by Dave Northup
When I started working in the trades some 10 years or so ago,
my first job was with a flooring contractor. I’m now a
general remodeling contractor, and when any of my projects call
for new wood floors, I typically do the work myself.
Customers frequently ask me to install wood flooring over
concrete slabs, both in basements and on slabs-on-grade. I
prefer, when possible, to glue the flooring down to avoid the
“hollow” feeling that a floating floor has.
Changing Industry Standards
Conventional wisdom in the flooring industry has long been
that solid wood flooring should not be installed below grade
and — except for parquet — should not be glued to
concrete. The rules are different, though, for engineered wood
flooring, which is dimensionally more stable than solid wood
because of its cross-laminated construction. If the substrate
is dry enough, gluing engineered flooring to concrete is
usually permissible — even below grade.
In recent years, manufacturers have developed adhesives and
moisture barriers that retard the flow of moisture to the point
where it’s now possible to successfully glue wood
flooring to substrates that once would have been too damp. Last
year, the Wood Flooring Manufacturers Association — NOFMA
— changed its position and began to allow NOFMA-certified
solid wood flooring to be glued to concrete that is on or above
It’s important to read and follow the installation
instructions, because not every flooring product is
NOFMA-certified, not every slab is dry enough, and not every
flooring manufacturer recommends gluing its product to
How dry is dry enough? The job shown
here is from a house in Northern California where I replaced
carpeting with solid bamboo flooring over a slab-on-grade.
Solid bamboo flooring — which is made from narrow strips
of wood glued together — is more stable than most solid
wood flooring, but I still needed to find out if the slab was
dry enough for a glue-down installation. There are a number of
ways to test concrete for moisture (see “Testing for
Moisture in Concrete Slabs,” 6/07); I prefer the calcium
chloride test because it doesn’t require an expensive
concrete moisture meter (see Figure 1).
Figure 1. The author uses the calcium
chloride test to check the slab for moisture. Dry calcium
chloride is weighed, sealed under a dome for a specified time,
then removed and reweighed. The weight gain is used to
calculate the moisture vapor emission rate (MVER) of the
The calcium chloride test measures the moisture vapor emission
rate (MVER) of the concrete, or the number of pounds of
moisture vapor emitted per 1,000 square feet in 24 hours. The
industry standard — which is referred to in most flooring
warranties — is that flooring should not be glued to a
slab unless the MVER is less than 3 pounds. Our slab tested out
at less than 3 pounds — dry enough to proceed with the
Choosing the Right Adhesive
The type of flooring and the condition of the slab determine
which adhesive I use. Also, if the slab isn’t dry enough,
I’ll have to use a moisture barrier. I typically avoid
adhesives that contain water, because evaporation can cause the
flooring to cup. I prefer moisture-cured urethanes, which
contain no water but set and cure in the presence of ambient
Adhesives are formulated for either dry-lay or wet-lay
applications. With the dry-lay method, the adhesive is spread
and allowed to flash (tack up) before the flooring is set in
it. The wet-lay method is faster because flooring can be
installed as soon as the adhesive is spread.
On this project I used Parabond 2010 (800/763-7272,
wet-lay moisture-cured urethane that grabs quickly, is
compatible with bamboo, and does not require rolling, as some
adhesives do. Bostik’s Best (888/592-8558,
www.bostik-us.com) is an
equally good product, though the manufacturer does recommend
the added step of rolling.
Moisture barriers. When the MVER is
higher than recommended for the flooring product, I have to
seal the slab first. In such cases, I like using Taylor’s
adhesive that also functions as a moisture barrier. The first
coat seals the concrete and needs only about four hours of
drying time before the second coat can be applied, for adhering
Other moisture barriers include Parabond’s UMB 3000 and
Bostik’s MVP4, both of which have a longer curing time
— about 16 hours — before the wood flooring can be
installed with a compatible adhesive.
Technical support. Regardless of the
product you use, always read and follow the
manufacturer’s instructions. If you have questions, call
the technical support line; I’ve found the reps often
provide information that is not in the published specs. For
instance, I recently installed flooring on a slab with an MVER
that was marginally high. Technically, I needed a moisture
barrier, but in speaking with the adhesive manufacturer’s
rep (who could have sold me one), I learned that the adhesive
alone would block enough moisture to handle the problem.
Prepping the Job
Before installing any type of wood flooring, it’s
important to bring the material into the house to let it
acclimatize. So I stacked the bamboo in another room and then
removed the carpet, tack strip, and baseboard. Since adhesive
won’t stick to a dirty slab, any paint, concrete sealer,
or old adhesive that’s present must be scraped or ground
away. Fortunately the slab shown here was clean, so I went to
the next step — checking it for flatness with a long
With a glue-down installation, there should be no more than
3/16-inch variation in flatness over 10 feet. Significant
ridges and high spots can be removed by grinding — though
in my opinion the dust is such a serious problem grinding is a
last-ditch measure. Where there are low spots, I fill them with
an approved floor leveling product like Bostik’s Webcrete
95 (Figure 2). After the leveling material sets up, I undercut
the door casings and jambs so flooring will fit
Figure 2. Slab flatness should be within
3/16 inch over 10 feet. High spots can be ground down when
necessary; low spots are filled with a leveling compound
(left). After flattening the floor, the author uses a Fein
MultiMaster (right) to trim jambs and casings so the flooring
will slip underneath.
