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Moisture in Wood
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Moisture in Wood

Moisture Content

Knowing the exact moisture content of the wood you are working with is very important. Using wood that is not at the appropriate moisture content can result in warping, swelling, cupping, splitting, or loose joints in your finished work.

Over 50% of a living tree's weight is moisture. This moisture content is expressed as a percentage of its "oven-dry weight". This means that to determine the moisture content of a piece of wood you must first weigh it, and then oven-dry it until it no longer loses weight. This oven-dried weight is then subtracted from the initial wet weight and the difference is divided by the oven-dry weight.

(wet weight - dry weight /dry weight X 100 = % moisture)

Although this method is the most accurate, it is not practical for anyone other than researchers. Your Timber Check Moisture Meter will give you instantaneous readings, accurate to within 1% in the critical 6 to 12% range.

Moisture in wood is contained both within its cell cavities and in the cell walls. As wood dries, moisture first leaves the cell cavities (free moisture). When the cells are empty, but the cell walls are still saturated (bond moisture), it is said that the wood has reached its "fiber saturation point". This is about 30% for most woods.

Wood will not change dimensionally until it is dried beyond the fiber saturation point. As the moisture is removed from the cell walls, the cells begin to shrink. Maximum shrinkage takes place as the wood dries from 20% down to 10%.

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Seasoning Wood

The object of drying or "seasoning " wood is to bring the moisture content of the wood down to an acceptable level quickly, but gently, to avoid distortion. This results in wood that is lighter, stronger, and less susceptible to splitting, warping, cupping and rot.

There are two methods of seasoning wood, air-drying and kiln-drying.

Air-drying reduces the moisture content to 15- 20%. Usually it takes about one year for every inch of thickness to air-dry hardwoods. Soft woods will air-dry in about half that time. With exceptionally dry weather, air-dried wood may reach 12%.

Kiln-drying can dry wood to any given moisture content in a matter of weeks. Heated air is used to drive out the moisture and steam is used to control the rate of evaporation to avoid distortion and splitting. Dehumidifiers are sometimes used in small kilns as a method of drying wood without heat. This process is much easier on the wood.

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Wood is always susceptible to changes in surrounding humidity levels. It will try to reach a state of equilibrium with the surrounding atmosphere. When lumber leaves the kiln and is stored outside, it will gradually pick up moisture from the air until it reaches an equilibrium level. No matter how old the wood is, it will always respond exactly the same way to environmental changes.

Fortunately, wood responds very slowly, therefore day to day changes in humidity can usually be ignored and only seasonal changes considered.
When green wood has been dried down to an equilibrium level, additional waiting time will not dry it further, but aging wood does have certain advantages. Mature wood will have its remaining moisture more evenly distributed and will therefore be even less likely to distort.

For fine woodworking, lumber should be kiln-dried to a moisture content below the required level and then stored in an atmosphere that will permit it to reach an equilibrium level that will be maintained throughout the building and finishing process.

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An understanding of how wood shrinks is important if fine woodworking is to be undertaken. Wood does not shrink equally in all directions. It will shrink along the direction of its annual rings (tangential shrinkage) about twice as much as it shrinks between its rings (radial shrinkage). There is practically no shrinkage in length.

If a plank is cut tangentially, the greatest amount of shrinkage will be across its width. If cut radially, the greatest amount of shrinkage will be in thickness. Therefore, radially cut lumber is more stable, with less tendency to distort.

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Gauging Wood Movement

The amount of shrinkage varies considerably from one species to another. The included table lists the approximate amount of tangential and radial shrinkage for common species of wood over a 7% change in moisture.

Looking at the table we see that a piece of Ash, 12" wide, dried from 20% down to 13%, would shrink tangentially 1/4" and radially 5/32".
The table also gives you the ratio of tangential to radial shrinkage (T/R ratio). The higher this ratio, the greater the chance of the species "cupping". This type of distortion is also dependant on the direction the grain is running in the plank (cut radially or cut tangentially).

The table also provides an approximate rating for each species which refers to the likelihood of warping and twisting. Generally speaking, species whose tangential shrinkage and radial shrinkage rates are similar are less likely to distort.

To calculate the amount of dimensional change you can expect for a specific piece of wood, just remember that wood movement is directly proportional to width and the change in moisture content. An 18" wide board will move 1 1/2 times more than the amount stated in the table for a 12" board. Also, if the moisture change expected is only 3 1/2%, then the movement will be half of what is listed for a 7% change.

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Wood Rot

Your Timber Check Moisture Meter can be used to check for conditions that may indicate the presence of rot. This is particularly useful when inspecting older buildings for structural integrity.

For wood to rot three things are required, oxygen, heat, and moisture. If these three factors exist, living fungi (rot) will attack the wood.

There are two main types of wood rot. The first is "wet rot" (white rot). This is what is usually seen outdoors on rotting logs and stumps. This fungi feeds on both the cellulose and the lignin content of wood leaving a white, slimy residue.

The other common type is "dry rot" (brown rot). This is mainly an indoor type which feeds on only the cellulose content of the wood. This fungi leaves a brown residue with the wood spongy and deeply cracked.

Since oxygen and heat are usually present, moisture then becomes the deciding factor on whether conditions are suitable for rot. Your Timber Check Moisture Meter is labeled "DRY" from 0 -12%. In this range wood rot is impossible. In the range labeled "WET", >18%, wood rot is inevitable. In the mid-range, 12 - 18%, wood rot is possible but not likely.

These "DRY" and "WET" ranges can also be used for testing firewood. Wood that falls in the "WET" range will burn poorly and provide limited heat.

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About the Timber Check Moisture Meter (model B350) | Moisture in Wood | Pin-type vs. pinless meters | Locate a supplier | Warranty information | Replacement Pin Probes | About us | Contact us

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