Fire Rating. Sound Rating. Flat Floor.
If you build wood-frame multifamily, hotels, or senior living, gypsum concrete underlayment is doing three jobs in your floor/ceiling assembly at once:
Fire. Poured gypsum concrete is the topping in over 100 UL-rated floor/ceiling assemblies with fire ratings from 1 to 4 hours. Gypsum is chemically bound with water; under fire exposure it releases that water as steam, absorbing heat. The material has zero flame spread per ASTM E84. When your architect calls out an L-series UL assembly over wood framing, a poured gypsum topping at a minimum thickness is usually part of what makes that rating real.
Sound. The IBC requires a minimum STC 50 and IIC 50 (45 if field-tested) between dwelling units. Bare wood subfloor does not get there. Gypsum concrete adds the mass that blocks airborne sound (STC), and over an acoustic mat it decouples footfall impact (IIC). This mass-plus-mat topping is, in practice, how wood-frame multifamily passes code. (Full teaching on STC vs IIC lives on our Sound Control Systems page.)
Flatness. Poured at 3/4″ to 3″ thick, gypsum concrete self-levels over wood-frame deflection, plumbing grooves, and panel seams, handing your flooring installers a flat, monolithic substrate.
The numbers architects spec: compressive strengths of 2,000 to 4,000+ PSI per ASTM C472, density around 110–120 lb/ft³ — roughly a quarter lighter than standard concrete, which is why wood framing can carry it.
Specified on Paper. Lost at the Pump.
Here is the uncomfortable part nobody puts in a submittal: the spec is only as good as the batching. A gypsum underlayment specified at 2,500 PSI can leave the pump at 1,600 PSI if the crew over-waters or over-sands the mix — and hand-batching by bag count and eyeballed water makes that drift routine. The symptoms surface late: soft, dusty surfaces; flooring adhesives that release; cores that test under the number your UL assembly was engineered around.
We took the crew’s judgment out of the chemistry. Our Smart Batch® GC-1 batch plant weighs every ingredient of every batch against your programmed mix design — one half-ton batch every 75 seconds — and archives every batch to a cloud server. The QC report goes in your closeout file. Perfect pour, every pour, and a document that proves it.
FIRM-FILL®. Proven Chemistry. Documented Placement.
We install FIRM-FILL® gypsum concrete (Hacker Industries) — a family of gypsum underlayments with recycled content, formulated for fire-rated and sound-rated assemblies, radiant heat encapsulation, and high-strength applications. Depending on your spec, the product line covers low-profile pours, quick-drying formulations, abrasion-resistant surfaces, and extended working time. We are also a Formulated Materials partner for the Pacific Northwest, giving us direct access to their Treadstone® gypsum underlayments and acoustic mat systems where the spec calls for them.
Where we pour it:
- Wood-frame multifamily — apartments, condominiums, townhomes
- Hotels and hospitality
- Senior living and student housing (fire and sound ratings both non-negotiable)
- Radiant heat encapsulation — full teaching on our Floor Leveling page
- Renovation and change-of-use projects chasing a new fire or sound rating
