tiling onto plywood or OSB?


Can I stick ceramic (floor) tiles directly onto plywood or OSB? I'm a little tight for clearance on a potential project (at least if I use 3/4" for the subfloor, and I worry that anything less will flex too much and crack the tiles*) so it'd be nice not to have to use backerboard if I don't have to.
* same with the underlying frame; I'd rather use 2x6" than 2x4" even though the latter would give me lots more height to work with.
cheers
Jules
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Jules Richardson wrote:

google is your friend
http://www.google.com/search?q Ên+i+tile+on+plywood
2nd link: http://www.johnbridge.com/vbulletin/archive/index.php/t-537.html
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On Mar 11, 3:30 pm, Jules Richardson

You can. But it's hard to keep the flex down enough with just 3/4 ply or osb on 16" centers. That's part of why you add backer board. If it's a bath then the backer board also gives you extra water protection.
You mean 2x6 floor joists? What are you spanning with those? Again sounds like you might have a flex issue. Floor need to be rock solid for tile or your grout will be cracking out.
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On Thu, 11 Mar 2010 12:45:33 -0800, jamesgangnc wrote:

Yes, that's what I was worried about; I'd seen comments before that it can be done, but the bond isn't as good and the pros hate it - but I figured some group wisdom couldn't hurt :-)

It's not as bad as it sounds - I'm just doing this to raise the floor level in our back porch so that it's level with every other room in the house (and will give enough height to eventually build a deck outside). The existing porch floor is concrete slab, so I can support the floor joists at quite small intervals (I'm thinking every 2' would probably do); I'm not expecting them to span several feet or anything.
Total height difference between concrete floor and the floor in the adjacent room is 7" (minimum, it drops another 1/8" or so in places due to slight variations in the underlying slab). I'd lose 5-1/2" of that to the joists and 3/4" to the OSB or ply layer, and I want a 1/4" air gap beneath (and probably plastic sheet as moisture barrier just to be safe, although the room's dry and the concrete doesn't appear to sweat).
Ceramic tiles on top of all that will probably be 1/4", which gives me 1/4" left for the adhesive layer - in other words, I'd really need to magic up another 1/4" - 1/2" to accomodate backerboard too (unless I use thinner OSB / Ply, but then I'd be worrying about flex issues again)
The only other option* would be to go to something less than 2x6" for the joists - which might be OK, given that it can be regularly supported by the slab below, but it still "feels wrong" :-)
* other than just using plain old vinyl tiles rather than ceramic ones, which would be a last-resort solution (I wouldn't want 'em in the front porch, but as it's a back room usually inhabited by the pets it wouldn't be the end of the world or anything)
cheers
Jules
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On Mar 11, 4:49 pm, Jules Richardson

You need to use pt lumber where it contacts the concrete. I would frame directly on top of the concrete. A plastic moisture barrier is fine if you beleive there is not one under the concrete. It is common these days to have a moisture barrier under concrete but if yours is older that might not be the case. I would still use pt lumber even with the plastic. If you use 2x6 that is really 5 1/2 so you are left with 1 1/2 inches. A 3/4 ply plus another 1/2" or so for tile and mortar brings you close. I assume there is a door transom between the two rooms so exact matching is not needed. I would frame 12" on center instead of 16" to place the tile directly on the 3/4 subfloor. 2x6's are cheap. Insulate to keep the floor from being cold. You can use regular motar to adhere tile to plywood. Upgrade a bit to get one of the poly blends but you don't need to go whole hog with the top dollar stuff.
I tiled directly to ply in our sunroom 7 years ago and have not had a single problem. I used 1/2 ply on top of the 3/4 subfloor. On big rooms it is a lot of work to put down backer board. Ply is bigger and goes down faster. Use construction adhesive between layers of ply and a bunch of screws.
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On Fri, 12 Mar 2010 06:40:34 -0800, jamesgangnc wrote:

Yeah, good call on that - I think when I roughly priced things up last year it was for 2x6" PT (decided to wait until the weather got warmer - I didn't feel like fitting a new exterior door in -30 temps :-)

I can't see an obvious one, but the concrete doesn't seem to allow anything through - but I don't believe it's expensive to lay one down, so I may as well just in case.

Yep, it's pretty tight. I'm not sure what common practice is for framing direct onto concrete, given that there's some very slight variation in the concrete surface; do folk normally put a little cement down (and fit the frame before it's dry) to pick up the slack, or it is possible to buy some kind of flat shim for that purpose? I think I'm looking at about 1/8" max variation in the concrete height, so need something if I'm going to put the frame down directly on top of it.

The porch must have been added much later, so there's actually what would be once been the rear step for the house between the two rooms. That means I can be out height-wise by 1/4" or so without it looking (or feeling) strange, although obviously I'd like to get the heights between the two as close as I can get.

Yeah, good idea. I was going to go with 16" only because I have some 16" insulation rolls already, but I've got a later project in mind that can always use those, so I wouldn't be wasting anything.
The porch isn't big at all, around 7x8', so it's not going to be a particularly expensive job.
thanks for the tips!
Jules
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On Mar 12, 11:23 am, Jules Richardson

I'm not sure what the best answer is for leveling your concrete. Generally 1/8" is not much variation and it might not matter. They make leveling compounds that probably adhere to concrete. I'm not sure about using motar as it would be thin and you would be putting a lot of pressure on small areas where the 2x6's sit.
You could put some strips of pt 2x4 wide side down along the inside perimeter, nail them to the frame and the powder nail them to the concrete. Are you framing walls on top of this floor frame? That will put some weight down on it. Will you be able to attach the edge adjacent to the house to existing floor framing?
I would build it all and make sure it had all settled before tiling.
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OK, so you want to raise your porch floor about 6" or so. A couple comments:
With support every 2', a 2x4 is more than adequate for the job. [For a distributed load, this has the equivalent angular deflection to, for example, a 10.5" tall member spanning 6'.] You could also just continuously support the joist on the slab.
For rigidity of the subfloor, go with thicker plywood, like 1 1/8". This is over three times as rigid as 3/4" plywood (1.5^3). This saves you from the trouble of having to laminate multiple layers of thinner plywood. Then use 1/4" backerboard just as a tile base, not to stiffen the floor.
So your buildup is then: 3.5" joist, 1 1/8" plywood, 1/4" backerboard, and say 1/2" for tile and thinset. That makes 5 3/8" height. If you want it higher, you can screw some 1x PT blocks to the underside of the 2x4s every foot or so (use stainless steels screws), providing an airgap. That would get you to 6 1/8" height. Or you could just use 2x6 PT lumber and rip it to whatever width you want, just be sure to put the ripped edge up, since the treatment doesn't always fully penetrate the member.
Hope this helps.
Cheers, Wayne
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Jules Richardson wrote:

Disregarding the thickness/stiffness/flex...
Time was when tile was often (generally?) stuck to plywood with mastic; my kitchen counter tiles were, 14 years ago. I've also seen mastic used to stick them to concrete.
You could also use thickened epoxy (messy & expensive) or plain old acrylic caulk.
--

dadiOH
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