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Unread 02-26-2020, 09:32 PM   #1
Rman
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Thin backer board for tub surround

I have a question about using 1/4” thick tile backer board for the walls of a tub surround. I am adding 3/4” cdx plywood to the wall framing of my tub surround before installation of the tub. This will allow me to add support bars at any location, in the future, if they are ever needed, without having other blocking in place. The backer board manufacturers do not recommend using thin board on walls but I wanted to use them because I need to limit the final thickness of my tile installation because of an accessory that is mounted on the top of the tub. Is it OK to use 1/4” tile backer board on the wall since I will already have the 3/4” plywood behind it?
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Unread 02-26-2020, 09:40 PM   #2
jadnashua
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You'd need to be careful with your vapor barrier...CBU is not waterproof, but is not damaged by being wetted.

With a solid backing, similar to a floor, 1/4" stuff should work. FWIW, there is one CBU rated by the manufacturer for use on a wall without anything behind it if the studs are 16" OC or less...HardieBacker.

If your screw has a large enough gauge, it might hold well enough in 3/4" ply, but I'd prefer it to go further into blocking. Now, there are some specialty fasteners that should work in it.
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Unread 02-26-2020, 09:55 PM   #3
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Thanks. I’m thinking of using a brush on membrane for waterproofing over the backer board.
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Unread 02-27-2020, 04:03 PM   #4
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That could work. Make sure to make the board/tub junction waterproof.

Personally, with a solid backer wall, I'd consider using a 5mm (3/16") Kerdiboard panel. Waterproof the seams and the screw penetrations, and have a waterproof wall. Thinner, easier to cut, no carcinogenic dust, easier to carry home, and no risk of the cbu cracking when screwing it in place. Or, of pinholes or runs in the painted on material. A bucket of RedGard, or similar, along with the cbu and special screws starts to add up in cost, too.
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Unread 02-27-2020, 06:54 PM   #5
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1/4" hardi screwed to 3/4 ply will work.
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Unread 02-29-2020, 10:39 AM   #6
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Thank you for your replies. The 3/16" Kerdiboard may be hard to find locally. I have also been considering 1/4" Laticrete hydro board as well as 1/4" hardibacker board. The Laticrete seems to be similar to the Kerdiboard product, and may eliminate the need for a brush on membrane. I have also considered the use of a a kerdi membrane (Schluter Systems Kerdi 200 Waterproof Membrane) instead of any board. It can be bonded to the plywood with unmodified thinset material. This would be very thin and there would be no nail penetrations.

I also have a question re: how to detail the tile system at the tub flange. The flange is a factory applied piece of PVC that is bonded to the tub. The flange and the screws to hold it in add up to approx 3/16" depth beyond the plywood wall plane. It is my understanding that the board should overlap the flange, but that will not be practical with the thin board (can't notch it over the flange).
I have been thinking of stopping the board at the top of the flange and of using aluminum duct tape as a counterflashing. (see the attached photos). I like the aluminum tape because it is very thin but sticks well to the plywood and the pvc.
Attached Images
  
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Unread 02-29-2020, 10:57 AM   #7
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Quote:
Originally Posted by Ronald
It can be bonded to the plywood with unmodified thinset material.
Regardless what it is, Ronald, you cannot bond it to plywood with an un-modified thinset mortar. You must use a modified mortar meeting ANSI A118.11 standards.

And unless there's been a substantial change to Schluter's requirements, they do not recommend plywood as a suitable backing material for their Kerdi. And I'm not aware of any manufacturer of direct bonded waterproofing membranes that allow use over plywood backing in wet areas.

I personally think your plywood shower walls are a bad idea, but that's up to you. If you continue with those walls, though, you'll want to use a suitable backing material and waterproofing materials over them. I'd be inclined to agree with Jerry that 1/4" Hardibacker and an appropriate waterproofing membrane might be the better option. Any of the waterproof foam boards would likely work, too, but you'd still need to complete the waterproofing portion of the installation per the manufacturer's instructions.

I don't think the aluminum tape is a good idea in a wet area, either. If you were to stop whatever board you elect to use over your plywood above the tiling flange, you could use any of the sheet-type membranes to continue your surface waterproofing down over the tiling flange. Or you could use something like polyethylene sheeting in place of your aluminum behind a properly waterproofed wallboard, I suppose.

My opinion; worth price charged.
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Unread 02-29-2020, 11:34 AM   #8
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Kerdi can be applied over plain drywall or a cbu (and several other surfaces, none of them plywood). To make your walls compatible with Kerdi, you could install 1/4" drywall over it, then the Kerdi. TO make the seal to the tub, the best method is probably to use Kerdiband and some KerdiFix sealant/adhesive. Some use 100% silicon, but I prefer KerdiFix (a small tube is probably all you'd need, but it can be used elsewhere, so a larger tube might be useful).

A pail of Redgard is likely more than you'd need, so there'd be some waste unless you have another project coming up. Don't know how long it will last once opened.
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Unread 03-06-2020, 03:58 PM   #9
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Thanks for your replies. I saw the Schluter video showing the membrane being installed over sheetrock and I just assumed that it could be installed that way over plywood. Thanks for correcting my bad assumption. Looks like I will be using the 1/4 inch Hardi backerboard and waterproofing with either a brush on or a sheet membrane.
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Unread 03-06-2020, 04:59 PM   #10
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If you choose Kerdi, you could use 1/4" drywall over your plywood.
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