Working on a bathroom remodel project and found rot and water damage to the plywood subfloor around the shower area. The shower is 36" x 36", and the joists are 24" apart and his part of the subfloor actually spans from the bedroom through this bathroom and into the next room. Was thinking of cutting out damage, adding some perpendicular supports and laying down new 3/4". Would this be correct? We're putting down a new acrylic shower pan and will tile the rest of the floor. Would I need a vapor barrier with the new subfloor too?

  • How big is the floor joist?How much of it was damaged?What type of joist is it?Is it a regular 2x10 or a manfactured beam?
    – mikes
    Mar 25, 2012 at 10:44
  • Believe joists are 2x10. Haven't pulled up the floor yet. You can clearly see water stains / damage around the drain. But they is rot in the corner where the pan Ls have been.
    – gnome
    Mar 26, 2012 at 1:25

1 Answer 1


You got the right idea, but I'd opt for adding a double 2X? between the existing joists at either end of the bathroom span and fill in with an extra joist making your spacing 12 inches on center instead of 24. Add a couple of perpendicular stiffeners, and you will have a good stable base for your new 3/4" sub-flooring. Tile needs a good solid base or your grout will crack every time.

The vapor barrier is not so important on the floor, however, if you are doing new walls, a properly installed vapor barrier is very important. There are several methods of adding a vapor barrier, depending on what types of tile backer you plan to use. Think of it as a system and get all the right layers on in the right order.

  • I'd like to +1 the "you need a VERY solid base or your tile will crack" -- you cannot fix that with additional subflooring or by putting down panels of cement board backing. You MUST reinforce the structure underneath the subfloor. Mar 26, 2012 at 7:42

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