Fire-rated OSB: What architects need to know

Is fire-rated OSB an option for your next project? AIA partner LP Building Products outlines what makes this category unique

Meeting a building’s structural requirements while maintaining fire code compliance can sometimes be tricky. In many cases, fire-rated OSB can increase square footage and provide greater design freedom than common alternatives. This type of product offers the structural strength of traditional OSB along with ICC-certified fire code compliance.

An engineered wood product, fire-rated OSB sheathing such as LP FlameBlock sheathing eliminates a layer of gypsum by combining an ignition-resistant, cementitious coating with a layer of OSB. It carries an Exposure 1 classification, which means it can withstand normal exposure to moisture during construction.

The panel offers both flame-spread resistance and burn-through resistance, as verified in tests like the 30-minute Class A Flame Spread Rating (ASTM E84, UL723) and 15-minute Thermal Barrier Protection test (UBC 26-2). In contrast, FRT plywood has a flame-spread rating, but no rating for burn-through resistance.

Structural design values for LP FlameBlock sheathing are not reduced by the fire-retardant treatment, so the panel carries load/span and shear design values equivalent to an untreated wood structural panel in the same panel thickness category. In fire-rated wall assemblies, this attribute can provide more flexibility in designing wall openings and more options in framing and bracing to meet structural requirements.

In virtually all cases, engineered wood sheathing is certified for Type II, III and V construction. Some FRT plywood products are only certified for Type V construction.

It’s common for fire-rated OSB sheathing to be a code-compliant component of numerous UL- and Intertek-listed exterior and interior load-bearing wall assemblies. These efficient assemblies can improve construction timelines and reduce labor costs by eliminating the time required to install a layer of external gypsum.

For example, Grimm + Parker used fire-rated OSB sheathing for The Varsity, a 400,000-sq.-ft. multi-use dormitory building near the University of Maryland, College Park.

The stick-frame-over-concrete-podium construction required special considerations. The International Building Code allows for a height of up to five stories of wood construction for a residential building with two-hour-rated exterior walls. Typically, this wall uses two layers of exterior gypsum sheathing on the outer surface of the wall, according to Fernando Bonilla, senior associate with Grimm + Parker Architects. However, the architect was able to eliminate a layer of gypsum using LP FlameBlock sheathing.

In addition to reducing gypsum cost, the contractor didn’t need a specialty crew to install the fire-rated OSB because it installs just like standard OSB sheathing.

To see more ways architects are using fire-rated OSB to save time while preserving design objectives, download case studies here.

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