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Here in the Pacific Northwest, we typically frame our roofs with Douglas fir 2x12s because it's easy to get good-quality pieces up to 24 feet long. But when we need to span long distances, we usually use I-joists for rafters.
Uniformity and length. I-joists are manufactured in 60-foot lengths, so there is no upcharge as the lumber gets longer. Each joist is exactly the same depth and width — no wane, no hook, no defects. And we can order I-joists in 1-foot increments, which cuts down on waste.