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Building a Slate-Tile Deck - Continued

Some of these membranes come in liquid form and are troweled or painted on; others are sheet membranes that are applied to the substrate with thinset mortar. After the membrane is installed, the finish material is applied over it with thinset mortar. Membranes are available for interior or exterior use over a variety of substrates, including plywood, cement board, and concrete. Many can be used to create waterproof installations suitable for use over living areas.

We used Ditra (Schluter Systems, 888/472-4588,, a polyethylene sheet membrane with a distinctive gridlike surface. The indentations on the upper surface are undercut (like a dovetail joint) so that mortar will key into them. The underside of the membrane is faced with synthetic fleece, which sticks well to mortar while preventing it from clogging the interconnected channels on the membrane's grid.

If moisture or vapor gets below the membrane it can escape horizontally through these channels. According to the manufacturer, the vertical parts of the grid further isolate the finish material from the substrate below.

Ditra is 1/8 inch thick and comes in rolls 39 inches wide by 45 or 98 feet long. Full rolls cost about $1.55 per square foot, but our supplier will sell partial rolls for an added charge.

Installation guidelines. The installation handbook for Ditra provides detailed guidelines for many different types of installations. The specs for our installation — natural stone tile over wood framing not over living space — required a substrate of two layers of plywood sheathing, a layer of 1/2-inch exterior-rated cement backerboard, and Ditra that's been sealed at the edges and seams.

We installed backerboard over the sheathing with modified thinset mortar and galvanized roofing nails.

Once this was done, we installed edge flashings, and the glaziers installed the aluminum base channel for the glass-railing system.

Edge Flashings

There would be a slate-tile riser where the deck landed on the patio, but elsewhere we needed to install flashing to cover the edges of the cement board and sheathing; otherwise they'd remain exposed.

I had a sheet-metal fabricator custom-bend galvanized steel flashings for the outboard edges of the deck. For added protection we coated the flashings with Clearco's high-performance zinc spray (800/533-5823,, a cold galvanizing spray that produces a film that's 90 percent zinc.

We nailed the top edge of the flashing over the cement board so that the leg hung down and lapped onto the perimeter beam below. The drip edge provided a clean visual transition between the framing and the base of the rail, and kept water off the edge of the sheathing material.

Glass Railing System

After the flashings were installed, it was time for the glazing sub to install the base shoe for the glass-railing system. There are many glass-railing systems on the market; we chose one made by C.R. Laurence (800/421-6144, because it eliminated the need for posts, which would block the view.

The rail consists of a heavy aluminum base shoe, 1/2-inch tempered-glass panels that fit into the shoe, and a stainless steel cap railing that fits over the top edge of the panels.


After the cement board was installed, the glazing contractor bolted the aluminum base shoe over a galvanized flashing at the edge of the deck.


A galvanized flashing at the edge of the deck.

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    Later, when the tile had been set, he cemented 1/2-inch tempered-glass panels into the channel, then clad the channel with a stainless steel cover.

Installing the base shoe. Since this rail system was designed to be installed over a steel or concrete floor assembly, we had an engineer tell us how to attach it to a wood beam. His recommendation was to bolt the base 6 inches on-center with 3/8-inch-by-6-inch galvanized lag screws. The base shoe came drilled 12 inches on-center, so we had the glaziers drill more holes. The glaziers shimmed the base level over the metal drip edge, bolted it down, and then left while we did the tile work on the deck.

Completing the rail. After the tile on the deck was complete, the glaziers came back to install the glass and railing cap.

To enhance the view through the panels we purchased glass that had been treated with TekonUS Alpha (888/749-8638,, a chemical treatment process that reduces water spotting.

The glass panels were placed in the channel, wedged plumb with plastic wedges, and permanently secured with anchoring cement poured into the gap between glass and channel.

We purchased optional stainless steel cover pieces to hide the aluminum shoe. The stainless steel veneer is attached to the shoe with double-sided tape and sealed with silicone glazing sealant. The stainless steel cap rail has a built-in gasket that allows it to be friction-fit onto the edge of the glass.

Installing the Membrane

The tile-setter cut the membrane to length with scissors, applied it fleece-side-down on unmodified thinset mortar, and then pressed it firmly in place with a float. Unlike most sheet membranes, which lap at the seams, pieces of Ditra butt edge-to-edge. On the deck, the Ditra lapped onto edge flashings, but since there were no flashings on the slab, the membrane simply stopped at the edge.


To accommodate possible movement in the slab, the tile contractor installed an isolation membrane, applying unmodified thinset to the substrate.


Then pressing the membrane into place with a float.

Unmodified thinset. Schluter recommends using unmodified thinset under and over Ditra because latex-modified thinset must air-dry for the polymers in it to coalesce and harden.

Modified thinset takes much longer to cure when it's sandwiched between impermeable polyethylene and some other nearly impermeable material such as tile; although it will cure eventually, it could take 14 to 60 days, according to the Tile Council. And in the meantime, grouting the installation or allowing it to get rained on would be risky.

With unmodified thinset, the extended drying time is actually a plus, because the retained moisture helps it hydrate and form a stronger bond.

Waterproofing. Though the deck was built with pressure-treated lumber and was not over living space, we still wanted to prevent water from getting below the membrane; if the framing wasn't constantly going through wet/dry cycles, there would be less likelihood of the tile cracking. (Leaks would be an even greater concern in cold climates where water that gets below the membrane could freeze and break the mortar bond.)

To prevent leaks, Schluter recommends sealing the edges and seams of Ditra with unmodified thinset and a layer of its Kerdi-Band seaming tape, which is made from the same synthetic material as the Kerdi membrane used to waterproof pans and walls in showers. The Ditra handbook contains details that allow you to create fully waterproof installations, up to and including ones suitable for use over occupied space. We didn't go that far on this job; we simply sealed the seams.

Installing the Slate

We did this job during our rainy season. To avoid having to vacuum water out of the indentations before he could set tile, the tile-setter installed the membrane and slate a section at a time, tarped his work at night, and grouted it before moving on to another section.

He set the tile with unmodified thinset and used sanded grout in the joints, leaving a movement joint of matching silicone sealant every 10 to 12 feet.


The tile was set in unmodified thinset.


Then grouted.


The completed job adds an outdoor room to the house.

Cameron Habelis the owner of Cameron C. Habel Construction, Inc., a remodeling company in Oakland, Calif.