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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).

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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.

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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.

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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.

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