Product Info   :   Hardwood Stability

Why Stability is Important

When customers are looking at selecting a new hardwood product, one of the last things they probably consider is the dimensional stability. However, this is a very important aspect to consider, especially if you live in an environment that has drastic humidity swings between the seasons. One of the most frequently asked questions by our customers is, “why do my floors have cracks in the winter time”. Hopefully after reading this section you will have a better understanding of what causes these cracks and what can be done to minimize them.

How Moisture Affects Hardwood

Wood is dimensionally stable when the moisture content is above the fiber saturation point –which is typically when wood reaches a moisture content level around 30%. Wood will change dimensions when it gains or loses moisture when it’s below the fiber saturation point. How rapidly it changes depends on a metric called the dimensional change coefficient. The dimensional change coefficient can be used to calculate the expected shrinkage or swelling by simply multiplying the change in moisture content by the change coefficient number listed on the chart to the right.

This example will show how much a hickory floor (.00369) and a pine floor (.00263) will expand when the moisture content changes from 6 to 9 percent across a 15 foot span of hardwood.

Hickory:   3 x .00411 x 180 inches = 2.21 inches.
Pine:       3 x .00263 x 180 inches = 1.42 inches.

Not only do these calculations provide us with an estimate as to how much each floor will expand and contract but it also demonstrates how the pine floor will not expand nearly as much as the hickory floor.  Humidity and temperature changes have a major impact on the moisture content of hardwood floors and its not uncommon for moisture levels to fluctuate 3% or more from season to season. This example demonstrates why you may see cracks in the winter time and swelling or slight cupping in the summer months - especially with less stable floors like hickory.

NOTE: In actual practice the amount of change is diminished on a complete floor, as the planks proximity to each other tends to restrain movement.

What can be done to minimize movement?

NOFMA standards require mills to kiln dry their hardwood planks to somewhere between 6% and 9% but the optimal moisture content level may be higher or lower depending on where you live.  For example, in extremely dry places like Nevada, it’s not uncommon for the wood to have a natural moisture content of 4%.  Alternatively, on the east and west coast you may see normal moisture content levels as high as 13%.  Kansas City residents can expect to see moisture levels somewhere between 7% and 9%.  You can see why the acclimation process is crucial, especially on the coast where moisture levels tend to be much higher than NOFMA standards.  In order for the hardwood to properly acclimate, the job site much have the HVAC up and operating so that the wood can adjust to the normal living conditions.

Plainsawn or Quartersawn
The above picture demonstrates the grain structure of plainsawn hardwoods (Figure A) compared with quartersawn (Figure B).  Expansion and contraction follows the grain direction about 4 to 1, making the quartered boards significantly more stable.

Adding a humidifier to the furnace will introduce moisture in the winter months and help stabilize the humidity levels in your home. Stable temperature and humidity levels will keep the moisture levels consistent in your floor and minimize any movement.

Engineered Planks
The main advantage of using engineered hardwood floors is that they are more dimensionally stable than solid hardwoods. Engineered planks are constructed with several layers of hardwood that are stacked in opposite directions and this helps counteract the natural tendency of wood to expand and contract with different levels of moisture. This added stability makes engineered floors ideal for slab and basement installations