The shower drain flashing eased my paranoia about moisture management just a little. It will take a waterproofing membrane to put me more at ease.

We have worked with a waterproofing membrane on the floor and walls in the basement bathroom. That experience is good to have under the belt and should facilitate the work in the 1st floor bathroom.

Dual function

There is more to a waterproofing membrane, or liner, that meets the eye at first. The one obvious benefit is about – yes indeed – moisture management! But there is a second and significant advantage of using a liner. It serves as a isolation membrane.

Think about it this way. The tiles are glued to the concrete floor. If there is some movement in the concrete floor, it will develop cracks. These may be minute cracks – hairline cracks. Still, they would propagate all the way to the tile surface.

Because these cracks keep constantly moving, replacing the cracked tiles is really no solution. The new tiles will just keep cracking too.

The liner or waterproofing membrane, which is flexible in nature, will prevent small cracks from propagating from the concrete floor to the tiles, thus the term isolation membrane.

Prioritizing the functions

The concrete floor in the basement bathroom sits on rigid insulation and a sturdy gravel base. It has a very low potential for cracking. The priority there was on the waterproofing function of a liner. The isolation effect was a welcome added benefit. That allowed us to use the relatively thin Kerdi liner by Schluter that was easy to handle.

The concrete floor in the 1st floor bathroom is a very different beast. It sits in between and on top of floor joists. These will expand and contract depending on the temperature and humidity. Plus they react with some level of deflection to the load that is put on them.

Now picture the concrete floor in this assembly. This has high crack potential written over it. The priority here is to isolating the tiles from the floor, with the waterproofing as the added benefit. For this case we opted for the heavy duty 0.8 mm NobelSeal TS membrane.

Liner installation

As with the floor drain flashing, we again use beads of rubber sealant (NobelSealant 150) to waterproof at the overlap of the liner and around the floor drains. We also got nifty corner pieces that we sealed in place with the same product.

Because the walls of the bathroom have a low potential for cracks to develop, we switched back to a lighter gauge membrane, the Noble WallSeal. We place the membrane on the those walls that are exposed to water–behind the lavatory and the toilet, and of course in the shower stall.

Now we are almost ready for the tiles and a handy gadget that goes along with them. More of that in the next post.