Wait…What? Building science and a custom shower? Yep! As some of you may know from my Instagram feed, I can’t stay busy performing energy audits, energy assessments and building investigations in my area. I still do some hands-on work. A lot of the construction projects I’m involved with are custom tile work. I was fortunate to have the opportunity to work with a customer who trusted me when I told them I could build a wood shower. This shower works because of what I’ve learned in building science.
This project is unique for me in many ways. I rarely get to do the framing for any shower. I had the opportunity to not only choose the design, but also the framing material for the project. Tiling on crooked walls can be challenging so starting off with very straight framing materials is important. I choose LSL, or laminated strand lumber framing. LSL’s are basically a very thick piece of OSB, or oriented strand board sheathing cut into framing lumber sizes. Very straight and strong. They are also very hard, nailing through the products doesn’t work well so we screwed the framing together.
I use Schluter Systems tiling products to waterproof my shower assemblies. This system is a sealed system. The ceilings (in some cases), walls, pitched floor and drain assembly are all sealed with a membrane that keeps any products behind the system along with the framing dry. The waterproof membrane or fleece, which is orange, is stuck to the surface behind with thin-set mortar. After all the membrane, called Kerdi, is installed, the shower could be used without any finish material, and it would not leak.
Because the entire system is waterproof, Schluter recommends the surface material behind the membrane to be standard drywall. I have been using this system for several years and have yet to have a failure.
After all waterproofing is completed, I tested the system by flooding the shower pan (called a flood test) to confirm it does not leak. I do this by plugging the drain and dumping 10-15 gallons of water into the shower pan. I wait 24 hours and look for any leaks. I have yet to have a Schluter shower pan system fail a flood test.
Next, I installed tile to all the areas that were to be tiled. This included three walls and the floor. The ceiling and one wall are the only areas that received wood. The tiles that were chosen were a black 12 x 24 porcelain tile and a black pebble tile for the floor.
The round red things on the walls are a tile leveling system I use to keep all the tiles flat.
After the tiles were laid, next I installed a furring strip to attach the wood to the ceiling and wall. This is where the building science comes in. I wanted the wood used in the shower to have the opportunity to dry on all sides. I chose a system that we typically use for siding, a vented rain screen. By attaching a 3/4-inch board to the ceiling and wall, any moisture that should get behind the wood would be able to freely drain to the floor where it would end up in the shower drain.
I used a product called Kerdi Fix, which is Schluter Systems adhesive used to bond and seal materials to the waterproofing membrane. I used both Kerdi Fix and screws on the ceiling where I wasn’t concerned with the screw holes that poked through the membrane. The walls could not have any penetrations in the membrane below 78 inches, which could potentially cause a leak into the wall. This prevented any screws from being installed below the 78 inch point. In this case, I installed one screw at the ceiling and used the Kerdi Fix to attach the wall 1 x 4’s to the membrane below the screw near the ceiling. The 1 x 4 wall boards were held in place with the pressure of a few spring loaded poles extended to an adjacent wall until the Kerdi Fix cured.
After all the nailers were installed, I next needed to turn the waterproofing membrane black. My plan was to install the wood as an open cladding system, meaning there would be gaps between the boards. If the membrane was to remain orange, this color would be seen between the gaps. I chose to use thin-set, which is a concrete adhesive used for tiling, and dye it black. The thin-set was then troweled smooth over the membrane.
Next was the installation of the wood. Of course I couldn’t use just any wood. The wood needed to be resistant to moisture. I chose a thermally modified ash. (Arbor Wood is a local to my market source). Thermally modified wood is treated with heat, pressure and some moisture to change the wood at the molecular level. Though expensive, this type of wood can be used as siding, decking or in other exterior installs. Being very stable and able to be wet, it was the best choice I found for a shower. The wood is very hard and smells like smoke when cut. I did test a piece by placing it in my own shower for a couple months. It was set on end in the shower, which I discovered allowed some end grain absorption of water. Because of this, I installed all the wood horizontally with a gap between the end grain and any surface and treated the end grain with a product called Anchorseal End Sealer, which is a hybrid wax and polymer just to make sure there was no issues with moisture moving into the end grain.
The thermally modified ash comes the color shown in the photo. There is no finish on the product. The wood was nailed to the 1 x 4’s painted black with a stainless steel 18 gauge nail. The nail was 1 1/2 inch long and was installed at an angle so not to penetrate the Schluter waterproofing membrane. Both the walls and ceiling where installed in the same manner.
The shower features two wall to wall recessed niches. The upper niche is used for hair products and soap storage, the lower niche is used as a foothold to shave legs. A black granite was used for the sill of the niches and for the top of the curb which wraps up and over the top of the knee wall. The shower also feature an frameless shower door and a piece of matching glass at the top of a knee wall. We also used a mono-pitch shower pan which drains into a linear drain.
This shower was challenging, fun to build and has become my favorite. A very satisfying installation using building science in a different capacity.
I revisited this project 5 years after completion, the shower still looks great without any signs of failure. The only issue was one small stain on the wood, probably from some sort of hair product or soap. I would build another if given the chance.
(This post was updated on 6/29/2024)
Bad idea…it will promote mold..behind the wood