Curbless Showers and Where the Water Goes
Why this matters
A curb is a dam. Take it away so a wheelchair or a shower chair can roll in, and every drop that used to hit the curb and turn back now has a path across the bathroom floor, under the vanity and toward the hallway. A curbless shower that leaks does not leak at a seam you can caulk; it leaks because the floor was never sloped, recessed or waterproofed for where the water actually goes. Those three decisions are made before demolition is finished, and two different trades own them.
Why the wet zone grows when the curb goes
With a curb, the wet zone is the shower footprint. Without one, it is the footprint plus everywhere water reaches by three routes:
- Spray. A fixed head throws water past the opening. A handheld shower on a slide bar, which is the right fixture for a seated user, throws it anywhere the hand points, including out the entry.
- Sheeting. Water running off a seated body and off the seat lands on the floor near the entry and runs whichever way the floor falls. If the floor at the entry is flat, it spreads.
- Overflow. A drain slowed by hair, or a center drain whose grate is partly covered by a shower chair's leg, backs water up until it finds the lowest exit. In a curbless shower that exit is the bathroom floor.
So the slope and the membrane have to be designed for the bad day, not the clean drain on install day.
Slope: how much, and which way
The plumbing codes commonly require a shower floor to slope to its drain at about 1/4 in per foot; confirm the exact section and any upper limit in the plumbing code your jurisdiction adopted (the IPC or the UPC, depending on the state). A quarter inch per foot is exactly 1:48, which is also the maximum slope the 2010 ADA Standards use for a clear floor space (305.2), so a code-minimum shower floor is also as level as the reference allows a wheelchair to sit on. Steeper drains better and makes a chair, and a person standing on wet tile, drift. Hold to 1/4 in per foot unless the drain layout forces more.
The slope runs from the entry toward the drain. The entry is the high point, flush with the bathroom floor. That is what "curbless" means: no step, no lip, and a gentle fall inward. See related: Thresholds and Changes in Level, for the edge at the entry.
Center drain or linear drain
| Center drain | Linear drain | |
|---|---|---|
| Floor shape | Four-way slope to a point | Single plane, one direction |
| Fall at the entry | Largest at the corners, so the recess is deepest there | Uniform along the entry |
| Tile | Small tile, or large tile cut on the diagonals | Large-format tile works without diagonal cuts |
| Under a chair | The chair often sits on or near the grate | Grate sits at a wall, out of the seat area |
| Best location | Center of a small, square stall | Along the back wall, or across the entry |
A linear drain along the back wall keeps the grate out of the wheel path and lets the whole floor fall away from the entry in one plane. A linear drain across the entry catches water before it leaves the shower, which is useful in a tight room, but it puts a grate in the wheel path; choose a grate whose openings pass no sphere over 1/2 in, with long openings running across the direction of travel, per the reference at 302.3, so a caster cannot drop into a slot.
Recessing the floor: where the height comes from
The finished shower floor at the drain sits lower than the bathroom floor by the fall, and the assembly under that tile still needs its minimum thickness. The difference has to come out of the structure.
Wood framing. The usual method is to remove the subfloor over the shower and install a new, lower subfloor on cleats fastened to the sides of the joists, so the joists themselves are not cut. Cutting or notching joists to gain depth is a structural change; notching is limited by the IRC (R502.8 in the 2021 edition, as adopted) to small notches away from the middle of the span, and engineered I-joists generally may not have their flanges cut at all under the joist maker's instructions. Where the depth cannot come from cleats, that is the point to bring in an engineer or the joist manufacturer's technical line, with the joist size, span and spacing in hand.
Before any saw goes into a subfloor, open an inspection hole and look into the joist bay: wiring, a supply line, a drain or a gas line can run directly under the cut. Set the blade depth to the subfloor thickness so it cannot reach them.
Concrete slab. The slab is saw-cut and removed over the shower and the drain trench. Cutting, chipping or grinding concrete releases crystalline silica; on construction work that is controlled under 29 CFR 1926.1153, which for a handheld saw means a saw with integrated water delivery and the respiratory protection Table 1 specifies, under a written respiratory protection program per 29 CFR 1910.134. Never cut a post-tensioned slab without the engineer of record or a scan that locates the tendons; a cut tendon releases enormous stored force and can take the slab's capacity with it. Many houses built on slabs in the last few decades are post-tensioned, and the stamp is often on a garage slab or in the permit file.
Old bathrooms may hide hazards in the demolition itself. Sheet vinyl, its adhesive and old tile mastic in older houses may contain asbestos; have suspect material tested before it is torn out, and do not sand, scrape or break it until the result is back.
The membrane, and how far it has to reach
Two waterproofing systems are common: a traditional sheet pan liner over a pre-slope with the tile bed on top, or a bonded membrane (sheet or liquid) directly under the thin-set, tested to ANSI A118.10, a consensus standard that binds through the residential code where your jurisdiction adopted it by reference and through the membrane maker's installation instructions.
Whichever system, three things decide whether it holds:
- The drain has to match the membrane. A clamping-ring drain is made for a sheet liner; a bonded membrane needs a drain with a bonding flange made for it. A bonded membrane lapped onto a clamping drain is a leak waiting for its first slow drain.
