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Shower Walls Next After the pan assembly is completed and tested, I install the wall tile substrate down to the pan membrane, covering the upturned membrane that is fastened to the walls. Then I set the wall tile, positioning the bottom edge of the first course of tile 1/8 to 1/4 inch above the finished floor tile height to allow room for a control joint (backer rod and sealant). It’s important to use control joints wherever walls or walls and floors intersect (see "Preventing Tile Failures," 2/97, for more on tile control joints). The main reason I set the wall tiles first is that it would be hard not to damage the floor tiles while setting the wall tiles.

Floating the Mortar Floor

Before installing the mortar bed, I thread the adjustable drain screen into the body of the clamping ring drain, setting it at the finished height of the tile floor. Then I place a free-draining material like pea gravel or even plastic tile spacers around the base of the drain to prevent the deck mud from plugging the weep holes.

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Adjust the finish drain stem to the proper height (left). To prevent mortar from plugging the drain’s weep holes, circle the drain with plastic tile spacers (right). I float out approximately half of the deck mud, lay in a piece of reinforcing wire, then cover the wire with the remaining deck mud. For reinforcement wire I use 16/16-gauge 2x2-inch mesh, 13/13-gauge 3x3-inch mesh, or 16/13-gauge 11/2x2-inch mesh.

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After half of the mortar bed is placed and troweled level, reinforcing wire is laid on the mortar bed (left) and covered by the final layer of mortar. Then 1x3/8-inch screed strips are set and leveled at the perimeter of the shower (right). I compact the deck mud by beating it in with the wooden float, then float the bed to its final slope. The finished height of the mortar bed should position the floor tile 1/16 to 1/8 inch above the top of the drain, and 1/8 to 1/4 inch below the bottom edge of the wall tile. I’ve notched the top front edge of my wood float so that it rides along the bottom edge of the wall tile and leaves the proper space.

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The screed strips and the top of the drain are used as guides to accurately level the mortar bed (top). The screed strips are then removed and the voids filled with mortar (middle). To ensure that the tile finishes flush at the drain, carve a small secondary slope around the drain perimeter, using a sample tile as a gauge (bottom).

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Using a damp sponge, I’m careful to remove any deck mud that finds its way onto the wall tile. I use thinset mortar to bond the tile to the mortar bed. The floor tiles can be installed while the deck mud is fresh or the next day, when the setting bed has hardened. Since Portland cement shrinks slightly as it cures, setting tiles over a fresh mortar bed is tricky. For this reason, I prefer to let the mortar harden before setting the floor tiles. The thinset mortar should be allowed to cure for at least 24 hours before grouting (check with the manufacturer for details). Michael Byrne was a tile contractor for more than 25 years, and is now executive director of the Ceramic Tile Education Foundation in Clemson, S.C.

To the untrained eye, properly mixed deck mud looks like it’s far too dry to be of any use. Most tradespeople assume that deck mud should be plastic and pliable — similar to brick mortar. But good deck mud should have just enough moisture to hold the mixed ingredients together when squeezed into a ball. This stiff consistency allows the mortar to be tightly compacted, which is critical for tile floors. The working characteristics of deck mud also differ significantly from other types of mortar. I "carve" (rather than trowel) deck mud to final height, using the lead edge of my wooden float. Students at our classroom facility in Clemson, S.C., are taught to test their finished floor setting beds by standing on the bed immediately after it has been floated and finished smooth. If the setting bed is mixed and compacted properly, the fresh bed should not show any heel marks after it’s been walked on. Setting bed mortar contains two ingredients: Portland cement and sand. An acceptable mix consists of four to six parts clean, sharp sand to one part Portland cement. Sharp sand particles compact better than rounded ones. I mix my deck mud in a large mortar box. I place all the sand in the box, sprinkle some water on top, and use a mason’s hoe to pull the sand back and forth to lightly coat the particles with water. I add the Portland cement and pull the mixture back and forth until it’s thoroughly mixed. Finally, I add just enough water to achieve the required consistency, then mix it thoroughly again.