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Old 04-05-2011, 09:22 PM   #1
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How to install a Second subfloor?

I have a 5/8" T&G plywood Sub floor in my kitchen. I want to put 18" porcelain tile on it. I re-screwed the 5/8" ply but i was told i need 1 1/8" total sub floor before i install the tile and cement board. I plan on putting down 1/2" ply over the 5/8" ply for structure.
1. should i butt 1/2"x4x8 sheets of ply tight end to end and side to side?
2. Should i glue them and nail only through the 1/2" ply and 5/8" ply and the joist? What pattern?
3. Can i put 1/2" cement board on the 5/8" ply instead of 1/2"ply or should i put 1/4" fiberboard on top of the 1/2" ply?
I thank you for any suggestions.


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Old 04-05-2011, 09:55 PM   #2
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The little I know, leave a gap between sheets of 1/2 plywood, 1/8". This is what i was told, for expansion so the tile won't heave.

I put tar/felt paper between the 5/8 and the 1/2 to stop any squeeks and I used the paper to shim the floor flat.

I didn't glue sheet to sheet.

I screwed 3" through 1/2, 5/8 to floor joists and used 1 1/2 screws in between the floor joists, 8" of the joists. This is probably overkill.

I don't know if this was correct, thought it was, but would like to hear opinions.


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Old 04-05-2011, 10:59 PM   #3
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Hi all,

I have a 5/8" T&G plywood Sub floor in my kitchen.
5/8" is considered good enough in the industry. Scares the heck otta me though. If it was 3/4" I'd say good. So go ahead and add an underlayment. The more the better.

i was told i need 1 1/8" total sub floor before i install the tile and cement board

As I said above, the more the better. But, the 1 1/8" suggestion is if you're gonna tile direct to the plywood.

1. No, leave 1/8" gap. Check with the thin set manufacturer cuz some want the gap filled when installing the concrete backer, some may not.

2. Re-nail or screw the 5/8" subfloor first. Fasten the underlayment to the subfloor only, not the joists. Use 1 1/2" screws or ring shank nails. No cheapo drywall screws. Underlayment should NOT be glued to the subfloor. Pattern? for 1/2" 4-5" on perimeter, 5-6" in field. Of course the sheets go across the joists, same as the subfloor, but offset both ways.

3. I recommend adding the ply, then 1/4" tile backer or if you wanna go premium go with Ditra.

TILE GUY - retired- TROY, MI - Method & Product suitability consulting.

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