1-hour flat roof overhang detail - minimal roof thickness desired

[ScottB]

We have a flat roof carport with a two foot overhang which is permitted to encroach into a required yard, however the underside of the roof within the encroachment is required to have a 1-hour fire rating from the underside. The proposed assembly (attached) of the roof is plywood over metal decking with rated gypsum like DensArmor Plus. We desire to keep the roof thickness as minimal as possible, around 4 inches thick max. The closest approved detail for this which we have found is the GA600 File FC 4503 from the Gypsum Association Fire Resistance Manual. This assembly (7.75") is thicker than we prefer. I am hoping that someone knowledgable on this topic can suggest a thinner approved assembly other that the FC 4503. Thank you, Scott

 

[ICE]

In my experience the overhang does not have an Hr. Rating and is usually Class A roofing with drywall and stucco on the fascia and underside. Of course LA City might have a different take on it.

 

[ScottB]

In my experience the overhang does not have an Hr. Rating and is usually Class A roofing with drywall and stucco on the fascia and underside. Of course LA City might have a different take on it.

Thank you Ice for your reply. I posted here in commercial as I believe this group would be more knowledgable in fire rating assemblies. The project is in Summerlin (North Las Vegas). The referenced code is 2018 IRC, and per Table R302.1(1). For Projections (roof overhang); 1 hour on the underside, or heavy timber, or fire-retardant-treated wood.

 

[my250r11]

I have seen a cement board for the underside used also.

 

[Builder Bob]

Hardee Plank (cement Board) does not carry the 1 hour rating due to heat transfer from the fasteners - Use FRTW and you can eliminate any coverning adding thickness....