Is gypsum underlayment fireproof?
No. Gypsum underlayment is not fireproof, and the spec writers who detail fire-rated floors do not use that word, because nothing in a building clears the bar it implies. Gypsum underlayment is non-combustible and fire-resistant, and it is a working layer inside fire-rated floor-ceiling assemblies. That distinction is not pedantic. It decides what you can claim on a submittal and where the fire rating comes from.
This piece sorts the terms that get conflated, then shows where the rating actually lives.
Fireproof is not a spec term
In building code, “fireproof” is not a classification, and the people who write fire specs do not reach for it. The terms that carry meaning are non-combustible and fire-resistance rated, and they describe different things.
Non-combustible is a material property. A material is non-combustible when it does not ignite or contribute fuel to a fire, tested per ASTM E136. Gypsum is non-combustible. It does not burn.
Fire-resistance rated describes an assembly, not a material. A floor-ceiling assembly earns a rating, one hour or two hours, by surviving a standardized furnace test, ASTM E119 or UL 263, for that duration without passing fire or excess heat to the far side. The rating is how long the whole assembly holds, tied to one specific tested design.
Fireproof would mean a material fire cannot affect at all. Nothing in a building meets that bar, gypsum included. Under enough heat for long enough, gypsum loses strength and breaks down. The honest claim is that gypsum buys time, and the rating is how much.
Why gypsum slows a fire
Gypsum is calcium sulfate dihydrate, and roughly 21 percent of its weight is water chemically bound inside the crystal, per the Gypsum Association. That water is the mechanism.
When the gypsum heats up, the bound water is driven off as steam in a process called calcination. Releasing that water absorbs heat instead of passing it through, so the side of the assembly away from the fire stays around 212°F, the boiling point of water, until all the water is gone. Steel does not lose meaningful strength and wood does not ignite at 212°F. The gypsum is spending its bound water to keep the structure below the temperature where it fails.
This is passive protection. There is nothing to activate and nothing to maintain. The same chemistry that lets a gypsum ceiling protect the framing above it protects the floor-ceiling assembly a gypsum underlayment sits in.
Where the fire rating actually lives
This is the part that trips up a spec. A gypsum underlayment is one layer in a tested floor-ceiling assembly. The rating comes from the whole stack working together: the ceiling membrane below, the framing, the subfloor, the gypsum concrete topping, and any sound mat. Change a component and you are outside the tested design.
So there is no such thing as a “one-hour gypsum underlayment.” There is a one-hour or two-hour floor-ceiling assembly, identified by a UL design number, that includes a gypsum underlayment poured to a specified thickness over a specified subfloor. A spec writer cites the UL design, the assembly, rather than the product by itself. The underlayment manufacturer’s job is to be listed in the designs the project needs.
Where Treadstone fits
Treadstone is a fire-resistive gypsum underlayment, FR25 and FR30, and the FR is for fire-rated. The line is listed in more than 110 UL fire-resistance designs, which is the number that matters on a submittal. It tells the code official the gypsum pour is part of a tested assembly that carries a rating.
Treadstone does not make a floor fireproof on its own, and no gypsum underlayment does. It contributes non-combustible mass and the calcination buffer to an assembly that, as a whole, holds a fire-resistance rating. On a project that needs a one-hour or two-hour floor, the questions are which UL design the assembly is built to and whether the specified underlayment is listed in it. For how Treadstone’s fire-rated formulations compare on the other multifamily metrics, see Treadstone vs Gyp-Crete.