2pt question: reducing overall stack height of underlayment & stronger plywood [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-19-2010, 04:12 PM

I'm looking for advice on getting the proper product for the underlayment on my main floor.

We are replacing approx. 1000sq. ft of existing tile. The current installation is 12x12 porcelain tile on 1/2" plywood underlayment, over 5/8 T&G OSB subfloor. the subfloor is glued and screwed(I can see the glue from the basement)
Joists are solid pine, 2x10, 16" O.C., 13' max span. the house was built in 1996 in Ottawa, Ontario, Canada

The current installation has failed at multiple places where large cracks across multiple tiles are seen. no new cracks have former over the years, but these cracks have been there as long as I remember(7 years) Of course the grout is cracking at multiple places as well.

Now we are at the point where we are going to replace the entire floor. This floor goes into the kitchen, hallway, mudroom/laundry room, front entrance/foyer.

The installer advised he would use 1/2" ply over the OSB, but hasn't looked at the specific details I know to look for. He stated he would use whatever products I wanted. Now I know from reading this site, it all depends on the type of tile being used.

We have not chosen the tile yet, but we'll say stone as I think it would need the most re-enforcement.

I'm trying to avoid increasing the overall height of the floor.

Rather than 1/2" ply, ditra, thinset, tile, I remember reading about a plywood that is 3/8" or 3/16" but is as strong as 1/2".

This would allow the overall height to be reduced or not increased by a large amount.

Can anyone advise what this special underlayment is? I searched and came up with Halex 3/8"m but it stated that it is only to replace 3/8" and is not suitable for 1/2" substitution.

Any help is appreciated



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02-19-2010, 06:56 PM
Hey Eric, welcome to the forum!

Not sure what underlayment you are speaking of but I'm sure someone around here will know.

Just wanted you to know that you aren't being ignored. :D Hang tight.

02-19-2010, 06:59 PM
Thanks Roger,

I know I've read about it on here. I just can't find the name...lol


Jim wood
02-19-2010, 07:07 PM
I ran your numbers thru the Deflecto and it calculates a 458 which is fine.
Why do you need a second layer of plywood? I Don't see a problem just using Ditra over your subfloor.

Brian in San Diego
02-19-2010, 07:11 PM

The beefiest subfloor/underlayment in the world is going to help you if your joist structure isn't up to snuff. With "as new" materials your joist structure is only at L/458 and you need L/720 for stone. Was the porcelain laid directly to the underlayment without a tile substrate like hardiebacker? Unless the plywood used for your subfloor has face grades less than "C" I seriously doubt that is the cause of your problem. You have 1 1/8 " of subfloor/underlayment combo..that is essentially good enough for stone (although the preference would be for 3/4" and 3/8" rather then 5/8" and 1/2".

There's something else going on that is causing your problems IMHO.

If you do want to install stone you have some serious sistering to do before that will fly. If you sistered everything with like sized (2x10) sisters then you would be at a deflection of L/916.


02-19-2010, 09:27 PM
yes the current tiles are adhered directly to the plywood.

I think stone will be out of the question as it is impossible to sister anything. there is a ton of mechanical/venting in the joists.

We'll most likely be looking at a large 16x16 or slightly larger tile.

I wonder if the initial failure of the floor is due to the house settling over time. the cracked tiles seem to be parallel with the joists.

I want to keep the overall thickness low as we just had new hardwood installed. The HW was installed with a 3/16" PLy under to bring up the overall thickness to match the existing tile. existing tile and subfloor is 1.5"(measured from a heating register)

what I was hoping to use was 3/8" ply(with the strength of 1/2"-I know it's out there), ditra 1/8", then thinset and tile.

so overall 5/8" (subfloor) + 3/8"(ply) + 1/8"(Ditra) + 1/8"approx(thinset) + tile(5/16" uptp 3/8") Total= 1 5/8"

I just need to get my hands on that special plywood.



02-19-2010, 09:46 PM
I'd just use a decent 3/8" ply unless the existing subfloor is all torn up by removing fasteners. With wood floors, it's easy to make a transition out of matching wood to compensate for the differences in height, which wouldn't be much anyways. 1/2" ply would be better, but with the Ditra you should be fine.

To successfully tile directly onto plywood, you need two layers, properly offset and fastened, and a high quality, modified thinset. If you don't, you'll have problems.

Your biggest problem will be getting the screws to hold in the 5/8" ply. Make sure to use a driver with an adjustable torque stop.

Brian in San Diego
02-19-2010, 10:47 PM
yes the current tiles are adhered directly to the plywood.Therein lies your problem. You do not have to go to any extremes here, Eric. Schluter says the minimum subfloor thickness over 16" centers is 5/8". I would like more than that but if you have 5/8" and add 3/8" you will have PLENTY of subfloor/underlayment to have a successful ceramic tile installation over Ditra. The problem with your existing floor is that it was more than likely installed improperly. Installing directly over plywood is tricky business and most pros would not attempt it.