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Unread 02-24-2021, 08:53 PM   #1
Lou_MA
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Why not tile stair risers right over ply?

I’m tiling some interior stair risers, each one sheathed in a single piece of 3/4” exterior glue plywood. Riser width is 33”, set over 3 stringers.

I had planned on setting directly to ply with a Laticrete 317 unmodified / 333 admix combo. But I see many of the forum recommend cbu first.

Why?

CBU doesn’t offer any structural strength, just provides a stable bonding surface that bonds well to thinset. Well, ply is extremely stable and the 317/333 combo has a tenacious grip to ply. So it seems like cbu doesn’t offer any benefit that the ply or thinset selection doesn’t address
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Unread 02-24-2021, 10:28 PM   #2
CaliGrown
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CX will have your industry approved answer.

We’re it my job I’d use a peel ‘n stick anti fracture membrane over the wood surface. Green skin, if not a primer then skim coat your 317/333 combo then tile. Maybe epoxy adhesive, or a sikaflex adhesive for direct bond to wood. so it gives you some play with movement..

Purely opinion, no warranted advice given
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Unread 02-24-2021, 11:24 PM   #3
cx
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Actually, I'm not sure that's the case, Christopher.

There is certainly no doubt that the 317/333 mortar will bond to the plywood just fine. Something would be destroyed in removing the tile after a week.

Small area such as that with essentially no load? I think I'd expect it to hold up just fine and I'm at a loss to technically qualify an answer either way.

Better to add a mechanically fastened CBU? Maybe, but then you're looking at a lack of CBU manufacturer's instructions for the application. We just had a similar project in the Advice forum recently wherein the plywood was installed over solid dimension lumber and my recommendation was for a CBU if there was room. But would a direct bond to the plywood with something like the 317/333 been just as good? I dunno.

I think this falls in the category of just because it ain't in the ANSI standards or product manufacturer's instructions don't necessarily mean it won't work, eh?

Maybe Gobis will stop by and 'splain me the error in my ways.
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Unread 02-25-2021, 12:44 AM   #4
Lou_MA
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It’s interesting.

My curiosity was piqued reading Donato Pompo’s opinion that CBU is needed because it provides a better bond to the tile than direct set to plywood. If bonding is the main issue, I’d have no problem relying on the 317/333 combo.

In any case, I’m leaving an open joint around the entire perimeter that’ll get siliconed.

https://ctasc.com/expert-answers/rec...ircase-risers/
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Last edited by Lou_MA; 02-25-2021 at 12:54 AM.
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Unread 02-25-2021, 02:31 AM   #5
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Thanks CX!!! I too value Gobis’ word as the bond for anything Tile!!
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Unread 02-25-2021, 09:11 AM   #6
speed51133
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some treads and risers have a considerable amount of flex, especially older homes. Verify yours do not.
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Unread 02-25-2021, 11:02 AM   #7
Dave Gobis
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Stairs always make me cringe. 317 w/333 will stick to anything and destroy several layers of wood veneer on removal. I have used a powder liquid combo many times in the case of "I really shouldn't do this" and thus far have been fine. Have done it in my own house 26 years ago and all is well. I would use backer over dimensional lumber but not plywood in this instance. That is my non-industry approved comment. Anything that has bounce will either break or pop the tile. That is the biggie in stairs. And, the bigger the tile the greater the risk.
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Unread 02-25-2021, 05:14 PM   #8
Lou_MA
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Thanks Dave

Quote:
I would use backer over dimensional lumber but not plywood in this instance
So direct set to plywood (no cbu) would be acceptable to you?


Or are you saying you wouldn’t use plywood at all?
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Unread 02-26-2021, 09:28 AM   #9
John Bridge
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As long as the face grain of the 3/4 in. plywood is horizontal, I'd go with it.
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Unread 03-16-2021, 11:29 AM   #10
duneslider
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I have done this a number of times and I have always nailed a little chicken wire or lath over it first then then tiled away. Right or wrong, that's how I have always done it and haven't had a problem yet. I don't usually have 317/333 on the truck. We also don't always see plywood on risers here, usually an engineered product or dimensional lumber. 317/333 sticks pretty good.
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Unread 03-16-2021, 12:11 PM   #11
Davy
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I did these back in 95. The builder wanted a 1/2 inch of lath and mud so that's what we did. After the mud, the carpenter set the treads leaving me just the right size for the tiles. I also left a caulk joint all the way around each riser.
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Unread 03-16-2021, 01:45 PM   #12
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Interesting, Davy....I think I like it
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