My company specializes in kitchen and bath remodels, so we
install a lot of tile floors. We sometimes have to begin the
job by addressing structural deficiencies in the floor. But
even sound wood subfloors and concrete slabs may not be flat
enough for tile - a problem we most often solve with a
self-leveling cement underlayment, or SLU.
SLUs can be poured to a feather edge, so they're well-suited
for smoothing out uneven, poorly finished slabs. They can also
be used to fill low spots, making them a good choice for fixing
sagging, out-of-level wood-framed floors. And SLUs are fast: We
can usually prep and pour the underlayment in one day, then
come back and start installing tile the next.
We still prefer to correct floors that are more than a couple
of inches out of level with a traditional mortar bed - but SLUs
are our first choice for flattening most other subfloors.
SLUs are versatile and easy to use, but they're also finicky,
so we're scrupulous about following manufacturer instructions.
Getting even one step wrong can lead to the failure of the
Wood floors. We always begin by checking the size,
spacing, and span of the floor joists and the thickness of the
subfloor. Tile industry standards call for a pretty stiff
substrate - deflection from a concentrated 300-pound load can't
exceed L/360 under ceramic tile - so we're especially careful
if we see spans longer than 10 or 12 feet, sawn joists sized
smaller than 2x10s, or joist spacing greater than 16 inches
on-center. Engineered joists are easier to check because their
span ratings are usually stamped right on them.
Wood floors are also subject to concentrated deflection - or
deflection of the subfloor between joists (see Michael Byrne's
article "Tiling Over Plywood Subfloors," 3/11). We address this
by laminating 1/2-inch or 5/8-inch exterior-grade plywood to
the existing subfloor with full-spread yellow glue and closely
spaced screws. According to the manufacturer (203/393-0010,
laticrete.com), the LatiLevel 84 and 86 compounds we typically
use can't be installed directly over OSB. This is also true of
most other SLUs I'm familiar with. So we always cover an OSB
subfloor with an additional layer of exterior-grade plywood,
regardless of the structural rating of the OSB.
Any SLU substrate must be clean enough for paint. After making
all needed repairs, we thoroughly vacuum the floor, then wipe
it down with damp sponges or rags.
Concrete. Slabs can be structurally sound yet still
require a little more prep than a plywood floor. That's because
any contaminant like moisture, paint, or grease that would
cause adhesion problems under tile will also cause adhesion
problems for the self-leveling underlayment. As with a wood
subfloor, our goal is a clean and paint-ready substrate.
A new slab has to be fully cured before an SLU is installed
over it, but even a cured slab can have ongoing moisture
problems. If we have any doubts, we test the slab (see "Testing
for Moisture in Concrete Slabs," 6/07) and consult with the SLU
manufacturer before proceeding.
If there's paint on the slab, we remove it by abrading the
surface with a bushing bit mounted in a demolition hammer.
After vacuuming up the debris, we clean the slab off with Aqua
Mix Heavy-Duty Tile & Grout Cleaner (800/282-8786,
custombuildingproducts.com), a high-alkaline cleaner and
degreaser, then give it a clear-water rinse.
Movement joints. Self-leveling underlayments require
movement joints wherever the floor meets vertical surfaces -
walls, tubs, or pipe penetrations, for example. The thickness
of the joint depends on the room dimensions, but usually 1/4
inch is adequate.
We use strips of sill seal for movement joints around the
perimeter of the room - the material is inexpensive, widely
available, and easy to work with (see Figure
1). We can staple it to framed walls or use
double-sided tape or adhesive caulk to glue it to block walls
and tubs. To keep liquid underlayment from flowing underneath
during the pour, we caulk the joint between the sill seal and
the floor. After the pour, we leave the sill seal in place
until the tile has been set to ensure that we leave a clean
joint, free of thinset and grout.
Figure 1. Once the reinforced subfloor has been
cleaned, a worker installs sill seal around the perimeter of
the room to create a movement joint (A). To contain the
waterlike underlayment, seams and joints must be caulked (B).
