A curbless shower instead of a tub: what happens under the floor

A curbless shower, sometimes called a roll-in or zero-threshold shower, has no lip to step over: the bathroom floor continues into the shower and the water is carried away by a gentle slope to a drain. It is the most useful accessible change that can be made to a bathroom, and the one with the most going on underneath. Where it replaces a bathtub, the sequence that conversion follows starts at the drain and works upward.

Why the floor has to be recessed

Water runs downhill. For the shower floor to finish level with the bathroom floor and still drain, the shower area has to start lower than the bathroom. That means recessing the subfloor, which means working with the joists below.

Three situations, in ascending order of difficulty:

• A concrete slab. The recess is cut or the floor is built up around the shower. Predictable work.
• Joists running the convenient way, with depth to spare. The subfloor is dropped between them and the framing is reinforced where needed.
• Joists running the wrong way, or shallow. This is where the structural engineering enters, and where a cheap quote becomes an expensive discovery.

Which one you have is not visible from the bathroom, and any quote given without looking is a guess. That is the single biggest reason two bids for the same shower differ by a multiple, and the rest of what moves an installation figure is set out separately.

The waterproofing is the part that fails

Tile is not waterproof and grout is not waterproof. What keeps water out of the structure is a membrane, bonded, lapped up the walls, sealed at the drain and at every corner. On a curbless shower it matters more than anywhere else, because there is no curb holding the water back.

A quote worth accepting names the waterproofing system and includes a flood test before tiling. A quote that does not mention it is not cheaper. It is also the question that sorts installers fastest.

The drain decides the tile

A linear drain along one wall lets the floor slope in a single direction, which allows large-format tile with fewer grout joints. A centre point drain requires the floor to fall in four directions, which needs smaller tile to follow the shape. Both work. The choice is made before the tile is ordered, not after.

What it gains, plainly stated

Nothing to step over, room for a seat, room for a caregiver, and room for a wheelchair if that becomes relevant. What we will not do is attach a claim about falls to any of it. It is a floor with no step in it, and that is a physical description rather than a promise.

What the whole room needs to work is on the aging-in-place page.