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Region: Santa Cruz, California, USA

Do subfloors in bathrooms require any special sealant or moisture barrier prior to adding standard floor coverings (like tile)?

I know walls need a covering and special drywall but I wasn't sure about sub-floors (since there's already going to be something over it.

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I think a quality bathroom installation includes waterproofing and an isolation membrane. I'm a fan of Schluter's Ditra underlayment, but there are other good options out there. A typical installation has 1/4" backer board glued down with thinset and screwed down, the isolation membrane installed with thinset (and taped if necessary), and then the tile installed on top of that. A membrane like Ditra serves both as waterproofing and as an isolation membrane; it lets the subfloor move slightly without transferring the movement to the tile, preventing cracking. Make sure to read the installation instructions (found on the manufacturer's website) carefully; with Ditra, for example, you use latex-modified thinset in some cases and normal thinset in others.

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No that isn't needed. You'll likely have 2 more layers over the top of the sub-floor, backer and tile. Done right that seals things up nice and tight.

If you are talking about your local building codes your best bet would be to call a builder/plumber and ask.

  • Thanks. It was more of a general question so you answered it. – Mike B Sep 5 '14 at 19:49
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The only time I have ever heard of an inspector even talking about a bathroom floor is if there is another dwelling below you (basement apartment being rented out). There are no codes or laws for a normal one family dwelling. I am sure certain towns may have something for apartment buildings and such but the answer is no.

Should you use one? Seriously debatable. If you have a shower open to your entire bathroom or if what is below your bathroom is worth a lot then yes. So for example if under you bathroom was you 100K kitchen... then yes. But these systems are too expensive and laborious for your average bathroom. Do you really expect someone to pay an extra $500 for a membrane in a small bathroom?

However tile plus baseboard plus caulking under baseboard can hold a lot of water.

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