Ceiling installation question. Across trusses or with them?

[jtsboss]

Can't seem to find and answer. If you're sheeting a ceiling in the garage do you go across them or with them? Say 4x8 sheets of foam board. Mine are 4' on center and just have had plastic last few years.

Thanks
JT

 

[PCustoms]

Not sure if I'd use foam board for ceiling sheathing....

What foam? Could be a major fire hazard

 

[E1271]

Usually you'd run sheathing or sheating against the trusses.

 

[AC-WC]

Both of the above answers are correct.
Foam without some kind of drywall is a fire hazard unless you use intumescent paint. 4 feet is too much of a gap for drywall. It will sag, especially if you have insulation above. IF this is a pole barn I would recommend steel roofing. It's plenty strong for adding blown insulation (or any other type) after it is up. Your other option is to add more trusses at 24" OC. $$$$$$