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to answer your question, there is a book called "pocket ref" by thomas j glover. isbn1-885071-00-0. starting on page 50 and going on to page 69, there are maximum floor joist span charts that cover 2x6,8,10,12 in varying lengths and covering different species of wood. however, if you are in doubt, you can always double up the amount of wood that you would normally use. i'm not an engineer, so i would tend to overbuild. hope this helps.