Did You Know? (Lumber)
Some Interesting Effects of Moisture on Lumber
Sawn Lumber
Raising the moisture content in sawn lumber from normal use conditions (moisture content < 15%) to fully saturated decreases the stiffness about 25%, thus increasing deflection immediately by 33%.
Creep is increased deflection that occurs over time when the sawn lumber is under load.
Under dry conditions (equilibrium moisture content < 15%) creep will increase deflection about 50%.
Equilibrium moisture content greater than 15% long term will produce creep greater than 50%.
Creep increases in cyclical moisture changes.
The moisture content must be greater than 28% for fungi growth to be established.
A moisture content greater than 20% is required to support existing fungi growth.
Engineered Lumber
Unless coated, Microlams and Paralams absorb water faster than sawn lumber.
Swelling of saturated Paralam PSL can exceed 10%.About only half of the swelling will be lost during drying.
Swelling of saturated TimberStrand LSL from water vapor is less than 2%.
Swelling of saturated TimberStrand LSL from liquid water can be more than 15%.
I-joists exposed to abnormally high humidity (65% and 95% relative humidity tested) or rapidly changing moisture content have a greater susceptibility to creep deflection than sawn lumber.
I-joists exposed to a humidity increase from 65% to 95% increased deflections 2.2 to 3.2 times the elastic deflection. Webs with OSB in lieu of plywood were higher. These effects can occur in a few weeks. Sawn lumber increased “only” 1.6 to 2.1 times in the same conditions. The moisture content increased from 10% to 20% in these tests.
Manufacturer’s recommend that I-joists submerged for more than 48 hours be destroyed.