Common Moisture Problems in Wood

Wood is an ideal raw material: long lasting, sturdy, easy to shape, join together or glue and is continuously renewed.  Above all, wood is beautiful.  Everybody likes its durability, texture, looks, touch, color and unique design.

Wood is used in many different applications – from structural building components to hardwood flooring, from furniture to toys for kids, from cabinets to musical instruments.

The challenge of woodworking starts with choosing the right wood.  If the wood is not right for the application, the best tools and excellence in workmanship cannot guarantee a high quality product.  Wood species, grade and moisture content determine quality and usefulness of wood.

Common Moisture Problems

A chair with loose joints or a cabinet door that will not close.  A  butcher-block table with a big crack down the middle or a veneered surface that shows surface checks. Edge banding applications where the glue line fails.  A jewelry box with crooked joints and bent out of shape, cupped floors, foggy finishes, etc.

During air and kiln drying moisture defects such as case hardening, honeycombing, split ends or surface checks can occur. If wood is not properly stress-relieved, internal stress within the board can be suddenly released when a board is cut.

Moisture defects are irreversible. Immense stress within the wood permanently deforms or destroys the structure of the wood (cracking, checking, warping, honeycombing).    

Shrinkage between 18% and 6% in Red Oak.

At the beginning of the test both pieces ( 1 1/4” x 4”) had the same size and the same moisture content of 18%. The upper piece dried down to 6%, whereas the lower piece was kept at the 18% moisture level.

Why Moisture Problems Occur

Wood is not a homogeneous material, but structured by year rings, which form a series of more or less concentric cylinders. These cylinders consist of irregular tubes that transport nutrients and support the tree.Interaction with

Air:  When wood absorbs or looses moisture below the Fiber Saturation Point (28%-35%), the tubes expand or contract, causing wood to swell or shrink.  To the woodworkers dismay, shrinking and swelling is often accompanied by warping. Shrinking does not occur lengthwise along the tubes.

Changes in the wood moisture content occur until an equilibrium with the surrounding air has been reached.  At that point the equilibrium moisture content (EMC) of air is the same as the wood moisture content (MC).If wood pieces with different moisture contents were placed for a long time in a constant climate of 70⁰F and 35% relative humidity, all wood would end up with a moisture content of 7% independent of the wood species or the initial moisture content.

Since 70⁰F and 35% – 45% relative humidity represent normal in-home conditions, hardwood floors and furniture built with a moisture content of  6-8% is stable.

Shrinkage and Warping

1a) Arrows indicate radial shrinkage

1b) Arrows indicate tangential shrinkage

1C) Quarter sawn board with straight year rings parallel to the edges

Year Rings:  Each board has a unique year ring pattern (grain), dependent upon its position in the log.

Shrinkage and warpage vary with wood moisture and with the arrangement of year rings.

Shrinkage:  Whenever wood below Fiber Saturation Point looses moisture, it shrinks. By actually measuring the shrinkage in a block of wood, it has been found that shrinkage is not uniformly the same in all directions, but differs with the grain. Tangential shrinkage along the year rings is twice as much as radial shrinkage across the year rings. The different shrinkage factors within the same board cause warping by pulling the board in different directions.

Warping: In most cases shrinking is accompanied by warping. Warping deforms boards by cupping, crowning, bowing, twisting or a combination of all four.2a)   2b)    2c)   2a)    Twisting: Curved diagonally across the board2b)    Bowing: Curved along the length of the board

2c)    Cupping: Curved downwards along the width of the board                                           Crowning: Curved upwards along the width of the board. The drawings above indicate the main deformations of a board. For instance, when a wood floor gets excessively wet the floor planks absorb extra moisture and swell. Since there is not much room to expand problems become obvious very fast.