Did You Know? (Lumber)

Posted by in Wood | July 14, 2014

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.