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Dealing with wood movement
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Wood,
being a material that was once very much alive, and quite full of
water, never loses it ability to absorb moisture from its
surroundings. When it does absorb water, it expands and when it
dries again, it shrinks. If you do not plan for this movement, your
projects can be weakened, disfigured or even ruined.
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I learned the hard way.
(Do we always have to learn that way?) One of the first projects I
made, was a chest of drawers. I was making it with the remains of
one I found at the dump. The wood was maple and looked beautiful,
but I had a lot to learn. The crack that eventually formed goes
right up the middle of one side, right across the top, and down the
other side. The whole thing literally split in two. The good news
for me is, I learned right away, on that first project, and the wood
was free.
Without going into the details of why wood moves
in the directions it does, I'd like to just present the facts and
some methods to account for wood movement in various projects. This
of course will not be a complete list, and if you would like to send
me your ideas, I can add them to this article and let it grow. I am
still learning.
Wood expands and contracts across the grain significantly more
than lengthwise with the grain.
When joining boards with the grain running at 90° angles to each
other, never glue rigidly all the way across the grain. As the wood
expands or contracts it will weaken or break the bond or if the glue
holds, it could split the wood. Wood of the same species can always
be rigidly joined when their grain runs in the same direction.
When joining wood across the grain, affix the wood rigidly in
one spot (either end or in the middle) and allow the rest of the
board to move. To attach the two together, use a mechanical
fastener, such as a screw, with an elongated hole, which will allow
the wood underneath to move. (See fig. 1, next page)
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