Cottonwood: O.K. for rafters?
Cottonwood will work well for rafters as long as you have good ventilation to keep the humidity and moisture down. Moisture will degrade the cottonwood quickly. As a builder, I have seen several homes where cottonwood was used for floor joists over a damp crawlspace and the wood suffered severe damage and was weak and sagging.
Check your building code too. I very much doubt P. Trichocarpa will be allowed for rafters.
When looking at a species, you can compare the clear wood values published in the WOOD HANDBOOK or other sources. However, when considering the clear wood values and then relating them to lumber properties, there is another problem or technical gap. In softwoods, the knots are typically very small and often are tightly held within the wood structure. For some hardwoods (like cottonwood), knots can be very large and the cross-grain (weakness personified!) extend for a much large area. So, hardwoods often are much weaker than softwoods in-use, even though the appearance and clear wood values are similar.
Cottonwood should not be used for roof structures. I encourage you not to be too confident with its strength. This is especially true if the structure will be a dwelling. The building and insurance people are also important, as previously mentioned.
Gene Wengert, forum technical advisor
Another concern with cottonwood is its poor nail holding ability, particularly when it is used before it has dried to its equilibrium moisture content. My son has a metal-sided shed where cottonwood was used for roof purlins, and the roof nails have all loosened, most likely because of thermal expansion and contraction, resulting in a leak at every nail.
According to a list I have from Wood-mizer of the uses for different types of wood, cottonwood could be used for studding, roof decking, and wall sheathing. No mention of it being used for anything structural. Whether the wood is wet or dry, if you don't keep the humidity and moisture completely away, it will weaken. You won't know how the structure has turned out for a few years, when the wood has degraded and weakened. I would reconsider using it structurally.
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