Humidity, Lumber, and Wood Panels
Coping With Wood Movement
The structure of a piece of wood has been compared with a bundle of straws. The straws represent the cells and capillaries of the wood. When a tree grows, water and food are transferred through the cells and carried throughout the tree.
Once a tree falls, the free water in the wood is lost rather quickly, but the bound water trapped in the straw-like cell is slower to dissipate. As the level of bound water is reduced, these straw-like cells get narrower in diameter but maintain their length. This is the fundamental reason that wood expands and contracts across its width but has almost no expansion length-wise.
When working with wood, it is essential to allow it to expand and contract with changes in humidity. As humidity rises, the straw-like cells grow in diameter, with the opposite effect occurring as the relative humidity falls.
Ways To Adequately Accommodate Expansion
For the first example, let’s use a cutting board: a solid wood cutting board with a breadboard edge secured with a tongue and groove joint. One advantage has the breadboard edge is attached on both ends with a dowel through the language. The other breadboard edge is connected with the same tongue and groove joint, but this end only has a dowel in the center of the piece.
The end secured with two dowels will develop a crack in the main panel. Since the length of the edge piece doesn’t change, having it fixed across the width of the main board from expanding.
Alternatively, the opposite end of the cutting board is only fixed in the center. The central panel is free to expand and contract toward the ends. The sole visual difference would be that the ends of the edge piece are no longer flush with the visible edge of the main panel.
The Second Prime Example Typically Uses A Large Glued Panel Like A Tabletop
If the tabletop were fixed to the rails across the proper width of the panel, the gentle rise and fall of humidity would cause the board to split.
The most convenient method to overcome this is to use the wooden clip to attach the two key pieces sincerely. First, a 1/4″ x 1/4″ dado is milled on the inner upper edge of the rails. This universally accepts a wooden clip that fits into the dado and is then secured to the underside of the tabletop. This will securely grip the top onto the rails without restricting the expansion and contraction of the broad panel. Without this, the tabletop would slowly start to open up.
Predominantly, when you intentionally set one piece of stock across a large glued-up panel, measures must be taken to allow the board to move. Fondly remember it will progressively expand across its standard width but not its length. Efficiently utilizing one of the two prevalent methods mentioned above, you will be able to typically prevent your glued-up panels from splitting.