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best underlayment for bathroom stone/tile?

11 years ago

We've recently noticed a crack running about 30 inches diagonally across the stone floor in our master bathroom.
Near the crack, there's another spot that seems to flex (and I can hear a faint squeak) if I step on the wrong spot. We hope to have this bathroom remodeled soon and want to be sure that the new floor is properly supported. It's on the first floor, above an unfinished basement level garage. My guess is that the large stone tiles were laid directly on the wood subfloor with no reinforcement or membrane used.

We'd like to have radiant floor heat installed during the remodel. What's the best way to insulate the subfloor so that the bathroom floor is warmed and heat isn't lost to the garage below? What should we expect the contractor to do to reduce the flex in the subfloor? Finally, we're thinking about going from stone tiles that are 18 in. to something much smaller. Can we do that and still have it look sufficiently upscale/on trend for future buyers? We don't know if we'll be here for just a few years or much longer, but need the house to be appropriate for its price.

Thanks.

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Comments (6)

weedyacres
11 years ago
To support tile, you need deflection to be at least L/360 for tile and L/720 for stone. This measurement is a combination of joist size, spacing, and span. Google "deflectolator" for a calculator that will calculate yours. If the rating is insufficient, you can add or sister joists to reduce deflection.

Then you need underlayment. Tile needs 1 layer of plywood (OSB is ok); stone needs 2. Then the substrate: Ditra is best, as it's a decoupling membrane, plus way easier to install than cement board.

We just put fiberglass batts in our bathroom floor/garage ceiling, and we have radiant floors.

As for the upscale look, it's not just about the size of the tiles, it's about material and layout. So come on over to Bathrooms when you're planning your materials, and we can give you feedback on your choices.
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gildedcage
Original Author
11 years ago
Thanks, Weedyacres. That deflectolator was an eye opener. Our joists are definitely inadequate for the current stone floor according to that calculation. We'll get them beefed up even if we don't do the entire remodeling project.

It's good to learn that your fiberglass batts did the job. Dh put them in my garage and they do help.
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brickeyee
11 years ago
Make sure you have enough support (sob floor) between the joists.

"1 layer of plywood" is likely to be inadequate.

Think more like at east one solid inch of sub floor, and cement board underlayment does NOT count.

This post was edited by brickeyee on Thu, Mar 28, 13 at 11:22
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gildedcage
Original Author
11 years ago
Brickeyee, since this is a remodel and not a new build, how would we (or our contractor) deal with the resulting difference in floor heights if we add, for example, 1/2 inch plywood on top of the existing subfloor? The closets and adjoining hall have hardwood floors.

If we beef up the joists with both sistering them and then do blocking between and/or metal straps beneath the joists, would that remove the need for a second layer of plywood on top?

Whatever we do, I'll have to find authoritative websites from which I can print out explanations or instructions to show the contractors. I'm pretty sure what we have now is the norm around here.

Thanks.
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weedyacres
11 years ago
You only need the 2nd layer of plywood if you're doing natural stone. It's not needed for porcelain or ceramic.
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brickeyee
11 years ago
"how would we (or our contractor) deal with the resulting difference in floor heights if we add, for example, 1/2 inch plywood on top of the existing subfloor? "

You install a saddle in the door.

I have never seen a house that had inadequate joists fr stone but adequate sub-floor.

They generally go together (inadequate joists AND sub-flooring).
Probably 'value engineered' to save a buck at some point.

This post was edited by brickeyee on Mon, Apr 1, 13 at 15:51
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