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In a flat roofing system, MgO (Magnesium Oxide) boards act as a high-performance structural substrate or cover board. They are increasingly replacing traditional OSB (Oriented Strand Board), plywood, or gypsum-based boards due to their superior fire rating and moisture resistance.

Here is the technical breakdown of using MgO boards for flat roofing：

1. The Core Functional Role

In a flat roof assembly, the MgO board is typically installed over the metal or wood framing to provide a solid, flat surface for the waterproof membrane.

• A1 Fire Rating: Essential for commercial projects with strict fire codes. It acts as a fire barrier between the building interior and the external roofing membrane.
• Compression Strength: It provides a rigid base that protects the insulation layer from being crushed by foot traffic during maintenance.
• Dimensional Stability: Unlike wood-based sheets, MgO boards exhibit minimal expansion or contraction (low linear thermal expansion), which prevents the waterproof membrane from tearing at the joints.

2. Critical Specification: Sulfate-based vs. Chloride-based

For flat roofing, Sulfate-bonded MgO boards (Magnesium Sulfate) are the industry standard for high-end or industrial projects.

• The Corrosion Risk: Standard “Chloride” MgO boards can “sweat” (release salty moisture) in high-humidity roof cavities. This moisture is highly corrosive to the metal screws (fasteners), steel joists, and aluminized steel frames typically found in flat roof assemblies.
• The Sulfate Advantage: Sulfate boards are “chloride-free,” meaning they will not cause the fasteners to rust or the structural steel to corrode over time.

3. Common Technical Specifications