Resources & Guides / Building Codes & Standards / ASTM & UL Standards / ASTM C317
ASTM C317
Standard Specification for Gypsum Concrete
Overview
ASTM C317 covers a mill-mixed, lightweight construction material composed of calcined gypsum and suitable aggregate, such as wood chips or expanded perlite. Delivered as a dry powder requiring only water addition before being poured in place, its primary application is in poured-in-place roof decks and slabs, where its combination of low density, fire resistance, and smooth surface provides an effective substrate for roofing membranes.
Material Classes
- Class A: Maximum density 50 lb/ft³, minimum 500 psi compressive strength - for lightweight insulating fill
- Class B: Minimum 1000 psi compressive strength - for structural applications requiring greater strength
- Setting time: 20-90 minutes per Vicat needle test (ASTM C472)
- Typical system: 2-inch minimum thickness over formboards with wire mesh reinforcement
Fire-Rated Assemblies
Gypsum concrete is a key component in numerous UL fire-rated designs (such as UL Design P207), where its non-combustibility and thickness contribute to achieving 1-hour and 2-hour fire-resistance ratings. The material's fire resistance makes it ideal for roof-ceiling assemblies requiring code-compliant fire protection.
Critical Design Considerations
Gypsum concrete is fundamentally different from portland cement concrete and highly susceptible to moisture degradation. While used for roof decks, it's an interior product that depends entirely on the overlying roofing membrane for protection. It's not suitable for direct weather exposure, frequent wetting, or abrasive wear. Specifications must explicitly state "Class A" or "Class B" based on whether the priority is minimizing dead load or providing structural capacity.