· by Michael Blair, Nashville's Kitchen & Bath
A curbless or zero-threshold shower has no step at the entrance. The floor of the bathroom simply continues into the shower and slopes to a drain. It is easier to walk into, easier to clean, and the only kind of shower that works for someone using a walker or a wheelchair. It is also a structural detail, not a finish detail, and that is where most of the difficulty lives.
The floor has to drop
A shower floor slopes to its drain. With a curb, that slope starts at the top of the curb and the shower floor sits at or above the bathroom floor. Without a curb, the entire slope has to happen below the level of the bathroom floor, which means the shower area has to be recessed into the structure.
On a wood-framed floor, which is most of Nashville, that means either dropping the joists in the shower area, sistering lower joists, or building up the rest of the bathroom floor to create the difference. On a slab it means cutting and lowering the concrete. Either way, the decision has to be made before framing and plumbing, not when the tile arrives.
Slope and the linear drain
A standard center drain needs the floor to slope toward it from all four sides, which puts a shallow bowl in the middle of a curbless floor and complicates large-format tile. A linear drain along one wall or across the entrance lets the floor slope in one direction only, from the entry down to the drain, and lets full-size tile lay flat with one pitch.
The slope itself is modest, on the order of a quarter inch per foot, so a shower four feet deep drops about an inch from entry to drain. That inch is why the recess is needed. A bathroom floor sloped very slightly toward the shower, across the whole room, is what stops splash from running out the door.
- Linear drain at the far wall, or at the entrance
- Single-direction slope of roughly a quarter inch per foot
- Bathroom floor tile continues into the shower
- Glass panel or a partial wall to contain spray
Waterproofing covers the whole room
In a curbed shower the waterproofing stops at the curb. In a curbless one there is no line where the wet area ends, so the membrane runs out under the bathroom floor, up the walls and around the door threshold. A bonded sheet membrane or a liquid-applied membrane tied into the drain flange is the standard, and the whole assembly should be flood-tested before tile.
This is the step that fails most often when a curbless shower is built by someone who has only built curbed ones. The tile looks identical afterward. The difference shows up as a stain on the ceiling below or a soft floor at the bathroom door a year later.
Containing the water
Without a curb, spray has to be managed by the layout. A fixed glass panel, a partial wall, or simply enough depth that the shower head is far from the entrance all work. A hand-held shower on a slide bar helps because it can be aimed away from the opening. A door is optional and most curbless showers go without one.
For a bathroom being planned around aging in place, the curbless shower is the single most valuable detail, and it is far cheaper to build into a remodel than to add afterward. Plan the blocking for grab bars in the walls at the same time, whether or not they get installed now.