The expansion/shrinkage rate of engineered flooring is near 0.5% from green to oven dry for both width and length. With a 4% change in moisture, a 4″ x 48″ long board can shrink/expand about 1/32″ in length and have a .003″ change in width. Compared to a solid oak board would be .010” (<1/64”) in length and .059” (1/16”) in width. This 4% change in moisture can result in noticeable end gapping from shrinkage or end lift from expansion of engineered flooring.
Basically the product should be a consistent size and flat enough to be properly installed per instructions. There is a moisture content requirement of 5% to 9% at time of shipment in order for the flooring to perform in a typical indoor environment. There are exceptions for flooring that is specifically manufactured for arid climates or areas with generally high humidity.
What does this have to do with installation and performance of the flooring? Engineered floor-ing is marketed as being more suitable for environments with more adverse moisture condi-tions such as below grade slabs and basements fall into this category. With these applications the potential is in place for greater moisture change than solid flooring. The construction of the product makes it more stable than solid wood so the flooring should not react to the extent of solid wood where “extra” moisture is present. This is true for width of the product but may not be true for length. Extra moisture can result in end lift or extra drying can result in end gaps. Many instructions call for no acclimation because of the tight fit of the configuration so some moisture change is inevitable. Again, always make a moisture test on the installed product if you expect a significant gain in moisture; some slight spacing at ends may be necessary dur-ing installation. There is little you can do for a significant expected loss of moisture and related end gaps.

Another performance concern with engineered flooring is cracks in the top veneer. During the manufacture of the veneers cracks are inevitable. For rotary peeled veneers the actual cutting and flattening of the veneer will always result in latent veneer cracks or lathe checks. Even with sliced veneers, checks occur as the veneer is bent as the cutting knife slices the log. Fewer checks gen-erally occur as the bending is not as severe as a peeled veneer. After drying, assembly, and finish-ing these checks are filled and are no longer an issue. They can be noticed later if environmental change occurs, particularly when related to shrink-age. Many cracks can also manifest themselves if the finish is not flexible enough to accommodate the movement during seasonal change or acclima-tion. In addition, if the construction is unbalanced, particularly as related to moisture, cracks can oc-cur. The core stock is dryer than the face, as the face shrinks it is restrained by the core and frac-tures occur at the latent checks. Another cause of face checking is wet maintenance. When the floor-ing is wetted the water can soak into the latent cracks causing cyclical expansion and shrinkage as repeated exposure occurs. The finish breaks and the condition becomes worse. This almost al-ways shows at the ends of the boards more than along edges, as the end grain open cells can po-tentially absorb more moisture.
Using a moisture meter on the flooring before in-stallation can determine average moisture content and the expected changes during seasonal cy-cling. Too much change and the flooring will de-velop performance issues such as cupping, crown-ing, gapping, and cracking. In particular, cracking and cupping may occur in arid environmental situations as the flooring acclimates after installa-tion. Many manufacturers recommend that the relative humidity be maintained between 35% and 55% for standard product to properly perform. This equates to 6.9% to 10.1% EMC, an average of 8.5%. In many areas of the country this environ-ment cannot be maintained without installing hu-midification systems during winter heating.
Engineered flooring with an exotic face veneer can also have similar issues to the temperate North American hardwood species. Many are considered quite stable with a low percentage of shrinkage/expansion from green to oven dry. However, since they have a lower fiber saturation percentage, the expansion/shrinkage is more concentrated. For instance Brazilian Tigerwood shrinks approxi-mately 8% from green to oven-dry, considered a low shrinkage factor. However, its reported fiber saturation is about 20%. This translates into more concentrated movement for each percentage change in moisture content compared to temper-ate hardwoods. In addition, exotic species, particu-larly South American woods, may have chemical inclusions within the wood fiber that can create discoloration or finish interruptions as the flooring cycles seasonally. White spots that develop in Bra-zilian cherry after light exposure are an example of this. The adhesive bond performance between the flooring and the substrate is dependent on the proper choice of adhesive, the proper application technique, and the proper installation of the floor-ing. The flooring contractor is generally responsi-ble for these procedures. Clean, dry and flat is the mantra for the substrate. Proper spread rates and working times as the directions instruct are the guidelines for the adhesive. Any condition that is not properly followed can result in an improper bond and related noises and movement. Taking moisture readings of the flooring to determine suitability for the area’s geographic environ-mental conditions and advising the consumer if performance is in question can help with the final in-stallation decision. Consider with caution if the flooring is not at the final projected acclimated condi-tion and how it will perform during the acclimation adjustment. If a significant change is expected, not proceeding with the installation, changing the product, or changing the expected environment to a more suitable condition are options to consider. As stated in many instruction sheets, once the floor-ing is installed you have accepted all aspects of the product and site conditions as being correct. Any later issues can become your sole responsibility.

Special Thanks to JJ Haines for the article