- It goes up the walls. The IRC (R307.2, as adopted) requires a nonabsorbent wall finish to at least 6 ft above the floor in a shower; most membrane makers want the membrane itself at least to the head height. In a curbless shower, carry it up the walls past the entry as well, since that is where spray lands.
- It goes out across the floor. This is the step that gets skipped. A shop default worth setting: in a bathroom under about 60 sq ft, membrane the whole floor and treat the room as a wet room, turning the membrane up the walls a few inches behind the base; in a larger room, carry it at least 3 ft beyond the shower opening in every direction water can travel, and slope the bathroom floor toward the shower, never away.
Who owns what
| Trade | Owns |
|---|---|
| Plumber | Drain location and type, trap and venting, drain body set at the right height for the bed, the drain's own test, the mixing valve |
| Tile setter | The sloped bed, the membrane and its connection to the drain flange, the flood test of the membrane, the tile |
| Carpenter or GC | The recess: cleats, dropped subfloor, or slab removal, and any engineer's sign-off |
| Electrician | GFCI protection and keeping receptacles and luminaires out of the zone around the shower |
The joint between plumber and tile setter is where curbless showers fail. The drain body has to be set before the bed is built, at a height the bed's minimum thickness was calculated for. Get both trades to agree on the levels in writing before the plumber sets it.
The electrical zone moves too. The NEC (406.9(C) for receptacles and 410.10(D) for some luminaires and ceiling fans, in the 2020 edition and later as adopted) measures its exclusion zone from the shower stall threshold, which in a curbless shower is the floor. An 8 ft vertical zone measured from the floor covers most bathroom ceilings entirely.
A worked example: setting the levels
A tub alcove 60 in wide and 36 in deep becomes a curbless shower, over 2x10 joists with a 3/4 in plywood subfloor. A linear drain runs along the 60 in back wall, so the floor falls 36 in, or 3 ft, from entry to drain. The bed system's maker gives a minimum thickness at the drain of 3/4 in; tile plus thin-set measures 3/8 in. Those two are illustrative values for this job, read from that system's instructions. Every level is measured from the top of the existing subfloor.
LEVELS, from top of existing subfloor (0)
bathroom tile surface +3/8 in
shower tile at entry, flush +3/8 in
fall to drain: 3 ft x 1/4 in/ft 3/4 in
shower tile at drain 3/8 - 3/4 -3/8 in
tile and thin-set at drain 3/8 in
bed minimum at drain 3/4 in
dropped substrate -3/8 - 3/8 - 3/4 -1 1/2 in
bed at entry (3/8 - 3/8) + 1 1/2 1 1/2 in
So the new subfloor sits 1 1/2 in below the top of the old one. With 3/4 in plywood on cleats, the cleats' top edge sits 1 1/2 in below the top of the joists, and the joists stay whole. The bed tapers from 1 1/2 in at the entry to 3/4 in at the drain, which is the bed's minimum, met exactly at its thinnest point. The plumber sets the drain body so its flange meets the bed at the -3/4 in level (shower tile at -3/8 in, minus 3/8 in of tile and thin-set).
The bathroom is 5 ft by 8 ft, 40 sq ft, under the 60 sq ft default, so the membrane covers the whole floor. A fixed glass panel covers half the opening on the side where the shower head sits, and the handheld goes on a slide bar on the back wall facing the open half.
What changes the answer: a center drain in the same alcove would fall from the corners, which sit about 35 in from a central drain (half the diagonal of a 60 by 36 in rectangle), so the corner fall is close to the same 3/4 in and the recess barely changes. What changes is everything else: a four-way slope, diagonal cuts in the tile, and a grate right where the shower chair sits. A 2x6 floor system may not leave room for the cleats and the trap below, which sends the job to the engineer. A slab sends it to the saw.
How to verify
Before tile, flood-test the membrane. In a curbless shower the water will run out the entry, so build a temporary dam at the outer edge of the membrane (a strip of foam board sealed to it), plug the drain, flood to about an inch at the high point, mark the line and leave it 24 hours or the membrane maker's stated time. A dropped line with no visible leak means the drain connection, which is the usual culprit.
After tile, check the fall with a level at the entry, the middle and at each end of the drain, run the shower with a chair in place for ten minutes, and look at the floor outside the shower. Then have the person shower once with the plumber or tile setter's number in hand, because the first real use is the test that matters.
References
- International Residential Code, 2021 edition, R307.2 (shower wall finish) and R502.8 (joist notching), as adopted by your jurisdiction
- ANSI A118.10 (load-bearing bonded waterproof membranes for thin-set tile), as referenced by your adopted code and the membrane maker
- National Electrical Code, 2020 edition, 406.9(C) and 410.10(D), as adopted
- 29 CFR 1926.1153 (respirable crystalline silica, construction) and 29 CFR 1910.134 (respiratory protection)
- See related: How to Convert a Tub to a Low-Threshold Shower; The Shower Conversion That Flooded the Hallway