Unglazed porcelain -- Grout type? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-23-2008, 11:52 AM
I am laying 8x8 unglazed porcelain (colour goes right through the tile and it has a non-porous structure suitable for installation outside) directly onto 3/4 plywood. The plywood is screwed to the existing 3/4 subfloor for a total of ~1-1/2 inch of wood.

A couple of questions:

I think I will have to use latex modified thinset to get the unglazed porcelain to stick properly. I will plan to back-butter to ensure pretty full bonding coverage as well.

Are there any issues with what grout I choose to go with these tiles? Will any old grout stick adequately to the edges of the tiles, or will I have to use a polymer modified or epoxy one?

I left gaps between plywood panels as per instructions I've had from various places. The gaps are not supposed to get filled up with thinset. Is there any issue using foam rope to fill up the gaps to keep the mortar from going down into the gaps?


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10-23-2008, 04:44 PM
I think you may have trouble with the going directly over plywood,this takes some experience,i would use a membrane, and check your floor on the DEFLECTO above.Use a polymer modified sanded grout.

10-23-2008, 04:47 PM
I would urge you to seriously reconsider setting directly on the ply. It can be done, but the list of requirements to do it right are long and usually not achieved. It is a bad investment in most circumstances...

I would assume you will use a larger joint than 1/8", so that would mean sanded grout. IF you set on ply (and you really shouldn't :) ) I would suggest latex additive to make the grout more flexible and avoiding epoxy which is less flexible.

10-23-2008, 05:47 PM

The deflectolator says I'm golden. I'm in a hall where the spans under me are about 4 feet for most of it, with most of the joists spaced at 16, but the odd few closer.

The way I have my floor built up I'd say I'd be good for 12x12 tiles, if I wanted. The 12x12s are too big for the narrow hallway I'm in, and I like the look of the smaller tiles better.

I've put 6x6 quarries on a similar substrate before with to trouble. I used latex-modified thinset for that and it seemed to stand up. Porcelains are stronger -- are they more prone to detaching or cracking or something?

I'll have a look at my toilet flange and see if I have room for a layer of Ditra. What's the nominal thickness of Ditra with its thinset and all? Is there a comparable membrane system that adds less to the floor thickness? If I can squeek it in under the flange, I may try it out.

The joints will be 3/16ths. Polymer-modified sanded grout it is. Thanks!

10-23-2008, 05:58 PM
12x12-8x8-6x6 makes no difference, the requirements for the floor remain the same. If you prefer the aesthetic of an 8x8 that's great, but 12x12 or even larger in a hall would be the norm in my area.

Also, the joists are measured for the entire span, not just under the tile, it would be unusual to have a 4' span directly under the hall (not to say your wrong, I'm just clarifying). You can get flange extenders for your toilet, there is no need for ditra or tile to go under you flange.

10-23-2008, 06:33 PM
He's in Canada. They think this is normal to set on ply. Not sure why, but it is.
Sanded latex grout.
Ditra is 1/8" plus < 1/16" for flex.
Is CBU really that expensive up there?

10-23-2008, 06:38 PM
Welcome, gardner. :)

Let me pile on a bit on the issue of tiling directly to the plywood. It's a pretty sensitive installation method and both layers of plywood need to be correctly, and differently, installed for it to work well. You do have plenty of plywood thickness, which is in your favor, but the installation can be critical with direct bonding to the wood.

Did you do the installation on both layers of plywood and know the grades of the plywood and installation method used for each?

I think I will have to use latex modified thinset to get the unglazed porcelain to stick properly. I will plan to back-butter to ensure pretty full bonding coverage as well.The porcelain tile is less an issue than is the plywood. You must use a modified thinset meeting the requirements of ANSI A118.11 and it's preferable to use one that requires the mixing of the modifying liquid to the dry powder rather than one that mixes with water.

My opinion; worth price charged.

10-23-2008, 07:50 PM
Hi all. Thanks for thinking about my project.

The span really is 32 inches. I just measured it. The basement under this hallway has a poured foundation down one side and a 6x8 steel I-beam down the other side. The I-beam holds the second storey up (I'm working at ground level).

The bathroom at the end of the hall has poured foundations on both sides - again not over 3-feet apart. And don't get me going about that because I have no clue how the place got built with this one bathroom completely boxed in with no access from below. Frankly this is my biggest worry because any sort of pumbing problem in here will mean tearing out the tile floor.

The subfloor is 1x6 T&G which was mostly in very good shape. I screwed a 2x4 across under 6 boards to shore up one weak one (large knot) and shimmed a low spot back up. I've got 3/4 douglas fir plywood screwed down over that with screws on 6-inch pattern, 4-inch around the edge.

As far as I've read -- and I've read a few different sets of instructions -- I think I have a textbook plywood application.

The toilet flange is already installed, and it has been set for the height of the tile+thinset. It can be flexed a little bit higher and I think I could get an extra 1/8 for Ditra, if this is needed. Concrete backer would not be an option.

I expected the issue with plywood would be the expectation of dimensional instability, therefore a membrane would be wanted to de-couple the tile from the substrate. I was interested to see cx's concern being bonding to plywood. I can't see a membrane system really helping all that much on the bond.

10-23-2008, 08:05 PM
As far as I've read -- and I've read a few different sets of instructions -- I think I have a textbook plywood application.Not any textbook with which I'm familiar, Gardner (if that's not your first name, please enter one in a permanent signature line for us. UserCP/Edit Signature. How 'bout you enter it even if that is your first name.) :)

The only industry accepted method for direct bonding to plywood is to start with a layer of plywood attached properly to the joists (glue and fasteners) and a second layer of plywood properly oriented and attached only to the first layer. That provides some stability and a modicum of isolation from the joist movement.

Maybe y'all have a different standard up there?

As far as the bonding is concerned, it matters not what you are bonding to the plywood, be it a membrane or the tile, you must use an EGP rated thinset (A118.11).

You could have installed only half-inch plywood over your board floor if it was in good condition and reasonably flat, then a membrane or CBU.

Your "shimming" under the plywood layer you have, even though it's 3/4", may or may not have created unacceptably large voids, too. You generally want that plywood screwed tightly to the first layer, be it sawn boards or plywood. Any leveling at that stage should be done in the joist structure. If you want to level/flatten after the subflooring is down, you would do that under whatever membrane or on top of whatever CBU you chose to use.

My opinion; worth price charged.

10-23-2008, 08:20 PM
The height of your toilet flange is a non-issue, if you raised the finished height of your floor you could put a $3 extender on the flange; don't let that be the reason for not using CBU (you can have other reasons, just don't let the toilet be one of them).

10-23-2008, 09:09 PM
Shims are between the joist and the 1x6 T&G. One joist had a low spot in it.