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Wood Panels and Humidity

Posted on April 13, 2022 by Jim Neely

All wood workers have to be aware of wood movement when building furniture projects. Failure to consider this important factor can cause wood panels to crack or split.

My high school biology teacher compared the structure of a piece of wood with a bundle of straws. The straws represent the cells and capillaries of the wood. When a tree is growing water and food are transferred up through the cells and are carried throughout the tree.

Once a tree is fell the free water in the wood is lost rather quickly but the bound water trapped in the straw like mobile is slower to dissipate. As the amount of bound water is reduced these straw like cells get thinner in diameter but keep their span. This is the basic reason that wood expands and contracts across its width but has practically no growth length wise.

When working with wood it's extremely important to enable the wood to expand and contract with changes in humidity. As humidity increases the straw like cells grow in diameter with the opposite effect occurring as the relative humidity drops.

Ways to accommodate expansion

Lets use a cutting board for the first instance. A solid wood cutting board with a bread board edge secured with a tongue and groove joint. 1 advantage has the bread board edge secured on both ends with a dowel through the tongue Another bread board edge is secured with the same tongue and groove joint but this conclusion just has a dowel at the middle of the piece.

The end secured with two dowels will create a crack in the main panel. Since the length of the edge piece does not change having it fixed throughout the width of the main panel from enlarging.

Instead, the opposite end of the cutting board is only fixed in the middle. The main panel is free to expand and contract towards the ends. The only visual difference is that the ends of the edge piece are no longer flush with the edge of the main panel.

The next example uses a large glued up panel like a table top. If the table was fixed to the rails throughout the width of the panel the rise and fall of humidity will undoudtable cause the panel to divide.

The best way to overcome this is use wooden clip to attach the two pieces. A 1/4" x 1/4" dado milled on the inside top edge of the rails. This takes a wooden clip that fits into the dado and is subsequently secures into the bottom of the table . This will security hold the top hold the top on the railings without restricting the expansion and contraction of the large panel. With no table top would gradually begin to open up.

Generally anytime you're fixing one part of inventory across a large glued up panel steps will need to be taken to permit the panel to move. Do not forget it will expand across its width but not its length. Using one of the two methods mentioned above you'll have the ability to stop your glued up panels from dividing.