I am attempting to build a timber frame barn and have questions regarding how the roof should be constructed. The barn will be 30' x 48'. The construction is using 5 bents spaced 12' apart. The roof trusses for the bents are 8"x8". The bents will be connected by 4"x6" purlins connected inside the trusses. The purlins are 4' on center. Here is my problem. I planned on using 3/4" shiplap boards to create a deck or subfloor for my roof and then use a vapor barrier and then a metal roof over it all. I am worried that the 3/4" boards do not have enough support under them due to the purlin spacing and they will sag over time.

Am I fine with my plan as is?
Do I need more purlins?
Do I need more trusses?
Do I need thicker underlayment boards?

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