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Meeting Fire Codes with OSB

On average, a structural fire occurs almost once per minute. Fire-resistant construction aims to preserve the structural integrity of a building long enough to allow evacuation and prevent the collapse of key load-bearing elements. Assemblies incorporating fire-rated cementitious coated (FRCC) OSB sheathing can offer a cost-effective means of meeting fire-related structural code requirements.

FRCC OSB: The Basics

In order to gain its fire rating, FRCC OSB is tightly bonded on one or both sides with a layer of fiberglass-reinforced, non-combustible magnesium oxide cement. This non-combustible layer resists the spread of flames and the transfer of heat through the panel while the fiberglass reinforcement, along with a small amount of synthetic polymers, also improves structural performance and impact resistance. Water is chemically bound within the structure of the cementitious coating of each panel, which acts in concert with the inert elements of the cement and fiberglass to slow the progress of heat and flames.

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Code Compliance

Requirements for fire-resistant construction are specified in the International Building Code (IBC), International Residential Code (IRC), and state and local building codes. Standard fire tests are referenced in the IBC and IRC with regard to ratings of individual materials and entire assemblies.

To be considered code compliant, materials must pass a variety of fire and physical property tests. Agencies such as the International Code Council Evaluation Service (ICC-ES) consider the performance requirements of products in various construction applications and establish Acceptance Criteria (AC reports) to evaluate a product’s suitability for a given use, using test procedures that are standardized and regulated by organizations such ASTM, Underwriters Laboratories (UL), NFPA, and associations of manufacturers.

Once a product has been tested by an approved laboratory against the relevant acceptance criteria, its fitness for a given construction application can be evaluated. Organizations such as ICC-ES, through the use of committees and a public hearing process, review the test results and publish Evaluation Service Reports (ESRs) that establish fitness-for-use and are often referenced by manufacturers and required by the code authorities of most  municipalities.

Fire Ratings

There are two main categories of fire testing referenced and required by the IBC: Flame Spread and Fire Resistance. Flame Spread is a measure of the speed of travel of flame on the surface of a given product or material under a standard set of conditions. Fire Resistance, on the other hand, refers to the ability of a material or an assembly of materials to resist burning through, often while supporting a given load for a specified time period under standardized fire conditions. It is important to note that Flame Spread does not measure, and may not necessarily be correlated with, burn-through resistance. A material may achieve a Class A rating (the IBC required rating for fire-retardant treated wood) for flame spread, and yet exhibit  little burn-through resistance.


FRCC OSB is used primarily in Type III and V construction, particularly in roof decking adjacent to fire walls and in fire-rated, load-bearing walls. It can also be used, with the fire-rated treatment on both sides of the panel, in the roof decks of Type II structures and in non-bearing applications (such as curtain walls) in Type I and II construction.

The product may also be utilized for a number of additional applications, including roof and wall construction in areas subject to wildfire-related building codes, structural foam insulated panels (SIPs), and commercial roof decks. FRCC OSB is a UL-listed component in assemblies for Class A- and Class C-rated low-slope commercial roof deck assemblies under single-ply membranes.

About LP® FlameBlock® Fire-Rated OSB Sheathing

LP FlameBlock sheathing offers structural performance and fire resistance in one panel. Created by applying a proprietary non-combustible, fiberglass-reinforced Pyrotite® treatment to LP® OSB panels, LP FlameBlock is a PS2-rated structural sheathing with a Class A Flame Spread Rating. The product also carries an Exposure-1 classification, meaning that it is designed to withstand exposure to moisture during normal construction delays.

LP FlameBlock sheathing can reduce the time and cost associated with installing a combination of gypsum and structural sheathing. The product’s proven fastener-holding ability can also simplify installation of exterior facing materials, potentially providing additional savings.

The applications for which LP FlameBlock is code-compliant are listed in Section 4.2 of the FRCC OSB code report, ESR-1365.

Learn more by taking the Meeting Fire Codes with OSB course (HSW credit).


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