Notched Trowel Selection Critical to Proper Adhesive Use

By Ray Thompson, Jr.

Improper Trowel Notch Show-Through
Improper Trowel Notch Show-Through

The success, or failure, of any installation relies heavily on how well the hidden portions of that floor have been prepped. If concerns with the underlying concrete, moisture, and subfloors have not been addressed, then you’re setting yourself up for failure and a guaranteed call-back.

As such, one of the most misused components within an installer’s toolbox is the notched trowel. Today, with pressure-sensitive adhesives, the problem has reached precarious levels. To counteract this problem, the option for rolling or spraying adhesive is available; however, these too have their problems.

This presents a dilemma for manufacturers. If the material has too much texture, then they have a maintenance issue. If it’s too smooth, then they will have a show-through of the substrate and adhesive trowel notch.

The end user, especially hospitals and schools, are of the mindset that the glossier a floor is, the cleaner it is. Many flooring people installers, contractors, manufacturers, etc. ‒ are trying to get the end user to use a satin gloss, such as European flooring end users do, rather than a high gloss polish. Unfortunately, getting them to switch is a slow process.

Now, let’s take a look at some of the problems using the wrong trowel notch presents and the solutions.

Improper Trowel Notch Show-Through

As I mentioned above, some installers do not realize the importance of a trowel’s notch. I have seen attempts to use stand-up spreaders, but unfortunately, they leave puddles and the notching is subject as well. This is just one example of how critical the choice of a trowel notch can be.

Trowel Overlap

Trowel overlap is as big of a problem as leaving a gap in the adhesive. With adhesives using a fine notch, you are leaving about .032″ of the material on the substrate. When it dries to the touch, it shrinks to about .025″ of an overlap on the adjoining spread. Once the overlap dries to the touch, you now have .040″ to .050″ of adhesive. It takes only .015″ to show-through some materials.

Trowel Overlap
Trowel Overlap

Trowel Chatter

Trowel chatter is mostly caused by not carrying enough adhesive in front of the trowel. Rather than applying more adhesive, some installers continue to trowel on, leaving chatter marks behind. Another way issue is trowel chatter marks in the substrate. This is a floor preparation issue.

Trowel Chatter
Trowel Chatter

Improper Floor Preparation

Proper floor preparation is an absolute must for the success of the installation. You must know that adhesive shrinks when it dries. That, coupled with lighting, can highlight subfloor imperfections.

Improper Floor Preparation
Improper Floor Preparation

Expansion Joint Show-Through

Expansion joints should be classified as dormant or active. Dormant joints can be gone over; active joints cannot. The objective is to tell the difference between the two. Ask yourself: is the environment being heated or cooled, and if so, for how long? Also, is the concrete poured and stabilized? Remember, there are two things that affect concrete’s movement ‒ temperature and moisture (either water or humidity).

Expansion Joint Show-Through
Expansion Joint Show-Through

Spray Adhesive

The latest application method is to spray the adhesive, but this method is not without problems. The spraying of the adhesive must be uniform or you will have adhesive show-through, both by too much adhesive and not enough adhesive. To illustrate this point, I once saw a super-flat floor (FF90+) ruined by the spray of adhesive under linoleum.

Spray Adhesive
Spray Adhesive

Rolling Adhesive

Rolling adhesive seems to solve many of the adhesive show-through problems if the adhesive is troweled then rolled with a short napped roller. If you try to eliminate the troweling, then you will get a lumpy effect. Installers claim it takes too long, but if a call back is eliminated, then it is worth it the extra time and effort.

Rolling Adhesive
Rolling Adhesive

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