In this article, we’ll explore some of the design criteria and materials used.

Advances in building materials make it possible to design wall assemblies that are thinner and have lower dead load than common assemblies a decade ago. Architects are looking for fire-resistant materials that provide a better substrate for securely fastening exterior facing. They’re also concerned about product integrity during construction. Some building materials are classified Exposure 1, which means that they can withstand normal exposure to moisture during construction delays.

Exterior load-bearing walls with a one-hour fire rating typically use one sheet of interior-facing Type X gypsum, while those with a two-hour fire rating use two sheets. Interior load-bearing walls typically have a sheet of Type X gypsum on both sides of the assembly.

Exterior walls generally use just one layer of insulation, which can either be mineral wool or fiberglass depending on the application. Interior walls usually have two layers of insulation.

Some exterior load-bearing wall assemblies are designed specifically for “zero lot line” applications, where the walls are in close proximity to the property line.

The U350 interior load-bearing wall assembly is a cost-effective, space-saving alternative to area separation fire walls using Shaft Liner, which are often installed by specialty crews. The U350 assembly can be easily installed by a general construction crew.

Some architects are replacing the popular UL301 and U308 assemblies with newer assemblies that don’t require installing an additional layer of gypsum. This reduces labor costs and construction time on most projects.

Recent innovations in wall assemblies are finding their way into roof deck design as well. Newer fire-resistant materials are now being used for vertical continuity of firewalls in townhomes, condominiums and apartments in accordance with IBC Section 706.6.

Some fire-resistant materials are available in 7/16˝ thickness to provide continuity with 7/16˝ standard OSB where applicable. FRT plywood is not available in that thickness and must take deductions.

Panels made with OSB and a fire-resistant material are able to lie flat, which eliminates problems with delamination. Overall, they reduce labor costs and construction time compared to gypsum-over-wood structural panel options.

Design criteria for wall assemblies and roof decks continue to evolve, along with the cost-saving properties of the materials used to build them.