Fireproofing 2x12 wood joists

[kxa]

I have a situation where three A/C units were placed on top of a flat roof that has 2x12 wood joists. There is a hung ceiling and any fire could potentially cause the units to fall thru roof. The roof covering is rolled asphalt and the units that weigh about 500 lbs are placed on a couple of pressure treated 4x4's directly on the roof.

This is not how we do it now but 40 years ago, it was probably OK. Now the Town wants n engineer to OK it. The roof framing is OK for the snow loads and the units but have to make sure I get a 1 hour fire protection. Is there anything out there that would do that w/o imposing additinal or at least significant additional loads on the roof structure.

Thanks in advance.

 

[dik]

Encapsulate the wood framing in a layer of 5/8" fireguard drywall; this should provide the 1 hour rating.... or there may be an assembly that readily gives it to you. If you can't seal the area with drywall, can you use intumescent coating of the wood... you might want to talk to a couple of suppliers.

Dik

 

[Ron]

Agree with dik....encapsulate with gyp board.

 

[hokie66]

Agree with dik that you could use gypsum board to achieve a one hour rating, but protecting just the area under the new units is illogical...to me. If the roof burns and falls, it is bad news, whether there are units there or not.

 

[focuseng]

This make no sense to me. The 1 hour rating will be for the "assembly". 5/8 type x will not "fireproof" anything and only provides 40 min. of resistance to the side of fire spread to the actual structural members. Further, the fire rating is usually due to other issues (Building code requirements and fire/flame spread) and not to help protect an HVAC unit. Is this building being remodeled or something?