Striking the sheathing joints with a putty knife helps force
caulk into the joint (C). Penetrations through the flooring,
like this toilet flange, also must be dammed off
To avoid unexpected leaks (remember, liquid underlayment has a
waterlike consistency), we also seal seams and penetrations
through the subfloor. We squirt caulk into sheathing joints,
then strike it off with a putty knife to force it into the
joint. Depending on how wide the joint is between the toilet
flange and the sheathing, we use leftover material - felt
paper, backer rod, even cardboard - to prevent spillover. To
dam off entryways, we screw down a wood strip. Once all the
dams are in place, we caulk everything, essentially creating a
room-sized shallow tub to hold the liquid underlayment.
Every SLU requires a specified primer. Brushed onto the
substrate before the underlayment is poured, the primer helps
with adhesion and seals the substrate, allowing the SLU to
retain moisture until it has properly hydrated and cured
Figure 2. Primer is needed to seal the subfloor. A
worker rolls it on (left), then uses a stiff bristle brush to
force it into the fibers of the plywood (right).
Primers have roughly the same thin consistency as a latex
admix. They usually need to be applied within 24 hours of
pouring the SLU, but they also have to be dry when the
underlayment is poured. Because drying can take a few hours
(depending on temperature and humidity), we keep a couple of
fans on hand and sometimes circulate air around the room when
we're priming in humid conditions.
Concrete usually requires only one coat of primer, but for
absorptive substrates like plywood we dilute the primer with
water to make a sealer coat. We roll on the thinned primer,
agitating it with a bristle brush to force the liquid into all
the nooks and crannies in the plywood subfloor. As soon as the
sealer coat has dried, we apply the second coat, again using
the bristle brush after rolling it on. This may seem like
overkill to some installers, but we've never had an
Lath. In between primer coats, we cut and fit
3.4-pound-per-square-yard galvanized diamond metal lath, which
is needed to reinforce any SLU poured over a wood subfloor
(Figure 3). We don't wait for the first coat
to dry while we're doing this, but we do take care not to track
in dirt as we work. Because the lath is self-furring, we can
work on top of it without getting our shoes wet. Once the
second primer coat is down, we fasten the lath with staples so
it won't lift up during the pour, overlapping the seams by a
couple of inches.
Figure 3. Expanded metal lath is needed over
wood subfloors to reinforce the SLU.
Floor map. Before mixing the SLU, we use a laser level
to make a quick "contour map" of the floor (Figure
4). Identifying the low spots helps us decide where to
concentrate the SLU and minimizes the need to push the wet
material around after it's been poured.
Figure 4. Measuring down from the laser's benchmark
elevation, workers drive screws into the floor to identify low
spots and mark the final height of the slab.
On wood floors, we mark low spots with drywall screws, leaving
the heads about 1/8 inch proud of our target floor elevation so
that we can back the screws out after the SLU has hardened
(Figure 5). If we're pouring over a slab, we
glue pieces of scrap mosaic tile to each other and to the slab
with fast-setting Laticrete 310 epoxy, making tiny pillars that
will remain slightly lower than the finished level of the SLU.
After the epoxy dries - usually within a few minutes - we brush
primer onto the pillars.
Figure 5. Before pouring an SLU over a wood subfloor,
the author reinforces the framing as needed and installs an
additional layer of plywood sheathing to ensure that the floor
system meets minimum floor deflection requirements for tile
(above). Screws driven into the subfloor - or, in the case of a
slab, mosaic tile piers epoxied to the concrete (left) - help
guide the pour. An antifracture membrane isolates the tile from
any movement in the structure.
With experience, we've learned to quickly guesstimate the
number of bags we'll need for a certain amount of coverage, but
manufacturers also include a coverage chart with their
technical information and installation instructions. For
example, Laticrete says that one 55-pound bag of Laticrete 86
will cover 6.7 square feet at a nominal thickness of 1 inch.
Our rule of thumb is that a bag is equal to about 1/2 cubic
foot of material.
The trick with any SLU is to get all of the material in at the
same time. That's because there's a limited "self-healing"
period once the material is mixed with water, typically about
30 minutes. Within this time, different buckets of material
will flow and merge together, but once the half-hour is up the
material starts to clump and skin over, then quickly sets up
and becomes unworkable. We've learned the hard way that you
don't want to still be working the material after it starts to
go, so we make sure to stage the job carefully and work
Mixing. There's no need to guess how much water to use
or what the proper consistency should be. Each manufacturer
specifies the exact mix ratio, which we follow precisely. For
example, a 55-pound bag of Laticrete 86 requires 6 quarts of
clean water. When we're ready to pour, we measure out the water
needed for each bag in advance and dump it into a bucket. We
make sure to have a bucket ready for each bag that we need
(Figure 6). If there are any lumps of material
in the bag or if the bag is more than about six months old, we
won't use it.
