How much slope does a shower floor need?

The quarter-inch rule

A quarter inch per foot is the common minimum across North America, and it is what most drain and waterproofing makers ask for. At that fall, water runs to the drain instead of sitting in low spots, and the floor still feels flat under foot.

Less than that and water ponds, especially once the tile and grout add their own small highs and lows. Much more and the floor feels steep, large tiles will not follow it, and a bench or a glass panel no longer sits square. The bed is set out to the number the drain and membrane makers give, which is usually the quarter inch.

Working out the fall

The fall is measured from the far edge of the floor to the drain. A drain three feet from the back wall needs the floor at that wall to sit three quarters of an inch higher than the drain. In a four-by-five shower with a centred drain, the farthest corner is the one that sets the height, and every other edge comes down to meet the same planes.

The drain body your plumber sets fixes the low point, and the walls and the curb fix the high points. Before any mud goes down, the heights are marked on the walls and the screed guides are set to them, so the bed is packed to known numbers, not by eye.

Point drain or linear drain

A point drain in the middle means the floor slopes in from all four sides, like a shallow funnel. The tiles have to be cut along the valleys where the planes meet, which suits small tiles and mosaics.

A linear drain along one wall, or across the entrance, means the whole floor slopes one way, as one flat plane. That keeps large tiles whole and makes the bed simpler to read, which is why linear drains are popular with large-format tile and curbless showers.

Curbless showers

A curbless shower still needs its quarter inch per foot. Without a curb, the high edge of the shower floor has to meet the bathroom floor, and the floor falls from there to the drain. That means the shower area is recessed, or the rest of the bathroom is built up, so the slope has room. The dry pack bed is what makes those planes meet without a step.

The pre-slope and the bed above

In the traditional method, the slope is built twice: once in the pre-slope under the liner, so water that reaches the liner runs to the weep holes in the drain, and again in the mortar bed above it. With a bonded membrane there is one sloped bed, and the membrane goes straight on it. Either way, the slope has to be in the bed: thinset and tile cannot fix it later.

Checking it before the membrane

Once the bed has set, the slope is checked with a level and a straight edge from every wall and corner to the drain, and with a little water where it is hard to read. Water should run to the drain from every edge without stopping. A low spot is fixed in the bed, never left for the thinset to fill, because a thick patch of thinset under a tile is where cracks and hollow tiles start.