While building the deck I came across something you might want to know. 2 1/2" screws are great for screwing boards inside, but you need 3" screws for screwing on the ends of boards (where warping is most severe) and for the inside locations where vertical warping is present buckling the board from the support. Longer screws seem to have more grip when screwed into the support beams and can apply more force to allow the board to "suck up" the flex, flattening the deck. You'll probably need 10% 3" screws and 90% 2 1/2" screws.
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