Wood Panels and Humidity

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 by: Dave Markel

Coping with wood movement

All wood workers need 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 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 very important to allow the
wood to expand and contract with changes in humidity. As humidity rises
the straw like cells expand in diameter with the opposite effect
occurring as the relative humidity falls.

Ways to accommodate expansion

Lets use a cutting board for the first example. A solid
wood cutting board with a bread board edge secured with a tongue and
groove joint. One edge has the bread board edge secured on both ends
with a dowel through the tongue The other bread board edge is secured
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 panel from expanding.

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

The second example uses a large glued up panel such as
a table top. If the table top was fixed to the rails across the width
of the panel the rise and fall of humidity would undoudtable cause the
panel to split.

The best method to overcome this is use wooden clip to
attach the two pieces. A 1/4" x 1/4" dado milled on the inside upper
edge of the rails. This accepts a wooden clip that fits into the dado
and is then secures to the underside of the table top. This will
security hold the top hold the top onto the rails without restricting
the expansion and contraction of the large panel. Without this the
table top would slowly start to open up.

In general anytime you are fixing one piece of stock
across a large glued up panel measures need to be taken to allow the
panel to move. Remember that it will expand across its width but not
its length. Using one of the two methods mentioned above you will be
able to prevent your glued up panels from splitting.

About the Author

Dave Markel is the author of "The All Wood Working Journal". He has helped hundreds of individuals improve their wood working skills. Visit his site at http://all-wood-working-plans.com. Subscribe to the All Wood Working Journal at http://all-wood-working-plans.com/wood-working-tips.html.

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