The Science Behind the Expansion & Contraction of Wood

Why Wood Moves

Like many building materials, wood expands and shrinks. This is due to how much moisture is in the air.  If there’s high humidity the cells within the wood will expand.  If there is low humidity, the cells will shrink. Thankfully there is a way of predicting how much the wood will shrink or expand.  The equilibrium moisture content (EMC) is basically the length of time it takes for your wood to accommodate to its new environment.   It can tell you whether your wood is going to expand or shrink.  The EMC helps us understand the response wood has to relative humidity (RH).  For example a change in Moisture content (MC) will cause a 12 inch wide maple board to expand or contact by at least ¼ inch.  The graph below gives examples of MC versus RH.


Wood, not only moves, because of its environment but because of many other factors.  Here are six distinct factors as to why wood moves.

  1. Width of Material – The wider the board, the more the movement.
  2. Grain Orientation- Flat and quarter-sawn lumber can make a big difference in telling how much your wood will expand or shrink.  Quarter-sawn shrinks and expands roughly half as much as flat sawn.  Flat sawn shrinks and swells less in thickness while quarter-sawn shrinks and swells less in width.
  3. Moisture content of the wood at delivery – You should know the moisture content of the decking material when you receive it.  This is measured using a moisture meter.  If you do not check your moisture meter upon your woods’ arrival then you will not know if your lumber has already began to move.
  4. Humidity inside and outside the structure.
    1. Homes in the US that lack humidity control, typically experience levels of humidity from 25 – 65% relative humidity.  This will cause a 6% change in the MC of the wood.  This means if you have a 12 inch wide board it will shrink or expand by a ¼ inch.
    2. Exterior humidity is different.  Most of North America experiences humidity levels ranging from 60-70% in the summer and winter.  If your material is delivered at 6-8% MC then it will experience more than a 2% change in size as it gets used to the EMC.
    3. Species affects the amount of movement Most species of flat grain will change 1% for every 4% change in MC.  The movement values for quarter sawn lumber are approximately ½ the flat sawn values.
    4. Applied finish won’t stop size change- It will not stop the size change but it can slow the rate of moisture exchange down.  Finished wood will eventually accumulate to EMC levels.

Environment and wood species are the main reason as to why wood moves.  When buying your lumber make sure to check your supplier’s environment and to calculate how much your wood will shift in size.  If you want to play it safe, it’s best to buy your wood at least 3-4 days prior to starting your project, so you can let it get acclimated.

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