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,
www.schluter.com), 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
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
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
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
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,
www.clearcoproducts.com), a cold
galvanizing spray that produces a film that's 90 percent
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,
www.crlaurence.com) 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
Figure 5. After the cement board was
installed, the glazing contractor bolted the aluminum base shoe
(top) over a galvanized flashing at the edge of the deck
(middle). 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 (bottom).
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
To enhance the view through the panels we purchased glass that
had been treated with TekonUS Alpha (888/749-8638,
www.tekonus.com), 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
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 (Figure 6). 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.
Figure 6. To accommodate possible movement
in the slab, the tile contractor installed an isolation
membrane, applying unmodified thinset to the substrate (left)
and 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
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 (Figure 7), 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
Figure 7.A seaming tape bedded
in thinset prevents leaks at joints in the membrane.
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 (Figure 8), leaving a movement joint of matching
silicone sealant every 10 to 12 feet.
Figure 8. The tile was set in unmodified
thinset (top left), then grouted (top right). The completed job
adds an outdoor room to the house (bottom).
Cameron Habelis the owner of Cameron C.
Habel Construction, Inc., a remodeling company in Oakland,