I need to build a small door between my garage and my attic. The rest of the wall is plaster on button board and is about the equivalent of 5/8" sheetrock. If I doubled up two sheets of 3/4" plywood, would it offer the same or better fire protection?
To lead from a comment you made on another answer
In a hospital I used to work in, the fire doors were made out of wood and were rated at 1.5 or 2 hours of protection as a function of their thickness.
I think I get it now. The doors you speak of do not stop fire dead. It's not about fire protection but more about giving time for people to be able to leave the premises. Fire beats wood. That is a fact. It does however take time. A basic definition from a Quora post:
Fire Door specifications
Fire doors are not just the door itself, it includes the frame, ironmongery, glazing and smoke seal.
Point to take from that is just having wood alone does not constitute fire protection. There is a reason that these types of things have complete codes to define their production, testing, use and safety thresholds.
2 sheets of plywood alone I do not think are good fire protection by any means. The thickness of the wood is not the sole reason it suffices in other fire doors.
If I doubled up two sheets of 3/4" plywood, would it offer the same or better fire protection?
Unless it's plywood treated with flame-retardant, no. Regular plywood has basically zero fire rating. Type X 5/8" gypsum drywall has a 1-hour fire rating.
Why would you want to build a fire barrier out of something flammable?