The Effects of Moisture on Wood

By Dan Martel, P.E. | August 1, 2020

We all know that when trees are alive there is natural moisture held in the wood, a porous material. But did you know that the process of manufacturing wood used for structural purposes includes baking the moisture out of the wood? The moisture content in living trees can be as high as 200%, meaning that the weight of water in the tree is twice the weight of the solid material! When the moisture content of most wood species exceeds 30%, the wood is weakened. So, when structural wood is produced, it is dried, or seasoned, to a moisture content below 30% so it becomes stronger. That’s why common lumber is described as Kiln-Dried.

After structural wood is in service, it will continue to absorb and exude moisture with environmental changes, typically relating to moisture contents of 7%-14%. That is particularly true here in the northeast, where summers are humid and winters are very dry. When wood absorbs moisture it swells, and when it exudes moisture it shrinks. Over time we expect wood to dry, especially true for older, larger timbers. “Checking”, or cracking, is one symptom of non-uniform drying, and it is common and typically not structurally significant. Below is a case of checking that occurred in a large painted timber because the outside of the beam shrunk faster than the inside. In this case the beam was overloaded, so the checks turned into cracks in other portions of the beam that needed to be remedied.

You may be experiencing checking in large wood members in your building. Let us help you determine if the strength has been compromised.

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