For decades, architects have been using shaft wall for fire code compliance in area separation walls as well as multifamily stairwells and elevator shafts. But shaft wall has plenty of drawbacks. For instance, it’s very heavy and requires extra components like H-studs, C-channels, and breakaway clips.

That’s why architects are looking for economical, easy-to-install alternatives to shaft wall, like the UL-listed U350 interior load-bearing wall assembly featuring LP FlameBlock Fire-Rated OSB Sheathing.

Architects see the U350 assembly as a way to meet fire code and reduce dead load. Compared to shaft wall, the U350 assembly consumes 1 square foot less per 4 feet of wall length—and it requires no special parts or tools to install.

Contractors are switching to the U350 assembly because it installs faster for a lower cost than shaft wall assemblies, and U350 lowers the weight of a 4-foot by 8-foot wall by about 30 percent. Many framers also complain that shaft wall doesn’t cut clean. The U350 assembly eliminates that problem.

Habitat for Humanity of New Castle County, Del., recently built nine townhomes using U350 wall assemblies with LP FlameBlock sheathing. Because the townhomes were mainly built by volunteers working under the supervision of an experienced contractor, Habitat officials were looking for a lighter, simpler solution than the traditional assembly using shaft wall liner (listed as a U375 assembly).

“Our volunteers range from people who have never touched a hammer to semi-professional framers,” says Sean Allen, Habitat’s director of construction and real estate on the project. “The typical party wall uses a heavier material. Because we use volunteers, party wall construction was becoming especially cumbersome and posed safety issues.”

For the townhome project, Habitat switched from a shaft liner wall design and used 270 sheets of LP FlameBlock sheathing along with 5/8-inch fire-rated gypsum panels in the U350 wall assemblies. “The assemblies were built probably a month or two faster than shaft wall assemblies,” Allen adds. “If this job had production builders on staff, I can see how this assembly would greatly affect the cost of a build because of the time and labor savings.”

Although the U350 assembly is still not as common as shaft wall in New Castle County, code officials saw that it was a UL-listed assembly for partition walls and had no problem approving it.

Architects nationwide are taking a fresh look at the U350 wall assembly because it offers numerous advantages over shaft wall—reduced installation time/cost, less weight, lower dead load, and greater design flexibility.

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