Hardwood Floors December 2018/January 2019

By Dan Cassens & Brett Miller 1

The 4-inch-wide solid oak flooring expanded in width 0.064 inches or about 1/16 of an inch in going from 7.4 percent to 14.8 percent moisture content. This is 1.6 percent and very close to what would be predicted using Wood Handbook (2010) numbers. On the other hand, the engineered flooring expanded 0.02 inches or just 0.5 percent. This low expansion is due to the cross laminates in the base material restraining each other. Thickness swell is another important property. The solid flooring increased 0.013 inches or 1.7 percent as the moisture content increased from 7.4 percent to 14.8 percent, while the engineered flooring increased 0.019 inches or 2.8 percent. Again, Shmulsky and Jones (2016) indicate that if high pressures are used in the manufacturing process, thickness swelling can be more for plywood as compared to solid wood. Thickness swell is likely to be even more in fiberboard and particleboard products. Longitudinal shrinkage is the last property to consider. Normal wood moves very little along the grain, and any movement in that direction is generally ignored. However, it has been shown that juvenile wood in the softwood species can move enough along the grain to be troublesome in the wood truss industry. For both the solid flooring and the engineered flooring, expansion was just a little more than 3/100 of an inch or 0.1 percent. These measurements are for small samples of flooring. Flooring systems often cover larger areas and these small changes can become significant and should be accommodated by design features.

The larger flooring experiment at Purdue University included installing the 4-inch solid white oak and 4-inch engineered white oak over an OSB subfloor (provided by Huber Engineered Woods). Additional materials were donated by Fortifiber, MAPEI, Allegheny Mountain Wood Flooring, and WD Flooring. These panels were installed per NWFA Guidelines by Dean Hultman, Owner, Hultman Flooring (who is

ENGINEERED WOOD

DIMENSIONAL CHANGE

DATE (2016)

MOISTURE CONTACT % WIDTH (INCHES)

THICKNESS (INCHES)

LENGTH (INCHES)

MAY 1 MAY 9 MAY 16 JUNE 10

7.0

3.958 3.968 3.970 3.965 3.978

.699 .711 .714 .703 .718 .019

33.400 33.418 33.417 33.415 33.435

12.4 10.5

9.3

AUGUST 25

14.2

.020

.035

DIFFERENCE

.5%

2.7%

.1%

SOLID WOOD

DIMENSIONAL CHANGE

DATE (2016)

MOISTURE CONTACT % WIDTH (INCHES)

THICKNESS (INCHES)

LENGTH (INCHES)

MAY 1 MAY 9 MAY 16 JUNE 10

7.4

3.983 4.002 4.008 4.001 4.047

.761 .764 .766 .765 .774 .013

33.394 33.410 33.414 33.413 33.425

12.7 11.6 10.2 14.8

AUGUST 25

.064

.031

DIFFERENCE

1.6%

1.7%

.1%

Table 1: Moisture content and dimensional change for solid and engineered rift oak flooring with changes in moisture content.

the magazine of the national wood flooring association

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