How to Assemble a Two-Hour Fire-Rated Wall


All drywall is naturally fire-resistant. This building material was developed in 1916 by the U.S. Gypsum Company and is composed of a layer of gypsum plaster compressed between two sheets of heavy paper. Gypsum is a natural material with a high moisture content. When exposed to fire, the moisture in gypsum becomes steam, which helps suppress flames. Two special types of drywall, X and C, are especially fire-resistant and are called fire-rated drywall. Both have fiberglass added to the gypsum core; C has more fiberglass content and also a form of vermiculite, which stabilizes the gypsum core under heat.

Things You'll Need

  • Fire-resistant drywall
  • Portland cement gypsum plaster (Optional)
  • Fire-resistant insulation
  • Double the number of Type X sheets on the wall for a simple system installed over fire-resistant fiberglass insulation; each sheet of 5/8-inch Type X drywall is rated to resist fire for one hour, so two layers will produce a two-hour firewall. Put two sheets together on one side or use one sheet on each side of a wall. Do this only in situations where the added material will not affect other construction.

  • Use thicker drywall for a two-hour rating, if double-sheeting is not an option, with fire-resistant insulation between the wall studs. Cover wood-framed walls with a single layer of 3/4-inch Type X drywall rated for 120 minutes of fire resistance. Make sure the drywall is approved under ASTM standard E 119; check the markings on the drywall or ask the supplier if it meets that standard.

  • Build a two-hour exterior wall with steel studs, rather than wood, and special metal stud fiberglass insulation between the studs. Install 5/8-inch Type C drywall on the interior surface and 1/2-inch gypsum sheathing on the exterior. Finish the wall with a 1-inch coat of Portland cement gypsum plaster.

  • Use steel studs and two layers of 1/2-inch Type C drywall on each side of the studs for a two-hour fire rating on an interior wall. Mount a 1 1/2-inch fiberglass barrier inside the studs. Use special fiberglass rated for use in firewalls. The American Society for Testing and Materials and Underwriters Laboratory both have specifications for these materials.

  • Consider special wall assemblies from manufacturers who supply wall components that combine Type C drywall with special insulation for fire resistance and soundproofing. Some assemblies use three or four layers of Type X drywall in a double-stud configuration, so each wall segment has two vertical studs rather than one.

Tips & Warnings

  • Check building codes for specifics on fire-rated walls. Two-hour walls are not required on most conventional single-family houses but are standard on multi-unit buildings, such as duplexes, apartments and commercial structures.
  • Fire-resistant drywall in single-family homes is used most often around furnace compartments and similar areas with high heat potential.
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