What kind of fire rating does stucco provide?

Portland cement-based plaster, commonly called stucco, has long been and continues to be a popular choice for finishes on buildings. It allows for a wide expression of aesthetics, is a cost effective finish, is durable in all types of climates (especially wet ones), and offers fire resistance. Fire resistance is typically classified by a fire rating, but what kind of fire rating does plaster provide?

Things that influence the fire rating of a plaster system include the type of material used for the support member, size of the support member, presence/absence and type of exterior sheathing, aggregate in the plaster mix, presence/absence of insulation, presence/absence of interior wall finishing materials (gypsum wallboard, etc.) and thickness of the section. The type of member—wall, partition, ceiling, or other, and member classification (load bearing(LB) or non-load bearing (NLB)) also influences the rating.

In 1991, the Foundation of the Wall and Ceiling Industry published a reference guide on portland cement-based plaster/stucco systems used for fire protection, the Single Source Document on Fire-Rated Portland Cement-Based Plaster Assemblies. Designers, specifiers, building code officials, contractors, and general public are the intended audience. The information contained therein is “not intended as design or installation criteria,” but can help people determine how to assess their assemblies using the referenced publications, fire test reports, industry standards, and codes.

For example, a typical residential application might be a three-coat system of plaster over 2-by-4-inch wood studs using metal lath attached to the studs, either with or without a layer of sheathing, like plywood. On the interior side would be a layer of gypsum board. The detail for a system made with these components is assigned a one-hour fire rating based on 1988 Uniform Building Code information.