Ceramic tile shower floors have long been regarded as a sign
of high-quality construction. Unfortunately, improper
installation can turn this durable floor system into a
problematic one. In this article, I’ll explain the
methods I use to build a leak-free and long-lasting ceramic
tile shower floor.
Anatomy of a Shower Floor
Shower floor systems are unique in that they must perform
flawlessly under water. Since the curb at the shower door acts
as a dam, the floor drain is the only safety valve preventing
the shower floor from becoming a swimming pool. If the drain is
blocked (by a dropped washcloth, for example), the shower floor
ends up under water.
To prevent a submerged floor from springing leaks, I use a
multi-layered mortar bed system (see illustration).
In a typical tiled shower, the pan liner extends above
the curb height on walls and is sealed in place by the two-part
subdrain assembly. Any moisture that penetrates the tile floor
and accumulates will exit through weep holes.
The layers include a sloped subfloor, a continuous
waterproof shower pan liner, and a floated mortar bed.
Clamping ring drain.
There is only one kind of drain that I use with a shower pan: a
clamping ring drain. The upper half of this two-piece fitting
works like an ordinary drain, accepting water runoff from the
surface of the shower floor. The bottom half provides backup
drainage, allowing any water that seeps under the tile onto the
waterproof membrane to weep into the drain pipe. I place the
supporting flange of the drain housing directly on the plywood
subfloor. No fasteners are necessary for the bottom half of the
clamping drain, because the mud bed will hold everything in
place. The supporting flange prevents any downward movement
when the drain is stepped on.
Creating a Sloped Subfloor
I always place the shower pan on a sloped subfloor. The sloped
subfloor directs moisture that finds its way down to the shower
pan liner towards the weep holes in the drain. Since the weep
holes are located above the mounting flange of the drain, a pan
liner installed on a level subfloor actually ends up lower than
the weep holes, and moisture will collect in this shallow
depression. This condition is one of the primary causes of the
mildew and mold that often discolors grout joints.
The floor framing itself can be pitched to create the slope,
but I find it much quicker to mold the slope into a thin
latex-modified mortar bed. The minimum recommended subfloor
slope is 1/4 inch per foot. After calculating the proper slope,
I fasten 3/4x3/4-inch float strips around the perimeter of the
shower floor to serve as guides when I’m placing the
sloped mortar bed.
A clamp ring drain is set (but not fastened) to the
subfloor (left), and 3/4-inch float strips are nailed around
the perimeter of the shower floor (right).
With the float strips in place, I cover the plywood subfloor
with a layer of 15-lb. tar paper, followed by a layer of
galvanized expanded wire lath held flat with staples.
A layer of 15-lb. tar paper and galvanized
expanded metal lath (2.5 lb. per sq. yd. or heavier) is
stapled to the subfloor.
The last step is to trowel out the latex-modified mortar
(known in the trades as "deck mud"). To shape the slope, I let
the wood float ride over the float strips and the drain flange,
and compact the mortar by tamping it with the float. Since the
cross-section of this deck mud can be as thin as 3/8 inch at
the drain area, I always use a strength-enhancing latex
additive instead of water when mixing the mortar (see ). I let
the sloped mortar bed harden overnight before installing the
shower pan membrane.