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DEFEATING DEFLECTION Most residential and many lightcommercial floors are built using woodframe construction. This usually consists of joists with a plywood subfloor, but occasionally long-span parallelchord trusses substitute for joists. Prefab plywood I-beams are starting to appear as an alternative to both. Sometimes heavy plank-and-beam construction is used. Such floors are rarely designed using engineering formulas. Instead, builders use seat-of-the-pants guidelines or span tables (after which, the plumbers come along and make Swiss cheese out of everything). Yet this can produce squeaky or bouncy floors, which are a frequent source of callbacks. In this article you'll learn how to design a proper wood floor without recourse to an engineer. How Floors Behave It is first necessary to know how floors behave. A typical joist, resting