Fire Barrier in Type 5B construction

[PostFrameSE]

Fire walls need to be structurally independent. Fire barriers do not. Fire barriers need to extend to the underside of the roof deck. However, in construction that contains wood 2x4 purlins on edge, that makes for extending the drywall to the roof deck very hard. Has anybody had experience with terminating the drywall at the underside of the purlins and then fire-blocking between them and had building officials approve of that set-up? I have a 2-hr fire barrier requirement on a project and I would be willing to make a 3-ply 2x4 block on top of the studwall (in the purlin plane) so that the fire would need to burn through 4.5" of wood at the top 3.5" of the cavity.

I'd appreciate the group's collective wisdom on this topic.

Thanks.