Waterproofing a Window in a Tiled Shower

We want to add a window in the tiled shower enclosure of our master bath, but we’re worried about water damage. How can we add a window in a way that will protect the window and the exterior wall from moisture problems?

— Paul Kreitler, Columbus, OH

Tom Meehan, owner of Cape Cod Tile Works: A window in a tiled tub or shower enclosure is common, but you are smart to be concerned about the potential for moisture damage. There are a few different ways to approach this project. I like to continue the tile into the window jamb as if it’s part of the wall after I’ve added a pitched sill.

In these types of situations, I encourage homeowners to use vinyl-clad windows housed in a wooden jamb. The vinyl provides maintenance-free protection from moisture for the window itself. The all-wood jamb provides a flat and sound base. Make sure that you locate the window outside the range of the spray from the showerhead.

As with all tiled shower installations, run cementboard throughout the shower area and on the same plane as the window jamb. Caulk the joint between the jamb and the cementboard with a good-quality silicone caulk. Although the caulk provides an initial seal, it should not be relied on as the only defense against water infiltration through this joint. To seal the joint completely and to cover the exposed wood of the window jamb, wrap a waterproof membrane such as Schluter’s Kerdi (www.schluter.com) over the joint and into the window jamb. Bond the membrane to the wood with a latex-modified thinset.

I like to install a pitched sill in these windows to allow water to drain back into the shower. In lieu of wood, I use a piece of Corian, which I install with latex-modifed thinset. The simplest way to pitch the sill is to build up the thinset under the back part of the Corian; mixing it a little stiff will make it easier to work here. Once the sill is in place, I install tile over the entire area in the same manner as I do the rest of the shower.

When the tile is complete and the grout is sealed, use a good silicone caulk where any two planes come together, such as where the window meets the newly tiled jamb.

Remember that the window is on an exterior wall, so temperature changes will cause expansion and contraction in many of these materials. Check the area every year for cracked joints; caulk or grout as needed.

More on shower details:

Tiling Bathroom Walls: Windows and Niches – In this video, Tom Meehan reviews the steps for waterproofing a window sill with Kerdi-Board.

Curbless Shower: Build Up, Not Down – Contractor Jake Bruton describes his technique for building a curbless shower on top of an existing subfloor, simplifying the traditional approach to this job.