i have a 20 foot roof joists on a flat roof they are 4.5 " x 11 1/4" (wx h0and 5 foot on centerthey span connecting on each end to metal I beams what is the maxcimum O.C spacing i can use for doubled tongue and groove 3/4 inch plywood if it is to be used for the decking and the roof joist is required to carry 80 plf
OTHER STRUCTURAL ENGINEERS
If you can answer the above questions, I'll get you an answer pretty quickly. Also, if you happen to know the species and grade, that would be helpful.
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THE JOISTS ARE CLEAR SPANNING 20 FEET
YES IT IS POUNDS PER SQUARE FEET
THE RAFTERS ARE OVER 80 YEARS OLD AND LOOK LIKE SOME SPECIES OF FIR
THE FIVE FOOT ON CENTER IS THE SPACING OF THE EXISTING ROOF JOISTS
WHICH ARE 4" X 11AND 1/4" BY 20 FEET LONG
THE CURRENT DECK USES TONGUE AND GROVE THAT IS 1&/2" BY 6"
CONSIDERABLE DAMAGE HAS OCCURRED AT THE ROOF PERIMEETER
ROTTING SEVERAL TIMBERS WHICH HAS RESULTED IN CITY VIOLATIONS
Assuming that you have APA Rated Structural I sheathing, 24" is the max for the plywood's sake, and that it kind of pushing it 19.2" is more reasonable.
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