How to Glue Down Engineered Hardwood on a Slab

Why this matters

A glued engineered floor on concrete is only as good as the bond under it, and the bond is the one thing nobody can see once the boards are down. A slab wetter than the adhesive or the flooring will accept, a section spread too far ahead of the crew, or a bowed board nobody weighted all leave hollow spots that crack the finish, click underfoot and eventually let boards lift. The fix for any of them is cutting boards out of a finished floor. The steps below are ordered so the bond is checked while it can still be changed.

What you need before you start

• Two documents on site: the flooring maker's installation instructions and the adhesive maker's data sheet. Each states its own slab moisture limit, trowel notch, working time and cure time, and the job has to satisfy both.
• Slab test results by ASTM F2170 in-situ RH or ASTM F1869 calcium chloride, whichever method the two documents name. See related: Slab Moisture Testing for Flooring, and How to Run an In-Situ RH Test on a Slab.
• A flat, clean, sound slab. Flatness is owned by Floor Flatness Tolerances by Floor Type; grinding and patching by How to Grind Highs and Patch Lows Before a New Floor.
• Acclimated flooring, meaning it has reached in-service conditions with the HVAC running, checked with a meter (see related: How to Acclimate Flooring and Confirm It Is Ready).
• The trowel the adhesive maker names, a straightedge or chalk line, low-tack tape, weights (boxes of flooring work), the adhesive maker's cleaner, nitrile gloves, and clean rags.

The steps

1. Hold the slab readings against both limits

Write each test reading next to the flooring maker's limit and the adhesive maker's limit. The lower limit governs, and where one document lets you go higher with a named moisture-control system, the floor only goes in at the higher figure when that system is applied and the other document accepts it.

Why first: everything after this step is wasted if the slab fails. What breaks if skipped: adhesive bond loss and board cupping from moisture below, which no warranty covers when no reading was taken.

2. Prime or apply the moisture-control system if required

Some adhesives call for a primer on dusty, porous or ground concrete; others call for a moisture-control membrane above a stated RH. Apply it at the coverage and cure time the adhesive maker states and do not spread adhesive until that cure time has passed.

Hazard: primers and membranes may be solvent-borne or isocyanate-bearing. Read the SDS, ventilate the room, keep ignition sources out where the SDS lists flammability, and wear the gloves and eye protection it names. Any concrete grinding to open the surface before priming is a silica task; see related: Respirable Crystalline Silica and the Construction Standard.

3. Snap the line and set a starter

Lay out per How to Lay Out a Starting Line for a Plank Floor. On a slab, a straight starter board held by weights along the line works without drilling. If you pin the starter into the concrete, drilling concrete is a silica task: use a drill with a shroud and vacuum per the Table 1 entry in 29 CFR 1926.1153, and fill the holes before they are covered.

4. Spread only what you can set inside the working time

Use the notch the adhesive maker names for this flooring width; see related: Trowel Notches, Open Time and Adhesive Transfer. Hold the trowel at the angle the sheet shows, so the ridges are full height. Size each section to what the crew sets in well under the working time at today's room temperature and humidity, because warm, dry air shortens it.

What breaks if skipped: adhesive that has skinned over does not wet the back of the board. It looks like adhesive and bonds like none.

5. Set the boards into wet adhesive

Set each board down into the ridges rather than sliding it a long way through them, close the joints with hand pressure or a tapping block, and keep the expansion space at walls and fixed objects (per the maker, commonly about 1/4 to 1/2 inch, hidden by base or shoe). Work so you never kneel on boards that were just set; use a kneeling board on the set floor if you must.

6. Check transfer on the first boards and every section after

Within a minute of setting, lift one board and look at its back and the slab. You are looking for adhesive on the back across the full width and at both ends, with ridges collapsed rather than standing. If the back shows only ridge tops, the board is not bedding: the notch is worn, the section was open too long, or the slab is out of flat there.

7. Strap and weight anything that wants to move

Engineered boards with a slight bow or crown lift at ends or edges before the adhesive grabs. Run low-tack tape across joints where ends lift, and set weights on bowed boards until the adhesive's initial set. Pull the tape within the time the flooring maker allows, because some tapes mark factory finishes if left on.

8. Clean adhesive off the finish right away

Wipe any adhesive off the face immediately with the cleaner the adhesive maker names and a clean rag, turning it often. Urethane adhesives are close to impossible to remove from a factory finish once cured, and the wrong solvent can dull the finish. Wear nitrile gloves for every wipe; uncured urethane on skin is a contact and sensitization route.

