Tile is not waterproof. This surprises a lot of homeowners, but it's true. Grout is porous, and even well-sealed grout allows moisture to migrate over time. In a shower or tub surround, that moisture has to go somewhere. If the substrate behind the tile isn't properly waterproofed, it goes into the framing. In the Pacific Northwest, where homes stay damp for much of the year, that's a problem that compounds quietly for years before it becomes visible.

What happens when waterproofing fails

The most common failure mode is a shower that was tiled over cement board without a continuous waterproof membrane. Cement board is water-resistant, not waterproof. It won't dissolve when wet, but it allows moisture to pass through. Over time, that moisture reaches the wood framing behind it. We've opened up shower walls in Bellingham homes and found framing that was structurally compromised after ten or fifteen years of slow moisture infiltration. The tile looked fine from the outside. The repair cost was significant.

The Schluter Kerdi system

Schluter Kerdi is a polyethylene membrane with fleece webbing on both sides that bonds to the substrate and creates a continuous waterproof barrier behind the tile. It's applied to the walls and floor of the shower area before any tile goes down. The seams are overlapped and sealed with Kerdi-Band, and the corners and transitions get specific Kerdi corner pieces. When it's installed correctly, the waterproofing is continuous. There are no gaps at the transitions between wall and floor, which is where most failures start.

Why we use it on every project, not just expensive ones

We made Schluter Kerdi a standard practice on all wet areas in 2018, after seeing enough failed showers to understand that the cost of doing it right upfront is much lower than the cost of a repair five years later. It adds roughly $400 to $700 to a typical shower installation depending on size. We include it as a line item in every bathroom estimate so homeowners can see exactly what they're paying for. We've never had a homeowner ask us to remove it from the scope.

What to ask any contractor about waterproofing

If you're getting quotes from multiple contractors for a bathroom renovation, ask specifically what waterproofing system they use and where they apply it. 'We use cement board' is not an answer to that question. A contractor who can't explain their waterproofing approach in specific terms is worth being cautious about. The materials cost difference between a properly waterproofed shower and a poorly waterproofed one is small. The difference in long-term outcome is not.

Waterproofing is one of those things that's invisible when it's done right and very visible when it's done wrong. We'd rather talk about it upfront than have a conversation about it after the fact. If you have questions about a bathroom project, reach out and we're happy to walk through the specifics.