You are not the first person to ask this question recently. The short answer is that indeed, the weather this past year has been less than perfect when it comes to drying of lumber. But let’s back up a few steps.
 
When lumber begins to dry, the surface fibers dry first and begin to try to shrink, while the core remains full size. With the average outside humidity in much of North America averaging 65 percent RH, the amount of attempted shrinkage for these fibers is substantial. For example, about 4 percent with the oak species. Higher humidities that are used in a kiln can reduce this attempted shrinkage. With the difference between the shrinking shell fibers and the interior fibers, stress develops.
 
 
 
If the stress exceeds the strength of the wood (wood is weakest when wet), a check (small crack) develops. This crack will never heal and oftentimes will worsen even with reasonably moderate drying conditions. It is also important to note that the formation of checks occurs at very high moisture levels (above 50 percent MC) and not later, but existing checks can worsen later in drying.
 
 

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What all this means is that when we first begin drying, we need to avoid very low humidities. Certainly, it seems that in the past year, there have been short periods where the humidities were much lower than normal.
 
I recall several days this past summer and even in the fall here in Georgia where afternoon humidities were under 35 percent RH. In contrast, when kiln drying red oak, we start with 87 percent RH and do not go under 40 percent RH for three or four weeks. (I am not sure if this dry condition, followed by really high rainfall, is caused by El Niño or not.)
 
So, what can we do? The general answer is that we need to slow the air drying process and we do this by increasing the humidity around the lumber stack.
 
One specific option is to wrap the drying bundle with plastic mesh or plastic burlap cloth to stop much of the air flow through the bundle. This indeed does work, but what happens if, after a dry period, we get several days of heavy rainfall? The extremely high humidity inside the wrapped lumber stack during rainy periods can create conditions ideal for mold growth; that is, the wrap needs to be removed when it is too humid. Incidentally, the stack should have a roof on top to prevent much of the rain from entering the pile and rewetting the lumber.
 
Another option, and perhaps the best overall, is to dry lumber in an open shed. The shed roof keeps the rain and sun off the lumber; the openness (no walls) of the shed allows drying breezes. When the humidity outside is too low, then plastic mesh curtains (similar to a shower curtain in a bathroom) that can be pulled shut to limit air flow during these dry periods. With less air flow we are able to increase the inside humidity.
 
In addition to controlling checking of a species like oak, a shed also will, by keeping the sun and rain off the lumber, develop lower moisture levels more quickly than normal air drying. This in turn means shorter kiln drying times.
 
When drying a species that needs whiter colors (ash, maple, white pine, etc.), the sheds are equipped with fans (1 hp per 6 MBF) to get even faster drying than normal air drying.
 
If you need more specifics on shed designs and use, drop an email to the WoodDoc.

 

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