Poured Gypsum Roof Deck Benefits, Installation, and Performance

Poured gypsum roof decks are a durable, fire-resistant alternative to traditional concrete roof systems used in commercial and multi-family construction. This article explains what a poured gypsum roof deck is, how it performs for fire protection, structural loading, and acoustics, and what designers and contractors should consider when specifying or installing it.

Attribute	Poured Gypsum Roof Deck
Primary Material	Gypsum cementitious slurry with lightweight aggregates
Typical Uses	Commercial roof decks, plaza decks, fire-rated assemblies
Fire Resistance	Excellent — inherent non-combustible gypsum core
Thermal Performance	Moderate R-value; often paired with insulation
Acoustic Performance	Good mass-based sound transmission control
Installation	Poured in place over deck pans or composite metal decks

What Is A Poured Gypsum Roof Deck?

A poured gypsum roof deck is a cast-in-place roof topping made from a gypsum-based slurry often mixed with aggregates and additives to improve strength and durability. It is applied over formwork such as metal deck pans, insulation, or lightweight concrete toppings. The result is a monolithic, noncombustible roof deck that provides both structural and fire protection benefits.

Materials And Composition

Poured gypsum roof deck mixes typically include gypsum cement, portland cement or other hydraulic binders, lightweight aggregates (perlite, expanded shale, vermiculite), and chemical admixtures for flow and set control. Specialty blends can improve compressive strength and freeze-thaw resistance. These mixes are engineered to balance weight, strength, and curing behavior for rooftop applications.

Performance Characteristics

Fire Resistance

Gypsum contains chemically combined water that releases as steam when exposed to heat, slowing temperature rise and providing fire resistance. Poured gypsum roof decks are widely used because they deliver reliable, inherent fire protection without relying on combustible components.

Structural Capacity

When properly designed and cured, poured gypsum toppings provide adequate compressive strength for pedestrian traffic, mechanical equipment loads, and ballast. Strength varies by mix but can approach lightweight concrete performance while maintaining a lower dead load. Structural engineers must verify live load capacity, deflection limits, and connection details to supporting structural framing.

Thermal And Moisture Behavior

Gypsum has moderate thermal properties and is often paired with insulation layers for energy code compliance. Proper waterproofing and vapor control are critical because gypsum is sensitive to prolonged moisture exposure. Designs commonly separate insulation and waterproof membranes from the gypsum layer or protect gypsum with cover boards and effective drainage.

Acoustic Properties

Due to its mass and density, a poured gypsum roof deck helps reduce impact and airborne sound transmission. This makes it attractive in mixed-use buildings where rooftop mechanical noise or pedestrian traffic could affect interior spaces. Acoustic performance is enhanced when combined with resilient layers or sound-absorbing insulation.

Typical Applications And Assembly Types

Poured gypsum roof decks are used in commercial roofing, plaza decks, parking ramps (non-vehicle areas), and multi-family roofs requiring fire-rated assemblies. Assemblies may include metal deck substrates, rigid insulation, vapor retarders, and roofing membranes. Compatibility with roofing membranes and edge details must be coordinated early in design.

Installation Process And Quality Control

Installation begins with substrate preparation, including cleaning, priming, and ensuring the form deck is watertight. The gypsum slurry is mixed on-site or delivered from a central plant and poured to specified depths, slopes, and densities. Quality control includes mix testing, monitoring pour thickness, curing conditions, and verifying compressive strength through samples.

Curing And Weather Considerations

Curing time depends on mix design and ambient conditions; protection from freeze-thaw cycles and rapid drying is essential to prevent cracking. Cold weather concreting practices, heated enclosures, or temporary covers are often used. Installers should follow manufacturer recommendations for temperature and humidity during curing.

Code Compliance And Fire Ratings

Poured gypsum roof decks must meet local building codes and fire-resistance requirements. Tested assemblies provide hourly fire ratings for flame spread and structural integrity. Compliance often requires documentation from recognized testing laboratories (UL, ASTM, or FM). Specifying tested roof assemblies ensures predictable fire performance and smoother permitting.

Comparing Poured Gypsum With Lightweight Concrete

Both systems serve as roof toppings, but poured gypsum offers inherent noncombustibility and lighter weight in some formulations. Lightweight concrete may provide higher compressive strength but can be heavier. Choice depends on fire rating requirements, allowable structural loads, and the need for compatibility with roofing systems.

Feature	Poured Gypsum	Lightweight Concrete
Fire Resistance	High	High
Weight	Often Lighter	Often Heavier
Water Sensitivity	Higher	Lower
Compressive Strength	Moderate	Higher

Cost Considerations And Lifecycle Maintenance

Initial costs for poured gypsum installations can be competitive with other roof toppings when factoring structural savings due to reduced dead load. Long-term costs depend on waterproofing strategy and maintenance of roof membranes. Protecting the gypsum from moisture intrusion is the primary factor affecting lifecycle costs and durability.

Common Problems And How To Avoid Them

• Moisture Damage: Prevent by placing membranes above or isolating the gypsum with cover boards and proper drainage.
• Cracking: Use control joints, proper curing, and reinforcement as specified.
• Poor Adhesion To Membranes: Ensure substrate preparation and compatibility testing with roofing manufacturers.
• Insufficient Slope: Design adequate slope during pour or incorporate tapered insulation to prevent ponding water.

Best Practices For Designers And Contractors

Early coordination between structural engineers, roofing consultants, and gypsum material suppliers is essential. Specify tested assemblies, define required densities and compressive strengths, and document membrane compatibility. Include inspection and testing requirements in the contract to ensure consistent performance.

Specification Tips

Require manufacturer data sheets, UL or FM listed assemblies, mix design parameters, and field testing. Call for compressive strength tests and moisture content verification prior to roofing installation. Detail protection measures during construction, such as temporary covers and restricted traffic over fresh pours.

Environmental And Sustainability Factors

Gypsum is abundantly available and can be sourced from synthetic or recycled streams, which reduces embodied energy compared to some cementitious materials. Proper design reduces material quantity and structural demands, contributing to sustainability. Recycling old gypsum and using low-carbon binders are emerging trends to improve environmental performance.

Frequently Asked Questions

Is a Poured Gypsum Roof Deck Waterproof?

Not inherently. Poured gypsum must be paired with a durable waterproofing membrane or protective cover board to prevent moisture penetration. Waterproofing is a separate, critical part of the roof assembly.

How Long Does It Take To Cure?

Curing varies with mix and conditions but typically allows roof membrane installation in a few days to weeks. Compressive strength tests determine readiness for subsequent trades. Follow manufacturer curing guidelines and verify with field tests.

Can It Support Rooftop Equipment?

Yes, if designed to accommodate equipment loads and point loads through reinforcement, pads, or additional structural support. Load paths should be reviewed by a structural engineer before roofing or equipment installation.

Resources And Standards

Relevant standards and resources include ASTM testing methods for gypsum and cementitious materials, UL fire assembly listings, and roofing industry guidelines from NRCA and local building codes. Consult these standards when specifying poured gypsum roof deck assemblies.

For detailed product selection and certified assembly listings, contractors and designers should request manufacturer data sheets, UL reports, and project-specific test results prior to finalizing designs.