Basement Bathroom Renovation: Waterproofing

I’ve done a lot of renovating over the last 6 years, and there are two things that scare me: structural work and anything that can cause water damage. So I really wanted to make sure I got the waterproofing right so I didn’t cause any problems in the future. And I guess only time will tell if I do a good job.

I watched several videos on how to build showers from scratch and found the Kerdi membrane to be the best option for what I was doing. I also got a couple of other accessories, including kerdi band (for the seams) and corner pieces for my shower ledge and window sill. I also got some Kerdi-Fix to seal around the faucets since the Kerdi valve seals didn’t work for my setup.

With the backerboard in place, I was ready to apply the membrane to the entire shower area and prep for tile. I found several installation videos and watched them over and over again. But basically you use an unmodified thinset, apply it to the walls, and apply the waterproofing membrane like a wallpaper.

Since some of my cuts were complex, I took time to cut the membrane before I started installation. Once the pieces were cut, I mixed the thinset and let it sit for 10 minutes. Meanwhile, I wet the wall so the thinset wouldn’t dry up too quickly during the membrane installation. Then, I applied the thinset, notched it with a 1/8″ x 1/8″ trowel and set the membrane into the thinset by using a large drywall putty knife. I peeled back the membrane to make sure I had decent coverage on the back and added a bit more thinset where it didn’t feel adequate.

I then moved on to the hardest wall: the exterior window wall. My pieces were easier to manage because I installed them horizontally, but I had the shower ledge to deal with, as well as the window, which wasn’t easy. I followed the same process as the first piece (wet the backerboard, apply the thinset, notch, place membrane and smooth out).

Once all the main membrane pieces were on, it was time to waterproof the seams with the Kerdi band and corners. It wasn’t as easy or pretty as it is in the videos I watched, but I got my 2″ overlap and called it a day. I eventually sanded down some thinset clumps when it dried to help reduce any build up. When you have multiple layers overlapping, the thin membrane sheets start to create a bit of thickness to the wall.

After the waterproofing was done, I caulked the bottom of the shower using regular silicone caulk (which was part of the shower base installation step) to basically waterproof the shower base.

Finally, I used Kerdi-Fix around the faucet valves to waterproof between the valve and the membrane. I’m not sure the Kerdi-Fix is much different from regular caulk. It goes on and feels exactly the same. And at $30/tube for such a tiny amount used, it seems like a rip-off.