i am planing to build a w16xL40xh12 pole barn over my camper and slowly finish it in to a house over time then pull the camper out. im going to use 4x4 post 8' o/c. i think i can go with a 16 foot span useing 2x6x16 joist and 2x4purlins? i dont know if i should put the purlins and joist 2'o/c or 16"o/c? i want to use 3/4 ply wood decking with shingles or maybe i should use 2x8 joist but im on a buget. i just dont want my roof to sag i live in south east texas. no snow im not going to put a lot of slope to the roof. i will have shingles and ply wood on top and later sheet rook on the joist and maybe some a/c duck later. i know that 2x6 will work 2foot o/c with a 16 foot span useing only tin but im going to have decking shingles and later sheet rock. i may have to suthern enginer some trusses lol after all i am a field enginers helper i do basic lay out and off set lines for hi rise buildings should have been a structural enginers helper lol.