Is it required to have a Waterproof Membrane under a Floor Tile Installation in a Mud Room, Laudry Room or in a Bathroom floor


We are doing a whole house renovation. While doing a walkthrough, I noticed the installer had used 1/2” interior plywood underpayment over the sub-floor. He indicated he was covering the plywood with a mat that allows for expansion and contraction without cracking. Great, I thought. After the matting was installed I asked if it was a waterproof layer. It is not! So, I have no waterproofing between the tile and the plywood substrate. The tile will be installed in a mud room, laundry room and bathrooms. Is this an installation compliant with commonly accepted installation standards? Are there building codes or other references that prescribe installation standards regarding underpayment and waterproofing?


ANSWER - It depends on what type of mat was used over the plywood on whether it is an acceptable installation method.  Although it would be ideal to install a waterproof membrane in areas that would be subjected to residual moisture such as the mud room, laundry room and bathroom floors it isn't required.  It is optional.  It is required to have a waterproof membrane in a shower.

Installing over a plywood underlayment isn't the best method in the first place.  Wood floors tend to have excessive deflection and they are moisture sensitive to some degree.  It is better to go over a backer board or mortar bed that is more moisture resistant and more stable.   There is a installation method of a uncoupling mat over a plywood underlayment.  The mat is moisture resistant, and is only waterproof if they include other steps to make it continuously waterproof. So it is better than bonding directly to the plywood underlayment.

2 thoughts on “Is it required to have a Waterproof Membrane under a Floor Tile Installation in a Mud Room, Laudry Room or in a Bathroom floor

  1. Doug blake says:

    I’ve got plywood floors with concrete board on top wanting to put in a shower do I need to put a water barrier down before the tile and if so what is best

    • Donato Pompo says:

      There are two things that are important. One is you have to waterproof the floor (shower pan surface) and up the walls. You have to install a drain and the waterproof surface of the floor has to be sloped to the drain at least 1/4″ per foot.

      If you want a curbless shower floor you can purchase the kits for lowering the floors so they are flush with the floor joists. You then can put a trench drain at the back wall and have the shower floor sloped to the drain.

      You can also purchase one of the extrude high-strength Styrofoam prefab shower flowers that are presloped and designed to incorporate the trench drain in the floor.

      If you go with a traditional shower pan floor then you install at shower dam that is about 6″ tall. You put at least 12″ tall backing along the shower walls at the studs for the waterproof membrane to go against. You have to place your two part area drain or a trench drain and plumbing.

      You have to pre-slope the the shower floor with metal lath and mortar over a building paper cleavage membrane. You then apply your waterproof membrane that is either a hot mop application or a sheet membrane and clamp it into the two part drain and cover the weep holes in the drain with either pea gravel or a plastic weep hole protector. If you have building paper on the wall as a moisture barrier then it should lap over the shower waterproof membrane. You then float your mortar bed over that with wire reinforcement suspended in the mortar bed by maintaining the slope to drain.

      They you apply a secondary waterproof membrane over the cured mortar bed and this membrane extend up the walls for full protection.

      Make sure you install movement joints at all the changes of planes.

      So depending on what you want, you can see there is a lot of planning and work that needs to be done.

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