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.

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