Tile is not waterproof. Grout is not waterproof. Both are wear surfaces. Everything that keeps water out of your floor joists sits behind them, and once the tile is set, none of it can be inspected without demolition. That is why a bathroom can look flawless on handover day and be failing by its third winter.
Water gets behind tile. Assume it.
Cement grout is porous. Caulk joints move and open. Sealers slow absorption; they do not stop it. So the assembly is not designed to keep water off the tile. It is designed so water that passes the tile hits a continuous waterproof plane and drains back to the drain. Leaks are rarely about water getting in. They are about water having nowhere to go.
The substrate decides most of it
Paper-faced gypsum board does not belong in a shower or tub surround. Neither does the green kind, which is water-resistant, not waterproof. The Massachusetts residential code, which adopts the International Residential Code with state amendments, requires cement, fiber-cement or glass-mat gypsum backers behind tile in tub and shower areas.
Cement backer board is not waterproof either. It does not rot or lose strength when wet, which is why it is the right substrate. But it passes water freely. It is a base for waterproofing, not waterproofing.
The other substrate mistake is doubling up. If there is a bonded waterproof membrane on the face of the board, do not also staple polyethylene behind the studs. Water that gets between two impermeable layers cannot dry in either direction, and in a Massachusetts winter the vapor drive pushes it to the cold side and holds it there. Pick one plane and detail it properly.
Waterproofing is a plane, not a product
Two approaches work. Both have to be done completely.
- A bonded sheet or liquid-applied membrane over backer board, meeting ANSI A118.10, run continuously across seams, inside corners, the curb and the drain flange.
- A traditional mortar bed over a pan liner, with sloped fill under the liner and open weep holes at the drain.
What fails is almost never the middle of a wall. It is the transitions. Inside corners, the curb, the niche, the bench, the drain flange, the line where wall waterproofing meets floor waterproofing. Then the penetrations: mixing valve, shower arm, body sprays, glass hardware, grab bar screws. Every fastener driven through a finished membrane is a hole unless it is sealed as one.
Before any tile, flood test the pan. Plug the drain, fill to two inches at the threshold, and watch it. The plumbing code sets fifteen minutes as the minimum. Overnight tells you more. Photograph the water line at the start and the end. A contractor who will not flood test a pan has told you something useful.
Slope, and the part under the slope
The state plumbing code requires the shower floor to fall toward the drain not less than a quarter inch per foot and not more than half an inch per foot. Under a quarter, water stands. Over a half, it feels wrong underfoot.
The part people miss is the pre-slope. In a mortar bed shower the liner sits on fill that is already sloped. If the liner is laid flat and only the topping is pitched, water that passes the tile reaches the liner and sits there permanently. The bed stays saturated. You get efflorescence at the grout lines, a smell that never leaves, and tiles loose at the corners.
The weep holes in the drain body let that bed drain. Protect them with pea gravel or spacers before the mortar goes down, or they pack solid on day one. The curb slopes too, back into the shower, not out toward the bathroom floor.
Curbless showers
Curbless is not harder to build. It is less forgiving, because the thing that used to stop the water is gone.
The floor structure has to drop to make room for the slope and the assembly. That means an engineered detail for notching or sistering joists, a dropped subfloor bay framed between them, or building the whole bathroom floor up to meet the shower. Which one is right depends on joist direction, span and what is below. It is a framing decision made long before anyone orders tile.
A linear drain along one wall lets the whole floor slope one direction instead of four planes folding into a center drain. That is what makes large-format tile possible here. Waterproofing then has to run well past the opening, because splash and steam have an open route out. Treat the first stretch of bathroom floor as wet area.
Niches and benches leak more than walls do
A niche is a hole cut in your waterproof plane at chest height, which people then fill with shampoo. It needs blocking around the opening, a sill sloped back into the shower, and membrane wrapped through the box and lapped so upper layers shed onto lower ones. Put one in an exterior wall and you have removed most of the insulation in that cavity, which needs a plan rather than a shrug.
A bench should slope to drain and be solid, or a properly waterproofed frame. A hollow bench with a tiled lid is a reservoir with a lid.
Size the fan, and vent it outside
Code minimum for a bathroom is 50 cfm intermittent or 20 cfm continuous. That is a minimum, not a design. The Home Ventilating Institute sizes small bathrooms at 1 cfm per square foot of floor area, and anything over 100 square feet by fixture: 50 cfm each for toilet, shower and tub, 100 for a jetted tub.
Rated airflow is measured on a bench at a set static pressure. A long run of sagging flex duct with crushed elbows delivers a fraction of the number on the box. Use rigid smooth duct, keep the run short, insulate it through unconditioned space, slope it slightly toward the exterior, and terminate at a hooded cap with a working damper.
Then the part that matters most. Exhaust from a bathroom cannot be discharged into an attic, a crawl space, or any other area inside the building. The residential code is explicit. It is still one of the most common things found in an attic during a renovation: a flex duct lying loose, aimed at the roof sheathing. In this climate that means frost on the underside of the deck in January and a brown stain on a bedroom ceiling in March, which everyone diagnoses as a roof leak.
Put the fan on a timer or humidistat so it runs after the shower ends, and undercut the door so it has air to pull.
Blocking costs nothing while the wall is open
The accessibility standard ICC A117.1 expects a grab bar, its fasteners and the structure behind it to take 250 pounds of force at any point, in any direction. No drywall anchor does that. Solid blocking between studs, or three-quarter-inch plywood backing across them, does.
Put it in whether or not a grab bar is going in now. Same for the glass panel, a wall-hung vanity, floating shelves, and towel bars, which get leaned on harder than they were designed for. Then photograph the open walls with a tape measure in frame. Cheapest insurance in the job.
Why it takes three years
Because the failure is slow. A small amount of water gets past a bad corner on each shower. It wets the subfloor, the sill plate, the top of a joist, and dries a little between showers, less in winter when the assembly is cold. The wood sits above the moisture content where decay starts, and rot takes hold quietly.
The symptoms arrive in order. Grout darkening in one spot. A hairline crack at an inside corner. One loose tile at the curb. A musty smell in a closed bathroom. Then a stain on the ceiling below, or a soft spot at the shower door. By then regrouting fixes nothing. The repair is removing the shower, the substrate, sometimes the framing.
That is the argument for spending on the half of a bathroom nobody sees. The tile sets how the room looks. What is behind it sets how long the room lasts.
What to ask before the tile goes on
- Which backer board, and which waterproofing method. One method detailed completely, not two half-methods.
- Photos of the flood test with the water line visible.
- Confirmation of pre-slope under the liner, on a mortar bed pan.
- The fan's cfm and the exact route the duct takes to the outside.
- Blocking locations, photographed before the walls close.
Then the paperwork. Massachusetts (201 CMR 18.05) requires a written contract over $1,000, caps the deposit at the greater of a third of the contract price or the actual cost of special-order materials, and bars a contractor from demanding final payment until the work satisfies you. The same rule puts one line on the contract, worth taking literally:
DO NOT SIGN THIS CONTRACT IF THERE ARE ANY BLANK SPACES
Your contractor also pulls the building permit as your agent. Pull it yourself, or hire someone unregistered, and you give up access to the state's Guaranty Fund if the job goes wrong.
We build bathrooms across MetroWest and Greater Boston, and we document the parts that get covered up. Our workmanship warranty is {{WARRANTY_TERM}}. For a bathroom already showing symptoms, or one you have not started, we will come and look at no charge. Idea Inc, HIC registration {{HIC_REGISTRATION_NUMBER}}.