Hardwood Floors & Water Do Not Mix
Uncut trees contain a certain amount of water. The amount they contain depends on a number of factors (species, density and sapwood vs. heartwood, for example). Trees contain liquid or sap in cell cavities (free water). The largest volume is held in cell cavities, but it is the liquid “bound” within the cell walls that gives wood flooring people the most headaches.
When the tree is cut and properly air-dries, most of the free water evaporates, leaving behind water in cell walls. Kiln drying forces most of the bound water remaining from the wood. The amount of moisture left in cell walls fluctuates depending upon the wood surroundings.
When properly dried and marketed, wood flooring contains a small amount of water, this measurement of water is called “moisture content,” which is the weight of the water contained in a piece of wood’s “oven dry weight”
Flooring mills usually try to dry their material to moisture levels compatible with anticipated atmospheric conditions on the average job. This usually means 6 to 9 percent moisture content (NOMFA standards allow up to 5 percent of a shipment to be outside this range, up to a maximum of 12 percent).
However, what may be too wet in one area may be too dry in another. Prevailing atmospheric conditions at some locations advice the use of materials more dry or more damp than the average. Time of year also can make a big difference, as can the method of heating and cooling and the degree of insulation in a given house.
The equalized moisture content is a reflection of temperature and humidity of an environment. In all but the most captive environments, wood is undergoing constant changes from short term as well as long term swings in atmospheric conditions. All wood is continually striving to reach equilibrium in terms of its moisture content. Given a sufficient amount of time in a stable environment, it will do so.
Since all structural environments are subject to changes in temperature and relative humidity, as well as other sources of moisture, we must expect dimensional changes in the wood relative to those fluctuations.
Surface finishes help forestall dimensional changes in atmospheric conditions, but long term seasonal changes are inevitable and must be allowed for in nearly every installation.
The inevitability of dimensional change stems from the fact that wood does not become dimensionally stable when it reaches its “fiber saturation point,” which is approximately 28 percent moisture content for most woods. Unfortunately, moisture contents of 20 percent or greater foster dry rot, so structural wood must be maintained at a lower (dimensionally unstable) moisture contents. Below the giver saturation point, nay increase will result in the wood swelling, and any decrease will result in shrinking.
Wood expands at differing rates according to species. Teak for example, is quite stable in comparison to pine.
The direction and amount of swelling/contraction are functions not only of species but of every board cut. Although most hardwood flooring is plain or flat sawn, other cuts such as quarter/rift sawing or end grain sawing result in different changes in boards with variations in moisture content.
Most swelling or shrinking occurs tangentially to growth rings. About half as much occurs radially and only a slight amount longitudinally. Cupping, bowing, warping, checking and splitting are manifestations of these changes in dimension, which push and stretch the wood in different directions simultaneously.
Any shrinking or movement will open a space between boards. This coupled with the natural shifting of the house and subfloor, may result in some unsightly cracks. Homes in areas with excessive humidity or long heating seasons may require air conditioners or humidifiers to moderate these conditions.
Cupping of finished flooring is a result of an unbalanced moisture content in the wood. It occurs when the back of the flooring absorbs moisture at a much faster rate than the surface. The more rapid moisture pickup on the underside creates greater swelling of that part with the result that a distortion develops in each individual strip causing the upper surface to become concave.
A similar reaction frequently referred to as “cupping” takes place when the individual flooring strips absorb too much moisture from the surrounding atmosphere so that a dimensional change occurs throughout. Since the finish flooring, after it is laid in place, is confined or restricted (by the adjacent pieces and the counteractive holding power of the nails) compression ridges form at a junction of the lateral edges of the individual strips. This condition is also termed “cupping.” It is even more critical since it may remain for an undefined period unless effective measures are taken to correct the unfavorable conditions which are the cause.