Updated October 2026 · By Bencke’s Tile
Why tile and grout do not keep water out
Tile itself can be very dense. The problem is the joints. Every few inches there is a line of grout, and standard cement grout lets water through slowly. The thinset under the tile does the same. Sealing the grout helps with stains, but a sealer does not make a shower waterproof.
So some water always gets behind the tile. In a well-built shower, that water meets a waterproof layer and runs down to the drain. In a shower without one, it soaks the backer board and the wood framing, and you find out years later.
The layers, from the room to the studs
A tile shower wall is a stack. This is the order, from the part you see to the part you do not.
| Layer | What it does | What to know |
|---|---|---|
| Grout | Fills the joints between tiles | Cement grout is porous and can crack. It is not the waterproofing. |
| Tile | The surface you picked | Dense, but only as watertight as its joints. |
| Thinset | Bonds the tile to the layer behind it | Needs full contact with the tile back, with no hollow spots. |
| Waterproof membrane | Stops water that gets past the tile | This is the layer that keeps the wall dry. |
| Backer board | A flat, firm base for the membrane and tile | Cement board, fiber-cement or foam board. Not plain drywall. |
| Wall framing | The studs and plates behind it all | The part you most want to keep dry. |
The backer board
Behind shower tile you want a board that stays solid when wet. Cement board and fiber-cement board do that. Foam boards made for showers do too. Plain drywall and paper-faced green board do not belong behind shower tile, and the residential building code calls for cement, fiber-cement or glass-mat backers in tub and shower areas.
Cement board will not rot, but water can still move through it. The tile industry standard that covers it, ANSI A118.9, covers the board itself and does not make it waterproof. Treat the board as a base, and treat the membrane as the waterproofing.
The membrane: sheet, liquid or foam board
There are three common ways to waterproof. A sheet membrane is a thin plastic-like sheet bonded to the wall with thinset, with the overlaps sealed. A liquid membrane is rolled or brushed on in coats until it reaches the thickness the maker requires. A foam shower board is a board with the waterproofing built in, and its seams are sealed with the maker’s own tape and sealant.
All three can work when installed exactly as the maker says. The tile standard for the membrane itself is ANSI A118.10, written for waterproof membranes that sit under thin-set tile. The weak spot is never the product. It is a missed spot, a thin coat or a seam that was not sealed.
| System | How it goes on | Watch for |
|---|---|---|
| Sheet membrane | Bonded to the wall; seams overlapped and sealed | Wrinkles and unsealed edges |
| Liquid membrane | Rolled or brushed in coats over the backer board | Coats that are too thin or skipped |
| Foam shower board | Board and waterproofing in one; seams sealed with the maker’s tape | Seams, screw points and pipe holes |
| Traditional pan with liner | Sloped mortar bed, a flexible liner, then a second mortar bed | Liner folds and a clogged drain weep hole |
How high should it go?
The residential building code asks for a nonabsorbent wall finish, such as tile, at least 6 feet above the floor in a shower. Good practice is to take the waterproofing higher, above the showerhead, and many installers run it all the way to the ceiling. The bigger the splash zone, the more sense that makes.
Where leaks really start
Most shower leaks do not come from the middle of a wall. They start at the details: inside corners, board seams, screw heads, the holes for the valve and the showerhead, niches, benches, the curb and the drain. Tile hides all of them.
A careful installer seals these spots with fabric or tape, preformed corners and seals made for the membrane system, and then checks coverage. This is slow, boring work, and it is the work that decides whether the shower lasts.
The shower floor
The floor must slope toward the drain. The residential code asks for at least one quarter inch of drop per foot, with no flat spots where water can sit. The curb must be waterproofed on its top and both sides, and its top should slope back toward the shower.
The drain is part of the waterproofing, too. In a traditional pan the drain has a clamping ring and small weep holes, so water that gets under the tile can still drain. In a bonded membrane system, the membrane is sealed to the drain flange.
The flood test
A flood test is the proof. Before any tile goes on, the installer plugs the drain with a test plug, builds a temporary dam if the shower has no curb, and fills the floor with water from a separate source, not the shower itself. The water level is marked. Then it sits, usually for 24 hours. If the level drops, there is a leak, and it gets found while the walls are still open.
The exact water depth and hold time depend on the plumbing code and the membrane maker’s instructions, and a local inspector may need to witness it. A good installer takes photos of the marked level before and after. If anyone offers to skip the test because the membrane looks fine, remember that a leak is cheap to fix now and very costly to fix behind finished tile.
Signs a shower is already leaking
Look for a stain or soft spot on the ceiling or wall on the other side of the shower, a swollen baseboard, a musty smell, loose tile, grout that stays dark long after showers, or white powder on the grout. Any of these can mean water is going somewhere it should not. Call someone to look before the damage spreads.
Questions to ask before you hire
Which waterproofing system will you use, and is it installed to the maker’s instructions? Will you flood-test the shower, and can I see photos of the marked water level? How are the corners, seams and pipe holes sealed? What happens if a leak shows up after the job? A confident installer answers these in a minute. If you get a vague answer, that is information too.
