Why a shower floor needs a slope
A tiled shower floor must slope toward the drain so water never pools. Building codes and tile industry standards (such as the Tile Council of North America handbook) call for a slope of 1/4 inch per foot, with a minimum of 1/8 inch per foot in some cases. The slope is built with a mortar bed (floor mud or “deck mud”), on top of or under the waterproofing layer.
The calculation
- Distance to the farthest wall: half the longest side for a center drain, the full length for a linear drain against one wall.
- Rise = slope × distance.
- Height at the walls = minimum thickness at the drain + rise.
- Average thickness: for a center drain with level walls, the floor forms an inverted pyramid and the average is about the minimum + two-thirds of the rise; for a single-plane linear drain, the minimum + half the rise.
- Mortar volume = floor area × average thickness; divide by the bag yield.
Example
A 60 × 36 in shower with a center drain, a 1/4 in per foot slope and a 3/4 in minimum at the drain: the farthest wall is 30 in (2.5 ft) away, so the rise is 0.625 in and the perimeter sits at 1.38 in. The average thickness is about 1.17 in, so the bed needs 1.46 cu ft of mortar: three 60 lb bags at 0.5 cu ft each. The short sides end up steeper, about 0.42 in per foot.
Practical tips
- Mix floor mud dry, about 4 or 5 parts sand to 1 part cement, with just enough water to hold its shape when squeezed.
- With a traditional liner, build a pre-slope under the liner and a second mortar bed over it, so water reaching the liner drains to the weep holes.
- Linear drains allow a single-plane slope, which suits large-format tiles.
- Mark the perimeter height on the walls with a level line before packing the mud.
Check the drain manufacturer’s instructions and local code for the minimum mortar thickness and waterproofing method.