To strap or not to strap?

[new land owner]

Just finished helping my brother build a 12 x 12 addition to the back of his garage and he is going to use tin on the roof.  We covered the whole roof with OSB and roofing felt.

My question is should he strap the OSB before installing the tin? It has been suggested to us and we don't know if it is necessary?

[Redoverfarm]

Mine is not.  Professionally installed.  I think generally purlins are mostly used when you do not sheet.  I have looked at a lot of metal sites and most of them say that it is not necessary.  It is hard to seal out insects with purlins whereas with sheeting there is a seal at the bottom that can be purchased to eliminate entry.