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We frequently read about jobs where things went wrong after the fact, but this is a case where the flooring contractor wanted to make sure ahead of time that the wood flooring wouldn't fail. His company and the GC on a large job wanted to be sure the engineered flooring delivered to the job site was at the correct moisture content for the installation. The manufacturer stated its products were delivered at a MC between 7–9%, but the contractor and GC wanted assurance of that before beginning the installation, so they called me, and we ended up testing several thousand square feet of the engineered flooring.
Collecting Info
The flooring was 5⁄8-inch-thick, 4-inch-wide random-length engineered plank with a 5⁄32-inch clear red oak top veneer and a nine-ply Baltic birch core. The installation was to be a direct glue-down onto an old concrete slab subfloor. The flooring was manufactured in a nearby state.
We determined that moisture meter readings would be done on-site of several planks from different stockpiles of flooring delivered to the site. During the conversation the expected long-term conditions within the building became evident—the building, a university campus student dorm, would be closed up for the summer, with the HVAC system shut down. With the building being situated in New England, known for high relative humidity during the summer, we decided to add further testing. In addition to testing current conditions, it was now just as important to learn how this flooring would react under the anticipated summer ambient conditions.
Because of the differences in the readings, we suggested that oven-dry testing be performed with the sample planks.
The MC readings of several planks of varying lengths were taken first using a non-invasive meter from both the top veneer and the bottom ply of the core material. These readings showed an average of 10% MC. A second set of readings was taken using an invasive-pin meter in both the top veneer and the bottom ply of the core material. These readings showed a moisture content range from 5–8%. Because of the differences in the readings, we suggested that oven-dry testing be performed with the sample planks.
Oven-Dry Testing
With the advice of my colleague Howard Brickman of Brickman Consulting, we created a plan for the oven-dry analysis. The two sample planks were cut into six smaller pieces of varying sizes and weighed on a gram scale. They showed weight ranges from 78.6–111.3 grams. The oven-dry method was maintained with a temperature range of 212–220 degrees Fahrenheit, and the samples were weighed every two hours. Normally, weighing during oven drying takes place every four hours, but these samples appeared to be close to dry (as judged by the moisture meter readings). The drying process took 16 hours before the samples stopped losing weight. The finished oven-dry weights showed a range from 74.8 grams–103.8 grams. The calculations of the oven drying showed the MC of the samples at 4.84–7.22%.
Measuring the oven-dried plank widths using digital calipers consistently showed 331⁄32 inches. We expected that the planks might show cupping at their oven-dry weight, but the only sign of stress from drying was splitting of the top veneer at the point of the invasive-pin-meter holes. The lack of cupping may have been due to the sample's size.
The same samples were then tested for size gained during swelling as the MC was increased. All the samples were placed into a steam box at their oven-dry weight, having a temperature of 160–175 degrees F and 92–96% RH for 10 hours. All the samples showed a weight-gain range of 17–20 grams. Calculations with the weight gain showed the MC averaged 18–20%.
Measuring the width of the samples at 18–20% MC showed dimensions ranging from 41⁄32–43⁄32 inches. This shows each plank can swell close to 1⁄8 inch at 18–20% MC. If a room needed 30 planks, or was a 10-foot-wide room, then the overall width dimension of the flooring would be 122.8 inches, or 10 feet, 3 inches.
The planks did not show crowning at the high MC but, again, this could be due to the sample size.
What We Learned
In reality, this testing subjected the samples to much greater extremes in temperature and RH than the flooring would experience in a closed building having no climate controls during July and August. The MC of the flooring during the summer months would be closer to 12%, not 19-20%, but knowing the dimensional change due to extreme conditions helped everyone understand the size gain of the planks. The manufacturer's rep on the job site was very pleased to know how well his product performed.
The testing showed, first, the differences that can be recorded when using non-invasive meter readings versus invasive-pin readings, especially in an engineered product. It was evident that differing plies and adhesive lines can affect meter readings. This does make it difficult to accurately read MC of an installed engineered floor.
The testing also showed the strength and stability of this flooring is in the number of core plies the manufacturer has used. The plies are very thin, the bottom being less than 1⁄16 inch, but the number of plies gives the planks multiple strengths in all directions.
The information was useful to the contractor, as he now had an understanding of what would be needed to install the flooring without experiencing difficulties during the summer months.