9. Protect the cure

Keep traffic off for the time the adhesive maker states, then light traffic only until full cure. No furniture or rolling loads until the sheet allows.

Worked example: a condo living and dining room on grade

Floor: 640 sq ft on-grade slab, engineered oak 7 inches wide, factory finished. Adhesive: a moisture-cure urethane. Both documents name ASTM F2170.

Testing: F2170 calls for three tests in the first 1,000 sq ft, so three probes went in, set at the depth the standard requires for this slab, and were read after the equilibration time the standard requires.

The limits below are what this job's two sheets stated, shown to illustrate the comparison; your products will differ.

SLAB RH LOG    unit 4B, living/dining, 640 sq ft
probe  location            RH pct
  1    near patio door      81
  2    center of room       83
  3    by kitchen wall      87

limits read from this job's sheets
  flooring sheet, no membrane ........ 85
  adhesive sheet, adhesive alone ..... 90
  flooring sheet, with the adhesive
  maker's membrane applied ........... 95

governing limit without membrane: 85 (lower)
probe 3 at 87 is over 85: FAIL
with membrane, governing limit: 95
highest probe 87 is under 95: PASS

Reading the log: two of three probes were under 85 percent, and one at 87 was over. One failing probe fails the room, because the floor is one bond and the high reading by the kitchen wall is where the moisture is. The adhesive alone would have accepted 87, which is exactly the trap: passing the adhesive's limit does not pass the flooring's. The crew applied the adhesive maker's membrane, waited the cure time on its sheet, and proceeded under the 95 percent limit the flooring sheet allows with that membrane.

Sections: the two-person crew sets about 100 sq ft per hour, which is 25 sq ft per 15 minutes. The adhesive sheet gave a working time of 45 minutes at the room's conditions. The crew spread sections they could set in about 30 minutes, so 50 sq ft at a time: three rows of 7-inch boards is 21 inches, and 21 inches is 1.75 ft, so a strip 1.75 ft wide by 28 ft long, or 49 sq ft. That leaves about 15 minutes of margin for a cut or a problem board.

Transfer: the first lifted boards showed full coverage. In the second strip a cut at the kitchen wall held the crew up, and the board lifted at that end showed adhesive only on its ridge tops: that end had been spread about 40 minutes before the board went in, close to the 45-minute working time, and had skinned. They scraped the skinned adhesive, re-spread that end, and cut the strips to 20 ft (35 sq ft) for the rest of the job so a held-up cut could not eat the margin. Nobody lifted a board at the kitchen end of the first strip.

Strapping: eleven boards had a crown at one end; each got tape across the end joint and a carton of flooring on top until the next morning.

How to verify you got this right

Tap test once cured. After the full cure the adhesive maker states, tap the floor on a tight grid with a coin or the handle of a screwdriver, or drag a short length of chain across it, and listen. A solid floor sounds dull and even. A hollow spot sounds higher and louder. In unit 4B the tap test found one hollow area of about two boards at the kitchen end of the first strip, the one end nobody had lifted a board at, where the same kind of delay had happened unnoticed. One transfer check per section is a minimum, not a sample size: check the far end of every strip too. Small hollow areas are repaired with the adhesive maker's injection kit; larger ones mean replacing boards.

Also confirm:

• The expansion space reads at every wall, the kitchen cabinets and the door casings, and no base or shoe is nailed into the floor.
• No adhesive haze on the finish when you stand and look across the floor toward a window.
• Joints closed and end joints staggered to the maker's minimum.
• The RH log, the membrane's cure time and the sections are in the job record, with the probe locations drawn on the floor plan.

What changes the call

• Over radiant heat in the slab, many makers restrict products and require the heat run and then reduced before install. Read that section; it can rule out the product.
• Below grade, the flooring sheet may require the membrane regardless of the reading. See related: On-Grade, Below-Grade and Above-Grade Floors.
• Solid hardwood is usually not glued to a slab without the maker saying so in writing. See related: Nail-Down, Glue-Down or Floating: Choosing the Method.

References

• ASTM F2170, in-situ relative humidity testing of concrete slabs, as named by the flooring and adhesive makers
• ASTM F1869, calcium chloride moisture vapor emission testing, where the makers name it instead
• 29 CFR 1926.1153, respirable crystalline silica in construction (Table 1 for drilling and grinding concrete)
• The flooring maker's instructions and the adhesive maker's data sheet and SDS
• See related: Trowel Notches, Open Time and Adhesive Transfer