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Old 02-20-2011, 08:18 AM   #1
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Can I tile over old plywood with no gaps?


I am preparing to install cement board and tile over my 25-year old subfloor. It consists of a layer of 1/2" plywood over 5/8" plywood.

I am concerned because I see no gaps between the top plywood sheets. All the sheets are tight fitting.

Is there a chance the plywood will expand and break the tile or grout?

What can I do?
- Is it best to heavily screw down the plywood to the joists? (say ever 10"along the joists with a 2 or 2 1/2" galvanized deck screws)? I assume this will limit expansion or buckling of the plywood.

- Is it better to screw down and install backer board and tile in the summer when humidity is the highest and the plywood is more expanded?

- Perhaps I could saw cut a groove between the top plywood sheets to allow for expansion (but of course the bottom layer could still be tight fitting?

Am I worring over nothing?

The floor system is comprised of 2x10's at 16" spacing over a 12' span.
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Old 02-20-2011, 12:39 PM   #2
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Can I tile over old plywood with no gaps?


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I am preparing to install cement board and tile over my 25-year old subfloor. It consists of a layer of 1/2" plywood over 5/8" plywood.
I am concerned because I see no gaps between the top plywood sheets. All the sheets are tight fitting.
Is there a chance the plywood will expand and break the tile or grout?
I doubt you have any real issues. This is done all the time and when there are gaps they just get filled with thinset mortar anyway. No one really caulks any gaps the way they should. I wouldn't worry about it.

Quote:
- Is it best to heavily screw down the plywood to the joists? (say ever 10"along the joists with a 2 or 2 1/2" galvanized deck screws)? I assume this will limit expansion or buckling of the plywood.
No! The top layer in a case like this should never be fastened to the joists, only fastened to the layer below.

Quote:
- Is it better to screw down and install backer board and tile in the summer when humidity is the highest and the plywood is more expanded?
WOW! Are you overthinking this or what?

Quote:
- Perhaps I could saw cut a groove between the top plywood sheets to allow for expansion (but of course the bottom layer could still be tight fitting?
See above comment.

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Old 02-20-2011, 07:58 PM   #3
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Can I tile over old plywood with no gaps?


Well the existing second layer of plywood already has long nails into the joists below. Should I remove those nails and only fasten to the layer underneath?

What is the harm in nailing the second layer to the joists? Won't it just cause the two layers to act more as one thicker layer rather than two independent layers?

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Old 02-21-2011, 02:01 PM   #4
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Can I tile over old plywood with no gaps?


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What is the harm in nailing the second layer to the joists? Won't it just cause the two layers to act more as one thicker layer rather than two independent layers?
I can't argue your point. The industry recommendation is to nail a second layer only to the first layer intentionally missing the joists. The theory is this will help to further isolate the tile installation from the structure. Beats me how that is supposed to work but that's the recommended technique.

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Well the existing second layer of plywood already has long nails into the joists below. Should I remove those nails and only fasten to the layer underneath?
I personally wouldn't sweat it at this point. My imagination tells me that even if you were installing the second layer new now, you would still inadvertently hit some floor joists anyway.

I would say - Move On.
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Old 02-21-2011, 02:58 PM   #5
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Can I tile over old plywood with no gaps?


The theory about deliberately missing joists with the second layer is that the relatively sparse nailing between layer one and layer two, if it misses joists, allows layer two to move a little bit, since the nails can bend slightly. In practice, plywood (but not OSB, particleboard, or solid lumber) moves very little with humidity changes. Hence the gap, or lack thereof, is not significant if you have plywood. Totally different story with OSB and particleboard, if moisture seeps downward into OSB or particleboard, it expands significantly, and can easily buckle. Solid wood subfloor (I have 3/4 inch pine boards laid diagonally) is relatively common, however solid wood always expands/contracts across the grain, not with the grain, unlike particleboard which expands both directions equally. You need a gap between solid wood subfloor pieces.

All that said, you may want to consider installing ditramat or similar between the subfloor and the tile. It acts as a bond breaker and a waterproofing medium. If you use ditramat, you can skip the cement board. See the Schluter website (www.schluter.com) for more details on installation. Ditramat comes out to about $2 per square foot, which is somewhat more than cement board, but adds less thickness to the total installation. I used it in my kitchen, and was very happy with the result.
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Old 02-21-2011, 03:52 PM   #6
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Can I tile over old plywood with no gaps?


TWO POINTS:

1. Particleboard is strictly forbidden anywhere in any tile installation.

2. DITRA can not be installed over structural plank or slat sub flooring without first installing a layer of OSB or plywood, preferably exterior plywood of an underlayment grade.


I think we are back peddling.

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