Trusses Exposed To The Elements
Question:
We are having a house built and on a recent visit we brought along a friend who builds houses for a living. He noticed that the trusses were wet and a bit moldy. The wood seemed warped, brown, and had white splotches on it. We are worried that it would later make the roof uneven. How could we tell how long the trusses have been exposed to the elements? What options do we have from here? Do we request new trusses, or can these be repaired?
Answer:
If the metal plates are still embedded in the truss, you should not worry about the look of the lumber (i.e., brown with white spots). The metal plates will work their way out of the lumber from repeated wetting and drying cycles, so they’re the first thing you can look for to determine whether the trusses have been over-exposed to moisture. As far as uneven rooflines are concerned, this is most often caused by incorrect installation of trusses, not the moisture content.
That said, the moisture content of the wood members does have an impact on the overall integrity of the truss. Fungus and mold will grow on the trusses if the lumber moisture content exceeds and remains above 20%. This growth will cease once the moisture content drops to 19% or less. If the wood used in trusses is kiln-dried, the moisture content is equal to or less than 19%. Once the trusses are installed, the moisture content of the wood will drop to its in-service moisture content, which ranges from 8-12%, depending on where you live. Unequal shrinkage during the drying process and inherent lumber characteristics produce warped lumber; the trusses should not become warped due to being left out on the ground for a week.
During long-term storage, trusses need to be protected from the environment and moisture gain. Coverings like tarpaulins should be left loose to allow for ventilation. When storing the trusses horizontally, blocking needs to be used on eight to ten foot centers, or as required, to minimize lateral bending and moisture gain. As far as how long trusses can be exposed to the elements, a ballpark number is about a month. For more information about mold contamination and how to prevent it, see SBCA’s TTB – Mold on Wood Structural Building Components and TTB – Builder Advisory on Mold.