Moisture Content Equilibrium And Truss Movement
Question:
How long does it take the moisture content to stabilize in a wood truss? I live in southern California. Also, how much deflection should be expected in a scissors truss over a partition wall?
Answer:
SBCA has no information on how long it takes lumber's moisture content to reach equilibrium with its environment, but have heard that new homeowners can expect movement and settling in wood structures for up to two years. Most truss manufactures use kiln-dried lumber, which has a 19% or less moisture content, but trusses are designed with lumber values corresponding to a moisture content greater than 19%. In most parts of the US, including southern California, you can expect that an annual average of 8% moisture content after the building is closed in and finished.
Vertical deflection limits for trusses are outlined by most building codes as l/240 where l is the span of the truss in inches. However, flat bottom chord roof trusses generally exhibit much less vertical deflection than this. Horizontal deflection is usually more pronounced in scissors trusses. According to section 7.6.3 of the Truss Plate Institute's ANSI/TPI 1, the amount of horizontal deflection for a truss is generally limited to 1.25 in. for total load (which in southern California the truss will rarely see). Maximum live load horizontal deflection is limited to 0.75 in. Vertical deflection is addressed in section 7.6.2.