Figure 6. An exact amount of water is dumped into the
bucket, and then a bag of powder. Each bag gets its own bucket,
and is mixed for two minutes with a 1/2-inch drill equipped
with a special mixing paddle.
A 5-gallon bucket holds one bag of material plus water, but
just barely. To give us a little extra room for mixing, we use
the less-common 6-gallon buckets that some of the other tiling
products are packaged in. After we dump in the powder, we mix
each bucket for two minutes with a drill-mounted mixing blade,
then quickly move on to the next bucket. Hot water will
accelerate the cure, so we always try to use room-temperature
or cooler water. For bigger jobs, we make sure to have several
paddle-equipped drills on hand.
Once we've mixed the underlayment we plan to use, we give each
bucket another quick mix before pouring it onto the floor. We
fill the lowest points first, then work toward the high points,
using our screws or pillars as guides (Figure
7). If we have to move material around, we use a rake
or a spreader. A flat trowel is handy for areas where we have
to feather-edge the material.
Figure 7. Starting with the low spots, all of the
buckets of SLU are poured onto the floor (A). To move material
around, the crew uses a flat trowel-like spreading tool (B), a
standard garden rake (C), and a gauge rake with adjustable wire
Most of our repairs require between five and 15 bags of SLU.
With careful staging, we once mixed up 35 bags for a single
pour on a 300-square-foot floor, but that was about the limit
for our three-person crew. More often, we'll do larger pours in
a couple of lifts, but this requires that we allow time for the
first lift to cure and be reprimed, which adds a couple of days
to the process. For us, it's more cost-effective to sub out
large jobs to a company with a pump truck.
Curing. Air movement and direct sunlight both cause
self-leveling underlayments to start to skin over and
significantly shorten self-healing time. To avoid this, we keep
windows closed while we pour the material, and we tape kraft
paper over them if necessary to block out the sun.
Only a small fraction of the water in the mix is actually
needed for hydration; the rest of the water is there to help
the material flow. As the SLU cures, this water rapidly
evaporates and produces a lot of condensation. This can be an
issue, depending on whether the finish flooring is
water-sensitive and how soon it will be installed, so most
manufacturers offer underlayments with "standard" and "rapid"
curing times. Laticrete 86 cures in three to four hours, while
Laticrete 84 takes 12 to 16 hours. If our tile schedule is
tight, we use the 86 and pay the premium; otherwise we save
some money and use the less-expensive 84.
After a pour, it's normal to see minor spiderweb cracking in
the underlayment. If there's substantial cracking all the way
through the underlayment, that's an indication there was a
problem with the pour.
Though not required, we almost always install an antifracture
membrane over the underlayment before we tile (Figure
8). In addition to providing insurance against
cracking and other problems, this helps make the installation
Figure 8. To install this sheet-style
antifracture membrane, the crew spreads adhesive with a notched
trowel (left), then beds the membrane - already cut to fit -
with a roller (right). Once the adhesive has been cleaned up,
the floor is ready for tile.
On jobs where we can't wait for the multiple coats of a
liquid-applied membrane like Laticrete's Hydro Ban to dry, we
install a sheet membrane like NobleSeal CIS (800/878-5788,
noblecompany.com), a composite made from chlorinated
polyethylene (CPE) with polyester fabric laminated to both
sides. We simply cut the sheet to size, comb out modified
thinset mortar onto the floor, and bed the membrane into the
thinset with a roller. With either a sheet or a liquid
membrane, it's easy enough to waterproof the room by turning
the membrane up the walls and taping the joints. At that point,
we're ready to start tiling.
Right now, Laticrete 84 costs about $21 per bag in my area,
while faster-curing Laticrete 86 costs about $32 per bag. Some
contractors charge a fixed price per bag (say, $125) to install
SLUs, but I use a slightly different approach. I give my
clients a fixed price based on the specific number of bags I
think we'll need, then add bags on a prorated basis if we find
we need more.
Rob Zschoche is a tile setter and licensed remodeling
contractor in Chantilly, Va.