Cottonwood Lumber In Truss Construction
Question:
I am developing a sustainable community in southeast Iowa. We have an abundance of cottonwoods nearby. I plan to have them sawn into dimensional lumber on the site and dry it there as well. I would like to use this material for trusses. A truss plate manufacturer informed me that because he had no data for cottonwood, he was unable to design trusses using it. Can you offer any advice?
Answer:
There are two steps to using your locally milled cottonwood lumber in trusses: determine the strength of the lumber and determine the grip of the metal plates in the wood.
The first is to determine the strength properties of the lumber. The design values for cottonwood are supplied in The American Wood Council’s Supplement to the National Design Specification for Wood Construction (NDS). The grading rules writing agency that deals with cottonwood is the Northeastern Lumber Manufacturers Association (NeLMA). Contact the association to determine what is needed to grade stamp cottonwood before proceeding. The lumber will need to be graded and grade stamped by a lumber grading agency certified by the American Lumber Standard Board of Review.
The second step is to determine the grip strength in cottonwood of the particular brand of truss plate you will be using. Most plate manufacturers have already tested for more common dimensional lumber species like Spruce-Pine-Fir, Southern Pine, Douglas Fir and Hemlock. Contact your plate manufacturer in case they have this data already.
We also suggest that you contact the truss manufacturer before you cut the wood. The truss manufacturer may be able to determine the size of lumber you need for the trusses (2x4, 2x6, 2x8, etc.). If you are planning to cut and dry the lumber yourself on site, keep in mind the possible shrinking problems with cottonwood. You might find, after weighing the costs, that it’s more economical for you to buy pre-graded lumber.