Originally Posted by Hohn
I'd stay away from fire-rated plywood. It either isn't very fireproof (the WOOD part of plywood) or it will have tons of chemicals in it that aren't needed to accomplish what you seek.
I'd cover it with the thickest d/w you can find 5/8" or so and be done with it. Plywood makes an excellent base to attach drywall.
However you need to keep this in mind. Plywood delaminates which leads to fire spread. what is above the current plywood? are you attaching directly to the joists?
if you are you now have a void space. any fire in this void space with be able to spread very fast if it is lined with plywood. This fire will also go unnoticed for a while. Depending on what type on construction (balloon frame, platform, lightweight...) a small fire else where will easily travel to these voids and just take off.
do you have floor truss? do you have 2x6? Do you have wooden i beams?
if you have a wooden truss floor or a wooden I beam you might get 5 minutes
of fire on these members before they fail.
Also there is no fire rated drywall. I'll say this again THERE IS NO FIRE RATED DRYWALL
....one more time.....THERE IS NO FIRE RATED DRYWALL did you get that? THERE IS NO FIRE RATED DRYWALL!!!!!!!!!!
what you have a fire rated ceiling assemblies
. These are tested as a whole unit down to the type of nails and tape and if insulation is needed. make one change and you DO NOT
have a fire rated ceiling. you can not take one part of the assembly and say it it is fire rated.....no mater what the a contractor tells you. Drywall can be fire-resistant......this is not the same as fire rated. Sheet metal is fire resistant.....would you use that that a firewall? I hope not.
Don't belive me...... from one of the largest makers of Type X drywall
What is the fire rating of one layer of the fire-rated gypsum panels? A: There is no rating on a board by itself. Fire ratings are allocated only to systems.
a 1/16" gap between the drywall and it is the same as if you didn't have it. Fire will find this path. Light fixture punched in the drywall....yup fire will by pass your so called fire wall. This is why you have to follow the ceiling assembly guidelines and not just toss anything up there. Those metal screws that you decided not to cover correctly.....
Don't believe me? Do it anyway you want.....good chance no one will inspect it very well. This is what keep me busy....nothing like a good garage fire that is attached to a house.
want a good read......buy a fire service building construction book or just about any fire strategy book. They are normally filled with examples like this and the outcomes are never good.
PM me and I'll recommend a few....they will open your eyes....