Twists and Cracks: Wood Movement Re-Visited

Volume 16

I think it’s time to discuss the effects of moisture movement again.  This was the topic of the very first Wood Works Newsletter, but even though Volume I is readily available from our “Library” page on our website, it still comes up more frequently than any other. This is actually not entirely unpredictable!  I remember years ago talking to one of my most reliable academic sources about this subject, and at one point he said, “moisture movement and its subsequent manifestations of warp and splits (checks) probably accounts for 90% of wood performance issues.”  Four decades later, his words still ring true.

With Autumn upon us and Ol’ Man Winter not far behind, “cracking, checking, bowing, warping, splitting, cupping, shrinking, and growing” are words that will dominate our vocabulary, usually accompanied by a little cussing.  Soon we will be shuttering our windows, sealing up our doors, and cranking up the heat to counter any Arctic chills blowing our way. It is at this time that most of our problems best described by one or more of the foregoing barrage of “ing” ending nouns occur.  There is always—always a reason for this:  moisture in wood moves.  That’s it.  That’s the reason.

Now comes the “rest of the story”

When we get cold in an environment we can change, we will.  We will make it warmer.  Usually, this involves nothing more than punching a couple of buttons or icons on our thermostats.  That can be done in the home or office where a wood product may be placed in service, and THAT may be bad enough.  We can do the same thing in the work environment where the components for that product were fabricated into the cabinet, book case, or wall system in question.  What does that have to do with moisture moving in wood?

Wood is a hygroscopic material.  If you research the many resources available on wood movement and moisture, you will find pretty much the same words spoken in pretty much the same way, as it is difficult to state this much differently.  All it means is that wood freely gives off or takes on water at the molecular level as ambient conditions change.  The amount of water and the rate of exchange depends on the amount of water in the air and the temperature of the air.  There is always water in the air around us.  We generally say the air is either dry or muggy, but what we mean is the relative humidity of the air is either low or high.  Relative humidity is the actual amount of water in a given volume of air spoken as a percentage of the total amount of water that volume of air COULD hold at that given temperature. Because the warmer the air, the greater amount of water it can hold, heating cold air increases the amount of water it can then hold.  If the amount of water in the air remains constant, because that same volume of air can now hold MORE water, the RATIO of the constant water to the amount of water the newly heated air CAN hold (in other words:  relative humidity) goes down. If you jack up the heat without any means of adding water back to the air, the relative humidity plummets, leaving you with warm but very dry feeling air.

Ok…ok.  I know that sounds confusing enough, but it still isn’t the whole story.  This process of taking on or giving off water takes place through a process known as hydrogen bonding.  According to The Textbook of Wood Technology by Panshin and De Zeeuw, 4^th Edition, McGraw-Hill, “the forces of attraction between dry wood and water are so large that it is impossible to prevent. Conversely, energy (heat) must be supplied to wet wood to remove any water that is present.”  As water enters or leaves the cell walls of wood, the microfibrillar framework expands or contracts laterally in proportion to the amount of water that is introduced or discharged.  Roughly translated, this means that as wood takes on or gives off water in this manner, the actual volume of wood increases or decreases accordingly.  The result in physical terms is the wood shrinks or grows according to how much water move through it.

And what does that mean, again?

As covered in Volume I, the major complaints generated by this known, predictable property of wood are warp and splits.

Warp  is any deviation from a flat plane.  In other words, if any part of a panel or component does not touch a flat surface upon which that component laid, then that component is said to be warped.  This condition usually happens if the affected part is exposed to different conditions on different surfaces.  A great example of this is the tendency of the top sheet in an open bundle of plywood to lift up on the ends.  The top of that sheet is exposed to surrounding air, while the back is in intimate contact with the environment to which it has acclimated.  The surface flashes off moisture while the bottom of the panel retains its moisture, resulting in the all too familiar bowing we have all seen.  This force can be so great as to literally pull through fasteners or shatter glued joints (often pulling wood fiber in the process), especially where no provisions have been made in the design of the affected component to compensate for naturally expected movement.

This photo was taken at a very nice restaurant in a recently revitalized business district in a popular city with a climate generally absent of extreme weather, especially cold weather.  Silly question, but do you think this open joinery would have been approved if it were present in the final walk-through?  That leaves only one other possibility…it was tight at the time but has sense opened, and moisture movement is the most likely culprit.

Splits, also called checks, or cracks, occur when the forces generated by moisture movement are so powerful and abrupt as to literally shatter the natural bond between wood fibers.  It is for this reason that logs on a veneer yard are kept wet and their ends sealed before processing in order to remove the water under controlled conditions.  The same can be said of logs destined to be processed as dimension stock.  Understanding the optimum time/temperature/species recipe and adjusting the time in and temperature of a veneer dryer or lumber kiln means the difference in stable wood or wood with twists and cracks everywhere.

Life after the mill:

Solid wood as well as wood veneered panel products are subject to movement after manufacture, regardless of the quality of the process that produced them.  Unfortunately, these are almost always due to conditions outside the producing mill’s control, including transportation, storage, fabrication, finishing, finished goods handling, storage and transportation, and even in-service conditions.  Loss of control over the environment at any of these stops along the way may create conditions that result in splits, surface veneer checking, especially in maple, but also in other species such as oak, birch, or cherry to name a few.

Ok, so how do we avoid warping and checking? 

There are many things that can be done, but the most important (as copied from Volume I) are:

• Ensure proper gluing instructions are followed
• Avoid lengthy warehouse time at the manufacturing facility
• Avoid lengthy delivery times
• Store wood in climate controlled warehouses at the distribution level and fabricator site
• Allow panels to acclimate to any new environment prior to processing
• Where possible, stack components face to face on uniform thickness pallets or runners and weight the stacks down with a reasonable weight between processes
• Maintain operating conditions between 30 and 60% relative humidity and 65 degrees to 75 degrees F in production and service
• Avoid installing any finished product on site until the heating and cooling systems are up and running, and even then allow the panels or finished goods to acclimate to the new environment before installing
• Seal all surfaces equally
• Allow for linear expansion in the design of the finished product – avoid nailing, screwing, or gluing panels in place wherever feasible, opting for free hanging installation utilizing “Z” clip hangers or equivalent, especially on thicker panels.
• Make sure any drywall, mortar, concrete, or plaster surfaces are completely dry and sealed before installing panels, cabinets, or fixtures

Unfortunate for us all is the fact that even given the predictable nature of moisture movement and its effect on wood, like lightning, it never seems to strike twice in the same place.  Perhaps this explains the common rebuke, “I have been working with wood for ____(your number here) years, and I have NEVER seen this problem before.”  While that may be true in rare cases, and my heart really does go out in all cases to those dealing with the problem, all I can say is, “I have been around for 44 years and I have NEVER seen a season come and go without a great number of inquiries as to the cause of wood splitting, checking or warping.”  I wish it wasn’t so.

‘Til next time,

 

 

 

 

 

 

 

 

 

 

 

 

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