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Old 12-06-2008, 07:42 PM   #1
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subfloor for upstairs bathroon

I had a leak in my toilet water connection in an upstairs bathroom. After 15 years of two kids taking showers i knew the subfloor is bad. So I broke out the existing ceramic tile and am down to the subfloor. I think I have a 5/8 inch OSB covered by a 5/8 inch plywood.

I am going to take out the top layer of 5/8 inch plywood. If the OSB is not bad I would like to leave it in and replace the 5/8 inch plywood with new stuff. My question is this.

1. How should I attache the new 5/8 inch plywood to the 5/8 inch OSB. using screws? on the joists only? between the joists? what length screws?

2. Before laying new tile. Should I use the 1/4 inch backerboard or the mitra backer. I think that is what it is called.

thanks for you advice


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Old 12-06-2008, 09:50 PM   #2
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subfloor for upstairs bathroon

Hi Larry,

1. Verify the subfloor is OSB and not particle board. If it is PB, take it to the curb next garbage day. Re screw the old subfloor, (OSB?), then install underlayment grade plywood It's best to screw the new underlayment to the subfloor only, use 1 1/4"- 1 1/2" max. screws. If the OSB seems to hold the screw well, move on to the next step. Use course thread screws btw. Stagger the seams, leave 1/8" between sheets and 1/4" or more at perimeter.

2. You can use either 1/4" tile backer board or a membrane such as Ditra. I prefer Ditra, but you may not be able to buy Ditra in small amounts? Ditra is installed using a modified thinset over plywood, then the tiles are installed using unmodified thinset. If you go with backer board you install it into thinset and fasten per directions. Then tape and thinset all seams.

Please come back here when you decide which way to go. You might have more questions?



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