, and perform better. CBU works, but it's old school.