Plywood and Durock floor

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Old 06-10-07, 06:47 PM
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Question Plywood and Durock floor

Hi,

I am redoing a bathroom floor with the intention of tiling it once I get the subfloor done. I sistered the old joists as they were pointy on top. A friend who was helping me suggested using 3/8" plywood and then 1/2" Durock. We put down this size plywood. The floor has a lot of give to it in some places, between the joists. I'm concerned this plywood is too thin. He said we could just put down the Durock and then screw it down. A couple of websites I read about installing Durcok say to put thinset down first and then install put down the Durock and screw it down.

So, my questions are: Will this thickness of plywood be OK and Durock with or without thinset under it? I also have some 1/2" Hardi backer I planned to put on the walls to tile them. Would that be better to use on the 3/8" plywood? If the plywood is too thin, can I put another layer of plywood on top of the plywood I've already put down & what size?

Thanks.
 
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Old 06-10-07, 07:31 PM
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Your concern is valid, the plywood is too thin. You need a good grade 3/4. The concrete board is not what provides the strength in the floor, it's the floor itself. The concrete board is just there as a medium between the floor and tile to compensate for the expansion and contraction differences between tile and wood. 1/4 inch concrete board is made for floors. It's made that thin to help with transitions between tile and other floorings. 1/2 is made for walls. That's not to say 1/2 won't work, it just isn't necessary and will cause transition issues with other materials, such as carpet. It certainly won't hurt to go over what you have already put down. That would add even more strength. The potential issue is height. If the situation will tolerate the extra height, go for it. As for thinset under the concrete board, yes, it must be there. The purpose is not to glue the board down, it's to bed it and make sure there are no voids under it. Voids cause flex and movement in the floor, and tile does not tolerate movement.
 
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Old 06-10-07, 08:16 PM
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Follow up to response about thickness of plywood

Hi Smokey,

Thanks for the quick reply and I'll sleep a little better knowing how to proceed next.

Would you recommend putting another layer of 3/8" plywood down over what I have in the opposite direction that I put the first on to get up to 3/4"?

The tile I'm putting down is pretty thick and I'm going to have a higher floor in the bathroom than the wooden floor in the hall so I'll need some sort of transition.
 
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Old 06-11-07, 01:27 AM
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I don't think I'd do it that way. I've never tried that so I really don't know how it would work, but it makes me nervous. I don't believe you'll gain as much strength with two layers of 3/8 versus one layer of 3/4. The idea is to provide a solid enough surface between the floor joists to eliminate flex in the floor. It isn't just a matter of enough layers to equal a particular depth. You need the strength or your floor won't hold up and I'm just not hopeful you'll get the desired result doing what you suggest. If transitional height is an issue, remove the 3/8 and then install the 3/4. A good way to understand what I'm trying to get across here would be to span a couple two by fours with two layers of 3/8 and one layer of 3/4. Step on each individually and notice the amount of flex to them. The two layers of 3/8 will flex considerably where the 3/4 won't flex at all. If your floor joists are close enough together, you might be able to get away with 1/2 inch plywood, but that's on the edge. You're putting too much time and money into this thing to run the risk of it falling apart.
 

Last edited by DIYaddict; 06-11-07 at 10:47 AM. Reason: Removed verbage about duplicate threads/Threads have been merged
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