Careful layout. Before installing
any flooring, I draw the room on a piece of paper, measure the
width of the floor, and use a calculator to figure out how the
boards will intersect walls, openings, and other transitions.
Amateur installers often start with a full-width board at one
wall, then end up with a narrow rip at the other; I’m
always careful to lay things out so the first and last boards
are about the same width. Walls are rarely straight, so I chalk
the first row of flooring.
Racking and Staging
Once the layout is complete, I “rack” the floor
— that is, precut and preassemble it before spreading any
adhesive (Figure 3).
Figure 3. With the first course shimmed
to a layout line, the author starts dry-fitting the flooring
(top left), creating a random stagger by using starting pieces
of varying length. He traces an edge every couple of feet to
mark how far to spread adhesive as he works a section at a time
(top right). He checks layout as he goes (bottom left) and,
after racking the entire floor, stages the pieces neatly
(bottom right) so that they go back down in the same order over
the wet adhesive.
Staggered joints. Bamboo and
engineered-wood flooring typically come in a single length, so
creating a “random” stagger requires some thought.
Because there should never be any cuts in the middle of a
course, I cut only the starting and ending pieces, with the cut
ends going against the walls.
For racking the floor, I first cut several strips of flooring
in two to create the necessary random-length starting pieces.
Then, with an assortment of starter boards on hand, I dry-fit
the floor. The first row goes against the chalked layout line,
spaced off the wall with shims. To avoid excessive walking to
and from the cut station, I’ll lay multiple rows of
flooring before finish-cutting the end pieces.
I often rack the entire room before gluing the first piece.
This allows me to check the layout and cull any warped or
miscolored segments. Irregular milling is common, so I look
closely for pieces that do not lie flush.
Staging. After the floor is racked
it’s time to “stage” the flooring. This means
taking the pieces up in an orderly manner and stacking them in
groups for installation. As I take the floor apart, I trace a
line on the concrete every 2 feet or so — a comfortable
working reach — so I’ll know how far to spread the
adhesive as I lay the floor in sections.
Since the full-length pieces are identical, I don’t
worry about mixing them up. But I take care to stack the
starting and ending pieces in the order in which they went down
so that there’s no confusion about which piece goes where
when I’m laying the floor in wet adhesive.
Using the notched trowel specified by the adhesive
manufacturer, I spread glue out from the wall, working as far
as the first 2-foot line, and bed the flooring in it (Figure
4). Because getting the first row straight and on layout is
critical, I again shim along the wall to hold it in position.
Each successive piece of flooring should butt tight to its
Figure 4. The author spreads adhesive to
a line traced on the floor (top left), then beds the first
course of flooring, placing shims that position it exactly
parallel to layout (top right). Successive courses are laid
exactly as they were positioned when dry-fit
It’s important to get total coverage on the back of the
flooring, so periodically I lift a piece and turn it over to
make sure I’m using enough adhesive (Figure 5).
Figure 5. To make certain he is applying
enough adhesive, the author occasionally pulls up a piece of
flooring to check the coverage (top left). Because flooring can
slide around on the wet adhesive, he uses painter’s tape
to hold the edge and end joints tight until the glue dries (top
right). Squeeze-out is wiped clean with a damp rag
Securing the joints. To prevent the
pieces from moving until the adhesive sets — a common
problem with glue-down flooring — I push the joints tight
and snug several courses together with high-quality
painter’s tape. I also tape the boards end-to-end to hold
the butt joints tight.
After each 2-foot section of flooring is installed, I use a
rag dampened with water or mineral spirits to remove any glue
that finds its way to the surface. It’s best to do this
while the glue is still wet.
I continue spreading adhesive, installing pieces of flooring,
and taping joints together until I reach the opposite wall
(Figure 6). Sometimes, to get the last row to slip under the
door casing, I have to install it before the second-to-last row
Figure 6. Careful layout ensures that the
last piece of flooring is the same width as the first and can
be slipped into place under the door casing (left). Right, the
Once all the flooring is in I do a final scan for glue
residue, then remove the shims from around the flooring. In
rare instances, the adhesive will cause the flooring to swell
slightly; if it’s restrained by shims it could buckle.
After the adhesive has fully cured — normally 16 to 24
hours — I remove all tape and any remaining glue residue.
I finish the job by installing T-moldings at thresholds and
reinstalling the baseboard.
Dave Northup is a contractor in Homer,