Treated Lumber In Wood Trusses
Question:
Can one use pressure-treated lumber in metal plate connected truss construction? If so, are special plates or coatings required?
Answer:
Whether pressure-treated lumber is appropriate for trusses depends on the preservative used in the treatment process. There are three broad classes of preservatives used for the pressure treatment of wood products:
• Waterborne preservatives, where water is the carrier for the preservative chemicals, serve a wide variety of uses. These include residential, commercial, marine, agricultural, recreational and industrial applications.
• Oil-type preservatives are used primarily for industrial applications including utility poles, piling, posts, glulam beams and timbers.
• Creosote preservatives, including creosote/coal-tar mixtures, protect railroad ties, marine pilings and utility poles.
Typically only lumber treated with waterborne preservatives are used in construction. If the lumber treated with waterborne preservatives is kiln dried after treatment, most of the preservative chemicals are inert and normal plates can be used. The key is drying the lumber and keeping it dry in use. See section 6.4.9.3 of the Truss Plate Institute's ANSI/TPI 1. Design values for treated lumber may require strength reduction, like for incising.