This shows two issues. First, the ridges show the rate of spread and whether the correct trowel was used. Second, there is no wood fiber transfer in the adhesive. Good adhesive transfer would have splinters and pieces of the flooring imbedded in the adhesive.

  I have started this article three times and each time hit a snag with what I wanted to say. As I wrote it, I came to the realization that I cannot report specifically about the adhesives themselves, their make-up, or components. I am neither a chemical engineer nor a chemist. What I do know is that there are adhesives available to the wood flooring industry that perform as expected. I can also say that advances in adhesive technology during the last five years or so allow us to use a wood flooring adhesive for almost any wood flooring application. The most frequent use of adhesive is with engineered wood flooring. However, a recent NOFMA directive shows that under specific conditions direct glue down of solid wood flooring on a concrete slab is also an option.

Adhesives work, failure of the adhesive itself due to incorrect manufacture is extremely infrequent. If a bad batch gets out the manufacturer will be informed quickly. It is in the best interest of the manufacturer to take care of the issues promptly. All the manufacturers I know will do this to minimize the adverse fallout.

New formulation technology may be the one area where we have to be wary of some claims. However, in order to progress we have to give these new adhesives a chance. We’ve come a long way since the hot tar days. If we hadn’t tried new formulations we wouldn’t be where we are today with the high performance materials available. Most manufacturers introduce new products to a controlled set of knowledgeable users, and expand and modify the product as success is achieved. From these successes the products are made available to the industry.

The main issue with adhesives occurs most often down stream from the manufacturer. The issues typically involve common sense issues such as--- choosing the correct adhesive for the specific application; reviewing site conditions for acceptance; correcting inadequacies; and following the instructions of application.  Let’s review the basics of using a wood flooring adhesive.

What are some of the items to consider when choosing an adhesive for an application? Area environment can affect the application performance of the adhesive. For instance, moisture curing urethanes may not set up quickly enough in very dry environments and may set too quickly in very wet environments. Adhesives with a water component may affect factory finished flooring by causing cupping. Some adhesives may not be suitable for applications over radiant heating systems. This does not mean these products cannot be used under these circumstances; but, application technique may need to be adjusted for a particular application or environment. Check with the manufacturer. 

The primary function of an adhesive is to affix the flooring to a substrate with a degree of permanency. This means the adhesive must perform in the typical environmental conditions of the area and keep the flooring in place for its expected useful life. To accomplish this, the contractor is responsible for the acceptance of the substrate.

How many times do we have to say the substrate must be, CLEAN, DRY, AND FLAT? I continue to get consumer comments and see inspection reports where the floor is loose and the adhesive has peeled away from the substrate because some “stuff” was on it and simply not cleaned off. Mud, drywall compound, and/or old cut back mastic, etc. are not structural components of the subfloor. They must be removed.

The next consumer comment is that my floor is cupping.  What was the moisture reading of the substrate at installation? Unfortunately, more often than not, the answer is: “The slab looked and felt dry; it was OK,” or “The subfloor was where it should be.” And, flat? “I walked it and it was flat enough.” The flooring moisture content was “OK”.  Meaning no tests were performed but it all looked good.

Give the flooring a chance to perform as it should. As standard procedure on every wood flooring job, clean the substrate; check moisture conditions by an approved test method before installation; and measure the variation in flatness. Then assess the results. Are they acceptable? Unless conditions are acceptable don’t begin installation. Just because the flooring was delivered that day or the builder said it had to be installed today shouldn’t force you to begin installation if there are problem jobsite conditions. If the conditions are not acceptable, inform the responsible parties of the deficiency and the likely result of the deficiency. Follow the conversation with a written note or email to all the interested parties reviewing the situation.

This shows two issues. First, as in the other photo, the flooring was not adhered to the substrate as there is no wood fiber transfer in the adhesive. Second, the adhesive was applied over some “stuff” and did not stick to the primary substrate.

Another adhesive function is to firmly attach the flooring thus minimizing movement of the flooring itself. That is, the adhesive must be strong enough to hold the flooring under any reasonable environmental condition. Flooring is wood and will expand and shrink seasonally. The adhesive must accommodate this horizontal movement while keeping any vertical movement to a minimum. The actual application of the adhesive is critical to this aspect of performance.

Read all the directions for application. Look at the pictures. When site conditions are acceptable, begin installation. Select the proper trowel. Spread adhesive at the proper trowel angle for correct coverage. Spread only as far as you can install during the recommended working time. Use the proper technique for placing the flooring in the adhesive. Don’t “plop-n-slide.” Every now and then pull up a piece of installed flooring to check for proper coverage and adhesion to both the flooring and the substrate. Don’t install boards that are too crooked or warped. Protect the installed flooring from abuse. Clean extra adhesive promptly with the correct cleaner. Don’t wipe the excess with a rag you have already used to wipe other areas.

Remember, if the adhesive is not the expected color or consistency check with the distributor for issues that may be related to freeze thaw cycles or age. Don’t use questionable adhesive. A replacement costs you at least twice as much as the original. Adhesive is not a leveling compound, don’t fill the depression with adhesive. Too much adhesive affects the cure rate and later foot traffic can loosen the bond. This leads us to keep any people off the floor until the adhesive has set. Don’t cover the flooring with a material that slows or inhibits the off gassing or cure of the adhesive. If you put a lid on top an incomplete cure can result affecting overall performance.

Where solid wood is to be placed over a wooden subfloor, NOFMA recommends fastening with mechanical fasteners, cleats or staples, unless specifically directed by the manufacturer to use adhesive. The typical purpose for gluing a solid wood product to a wooden subfloor is to reduce noises where the flooring has a too loose tongue and groove fit. The issue with this procedure is that when repairing an area or removing a problem board, the subfloor is significantly damaged. The irregularities created can be very difficult to flatten when the repaired boards are replaced. This damage can also affect the integrity of the subfloor system if damage is excessive.

Final thoughts;  adhesive belongs on the bottom of the flooring not on the face. Customers should expect a clean floor. The occasional hollow sounding area is considered “OK” as long as the performance of the flooring is not affected. The flooring should not move, pop, or creak in these areas. When trying a new product, don’t select a 3,000- square-foot job as a test floor. Select the small application so you can spend the extra time and attention required and if a problem occurs, the repair cost will not break the bank.