The Fire Fighter

Typically, fire-rated building assemblies for wood-framed structures incorporate gypsum sheathing and fire-resistant treated (FRT) plywood. Both have been used for many years, but each has significant drawbacks. Fortunately, architects and builders now have a third option—and this one combines a Class A Flame Spread Rating with the structural strength, consistency, and other advantages of OSB.

When a strong coating of non-toxic magnesium oxide cement and fiberglass is added to a panel of OSB, the result is a product with nearly three times the burn-through resistance of untreated wood panels. What’s more, this product is ICC-certified (ESR-1365) for use in common roof designs as well as one- and two-hour wall designs in multifamily and commercial buildings. In other words, builders can now enjoy all of the benefits of OSB—including strength, value, durability, and consistent performance—while complying with building codes for fire resistance.

The cement and fiberglass overlay on an OSB product actually enhances the structural strength of the panel, resulting in higher allowable loads for certain panel thicknesses and spans. With FRT plywood, on the other hand, the fire-retardant chemicals introduced to the product weaken its structural strength and reduce its load-bearing capacity.

An OSB panel with a cement and fiberglass overlay retains its Exposure 1 rating, meaning that it can withstand job site moisture during normal construction delays. If FRT plywood gets wet, it must dry to less than 15% moisture before it can be installed. OSB is also significantly lighter and faster to install than gypsum systems.

One advantage of gypsum over FRT plywood is that it does not warp (often called the “potato chip effect” with FRT plywood). OSB is also well-known for its resistance to warping, and the fire-resistant overlay only increases its strength. This helps reduce waste materials on the job site as well as time wasted culling out unusable panels.

The chemicals used in this type of OSB panel with a cement and fiberglass overlay are non-toxic, non-hazardous, and landfill-safe. The same cannot be said for FRT, which can cause scrap to be classified as hazardous waste and refused by some landfills.

It should be noted that not all fire-rated OSB products offer the same advantages. Unlike the new LP® FlameBlock® Fire-Rated OSB Sheathing, products that use an intumescent paint are vulnerable to moisture, including rain or even high humidity, both before and immediately after installation.

Fire-rated OSB products like LP FlameBlock sheathing cost less than either FRT plywood or a combination of gypsum and structural sheathing. They are available from a large national network of distributors, they fasten with common nails in standard patterns, and they require no special handling for waste or scrap disposal.

For durability and strength, an OSB panel with a cement and fiberglass overlay can provide clear advantages over the traditional FRT plywood and gypsum options. The most common uses for these materials include multifamily residential roof decks, fire-rated wall assemblies, Class A rated interior wall panels, SIPs, and commercial modular roof decks. When builders consider the cost and environmental advantages, along with the ease of installation, OSB emerges as the superior choice in a wide range of applications.

For more information about fire-rated OSB sheathing, visit LPCorp.com/FlameBlock.


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