OK to tile under new tub? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-12-2006, 10:52 AM
Hi all -

Is it reliable to set a tub on tile?

I would like to replace our crumbling bath floor tile this year, and next year repace the 29 inch wide tub with a 32 inch wide, acrylic, deeper so heavier when full. The long side would be on the tile. Probably most of the weight would be on built-up subfloor.

subfloor: 2x8, 16 inch centers, 8 ft span, 1/2 " nom. ply. will add another 1/2" ply which worked well in kitchen, same span and stuff.

I last used these forums so long ago, I forgot the previous id. So, not a newbie and thank you for the suggestions that enabled us to do the kitchen floor so well - porcelain over Ditra (wonderful stuff) with proper strengthening of sub floor. Even got a little Felker saw out of the effort. :) Floor and saw like new after 3 years.

Thanks so much,

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10-12-2006, 11:48 AM
Welcome back, Doug. :)

I see no reason you shouldn't do that. Well, except it would be easier to do it all at once, of course.

My opinion; worth price charged.

10-12-2006, 01:27 PM
Thanks much, CX. Agreed it would be easier all at once, but thems the breaks.

11-20-2006, 08:18 PM
The current bathroom 1/2 inch plywood subfloor had a soft spot, so I scraped it down to solid ply. About 4 x 10 inches by 1/16 to 1/8 deep was removed. The rest is solid. I will top it with another 1/2 inch ply. I really do not want to replace it down to the joists.

Is it sufficiently safe to fill the low spot with something, then screw and glue the top plywood? What to use for the filler? Thinset? Fiberous Bondo epoxy? Nothing?

Thanks much,

11-20-2006, 10:55 PM
Hi Doug, if you are going over it with more ply, you can use some construction adhesive and smooth it out then put your ply over the old. Are you adding Ditra or cbu to this?

11-21-2006, 02:33 AM
mike is right. I find it best to "smooth" it with a small 1/8" v notch trowel. But straight liquid nails is too firm to do this with. I like titebond subfloor adhesive cause it is in just a slightly more liquid state. It comes out of the gun easier and can be troweled. I would give a good squirt in the hole you describe and screed over it with the trowel. The glue gets rock solid overnight.

I would recommend at least 5/8" on top of that if you are not using backer board. The minimum spec for the high dollar thinset rated for plywood is 1 1/8" combined thickness.

Good luck

11-22-2006, 07:16 AM
Construction adhesive sounds wonderful. I did not know it set hard. I will use the titebond because it is easier to screed.

Should I leave the trowled adhesive exposed to set up overnight, or just screw and glue the additonal ply over and let it all harden at once?

We have not yet applied the additonal ply, so if you guys think thicker is needed, we will give away the 1/2 inch ply and get 5/8? What say you?

Here are all the support details. The build-up plan is the same as the kitchen floor, which has held up quite well for at least 2 years and falls within the range of the deflect-o-lator. But the kitchen floor has very strong porcelain tile.

small floor area, 5 ft x 8 ft

2x8 Joists 16 OC, 7 ft span
(wall in bsmt, otherwise 10 ft span to bsmt midline)

1/2 in existing ply subfloor
1/2 in exterior B-C screwed and glued
Ditra membrane with Kerdi membrane over any joints
Suntouch warming mat
12 in sq ceramic tile of reasonable quality and strength

Thanks so much for you expertise in these forums,


11-22-2006, 09:13 AM
Hi Doug, you can put the ply down as soon as you spread the construction adhesive. I would use 5/8" ply also.

11-22-2006, 11:10 AM
you can put the ply down as soon as you spread the construction adhesive.

I would use 5/8" ply also.

Oh poo! Back to home-deposit-your-paycheck. Better to do it right then do it over. At least I have good 1/2 inch templates for the 5/8 ply :)



11-22-2006, 10:40 PM
I combined you with your previous thread, Doug. Please keep all the questions for the project here so we can keep up with the history. Bookmark it in your favorites so you can always find it. :)

While I disagree with some of the advice given, you probably can't hurt much trying to put down the 5/8ths ply to the existing damaged half-inch subflooring. I would strongly suggest, though, that you pre-drill the new layer before trying to screw it down. You're gonna find that all you're gonna get if you don't pre-drill is a lot of "screw jacking" and very little in the way of attachment of that thin bottom layer to the new wood. You're likely to strip some of the screws anyway, but pre-drilling will give you a much better chance at a good installation.

And since you're gonna use a decoupling membrane, we won't need to get into the discussion of installing your tiles directly to the plywood.

One thing on your list of steps that must be changed is the order of installation of the Ditra. Ditra must go on top of everything but the tile bonding mortar and tile, no matter what else you've installed on the floor.

My opinion; worth price charged.

11-27-2006, 11:54 AM
must be changed is the order of installation of the Ditra. Ditra must go on top of everything but the tile bonding mortar and tile, no matter what else you've installed on the floor.

Oh, good to know. With that info, let's see if I have the layers correct.
From top to bottom:

1) ceramic tile
2) unmodified thinset
3) Ditra membrane
4) modified thinset
5) Suntouch warming mat
6) 5/8 ply, clearance holes to prevent screw-jacking, then screwed and glued
7) titebond to fill 4 x 10 x 1/8 inch spot of ply (questionable, but may be OK)
8) 1/2 ply original subfloor
9) Joists 2x8 in, 24 in OC, 7 ft span

How could it be made more reliable? Use pour-stone or other thin cement instead of Titebond wood glue? Bite the bullet and pull the weakened subfloor? That bit of subfloor extends a little under the tub, and I have resolved the source of water leakage.