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Wood is naturally engineered to serve as a structural material. The tree is fastened to the earth with its roots, the foundation; the trunk supports the weight of the branches, as a column; and it bends when loaded by the wind, as a cantilever beam. Wood's mechanical properties are complex, but if you understand a few basics of lumber strength you can easily size uniformly loaded joists and rafters with span tables. Stiffness and Strength A good set of tables includes a number of variables, the most basic of which are stiffness and strength. A house frame has to resist dead loads (the weight of materials), live loads (the weights imposed by use and occupancy), snow loads, and wind