Guide
Curbless shower floors: what goes under the tile
A curbless shower has no step: the bathroom floor runs straight into the shower. It looks simple, but the slope that a curb used to hide still has to go somewhere.
The problem a curb used to solve
A shower floor has to slope to its drain, usually a quarter inch per foot. With a curb, the shower floor can sit higher than the bathroom and the curb holds the water in. Without one, the whole shower floor has to sit at or below the bathroom floor: its high edge meets the bathroom floor, and it falls from there to the drain.
Making room for the slope
On a concrete slab, the shower area is usually recessed, or the rest of the bathroom floor is built up with a mortar bed so the shower can slope down from it.
On a wood floor, the framing in the shower area is often lowered, which is work for the framer and sometimes an engineer, or the whole bathroom is built up.
Either way, the dry pack bed is what makes the floor: one plane from the bathroom into the shower, sloped to the drain, ready for the membrane and the tile.
Why linear drains are popular
A linear drain along a wall or the entrance means the floor slopes one way only. That keeps large tiles whole, and it makes the transition between the bathroom and the shower easier to get right than a point drain in the middle.
Questions
Does a curbless shower need a special drain?
Not always, but a linear drain is the usual choice because the floor then slopes in one direction. Your plumber sets it at the height the slope needs.
Will water run out of a curbless shower?
Not if the floor slopes correctly to the drain and the drain is sized for the shower head. Many designs also add a glass panel to keep the spray in.