Mystery Spots in Engineered Wood Flooring

wood flooring spots

When you install new engineered wood flooring, you don’t expect to wake up one day and find spots appearing throughout the floor. But that is what happened to a floor in a suburban home here in San Diego. The homeowner found me on the Internet and hired me to figure out what happened.

The floor had been installed professionally using the glue-down method over a concrete slab foundation. At first, everything was beautiful! The gorgeous natural hickory engineered wood flooring (veneer over plywood) made the family room look great. Then, slowly, over a period of several months, small light gray spots began appearing. At first, they were barely noticeable. The spots slowly grew in size and darkened until they were highly visible everywhere in the room. That is when I was asked to inspect the floor.

Instead of explaining what happened, I have duplicated my inspection report here. It explains what happened, what I saw, and my conclusion.


I found individual planks with irregularly shaped gray discolorations in random areas of the room. These gray “spots” range in size from 2”- 14”. They appear to be just under the top wear layer of each plank.

I determined that the floors are flat length and width to 3/16” across a 10’ span (in tolerance). End joint stagger is greater than 6” (in tolerance) throughout the installation.

I found that the flooring is locked in (no expansion gap) at the front entry hall tile/ family room molding and at both patio door slider jambs.

One plank has been removed by the loveseat, exposing the concrete subfloor. I used a Tramex Concrete Moisture Encounter to take a moisture reading of 5% (high).

I used a pin meter on uninstalled planks and found readings below 9% (normal).

I used a Drieaz non-destructive moisture meter (scale: 10-20) to take multiple qualitative subfloor readings throughout the room. All readings were 20+%.

I used a Tramex hammer pin meter to take a quantitative moisture reading through a plank that exhibits a large gray discoloration. The readings were: surface=22.3%; mid-plank=29.4%; plank base=32.1%.


According to the manufacturer’s Installation Guide, certain requirements have not been followed:

HVAC systems must be fully operational at least 7 days prior to flooring installation, maintaining a consistent room temperature between 60-75 degrees and relative humidity between 35-55%.

  • During the final pre-installation inspection, sub-floors must be checked for moisture content using the appropriate metering device for wood and/or concrete.
  • Hardwood flooring must acclimate for as long as necessary to meet minimum installation requirements for moisture content. Always use a moisture meter to monitor the flooring and job-site conditions as they acclimate, until the wood is neither gaining nor losing moisture.
  • Measure the moisture content of the concrete slab using a TRAMEX concrete moisture meter. If it reads 4.5% or above, then this slab must be checked using calcium chloride tests. Flooring should not be laid if the test result exceeds 3 lbs per 1000 sq. ft. of vapor emission in a 24-hour period. Please follow the ASTM guideline for concrete moisture testing.
  • As an alternative method of concrete moisture testing, In situ relative humidity testing may be used. Reading shall not exceed 75% of relative humidity.
  • Expansion space shall be left around the perimeter at least equal to the thickness of the flooring material… leaving appropriate expansion space around all vertical obstructions.

In addition, the NWFA (National Wood Flooring Association) states: “There should be no more than 4% moisture content difference between properly acclimated wood flooring and subflooring materials, taking into consideration normal living conditions and equilibrium moisture content (EMC).” This flooring exhibits a moisture differential between the surface and subfloor of 9.8%.

The adhesive used was listed as “Mapei Ultrabond Eco.” Mapei manufactures four types of Eco adhesive for wood flooring installation. Two of them are not suitable for subfloors with high moisture levels. However, XXXX Wood Flooring, Inc. recommends using Bostik’s Best/ BST or DriTac 7500 & 7600 adhesives.

The manufacturer also states in their warranty: “Moisture content over 4 pounds…from electronic moisture meters… require a moisture barrier. Deco-Rez moisture guard (2 part epoxy) and Bostik’s MVP are recommended.”

I cannot specify the exact cause of the gray discoloration in the wood flooring. However, it is my opinion that it is a direct effect of high subfloor moisture levels that have intruded into the wood flooring and is installation related only.

Glenn Revere IICRC #113098


This is a rare type of complaint. In this case, the moisture caused a change in the color of the wood. But moisture related problems are very common with wood floors. That is why EVERY wood flooring manufacturer REQUIRES a moisture test on the subfloor whether that floor is concrete or wood. Moisture related problems are good for one thing: they keep flooring inspectors like myself working!

Who is Glenn Revere?

Glenn Revere has been a carpet expert since 1973. He’s a certified flooring inspector and the author of All About Carpets, the only book written to protect and inform you about your carpet choices, from carpet buying and carpet warranties to carpet care and maintenance. You can find him on Facebook, Google+ and Twitter.


Photo: Glenn Revere

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