Association of the Wall and Ceiling Industry Logo

What Is Included in a UL Fire Rated Assembly?

“What is included in a UL fire rated assembly?” is a question that comes up on a regular basis.

The contract documents often contain just the Underwriters Laboratories, Inc. design number, such as UL U405 and nothing more. Where do estimators go to find out what materials were used in the tested assembly? This is information you absolutely need to know in preparing the bid. We all know that the assembly must be constructed using the same materials used in the test. While there are other compilations of tested assemblies, for our purpose we will use the UL Fire Resistance Directory. The directory is available in hard copy, the form most of us are familiar with. Years ago it was a single volume and every architect and contractor had a copy in the library or on the bookshelf for quick reference.

Today, UL has entered the information age full tilt. For those of us who are visually challenged this is a welcome format for finding the information we need. Underwriters Laboratories, Inc. has an excellent Web site to help in determining the pieces used in the assembly. The address is You should have your estimators and project managers bookmark this in their Web browser. The categories are clearly defined, but you can also do a search by entering the UL design number in the search feature. The search feature in located on the home page of the site. The information contains more detail than is in the printed version of the UL Directory.

The two most important items in any given design are the drywall and stud. The drywall is important because you must determine if it is a single source of drywall or if it is generic, meaning that it comes from several manufacturers. For the stud the important item is the flange lip. For the design in question, UL405, you will see from the description below that the stud has a 1/4-inch lip flange. The majority of stud manufacturers and ASTM C645 use a 3/16-inch lip flange. To my knowledge no one produces a stud with a 1/4-inch lip flange. This means that if the architect selected UL405, it cannot be constructed to match the tested assembly.

According to a member of the Steel Stud Manufacturers Association, his company has been fabricating studs with a 3/16-inch lip flange for 15 years. He also stated that he could not afford to fabricate a stud with a 1/4-inch lip flange due to increased costs. While an additional 1/8 inch of material does not seem like much when multiplied by thousands of feet of coil material, the additional 1/8 inch becomes significant in the basic material cost.

Here is the definition of the stud taken from U405: “Steel Stud — 2 1/2 in. wide with 1 3/8 in. legs, 1/4 in. folded back return flange in legs, No. 25 gauge galv steel with 1 9/16 in. square conduit cutouts spaced not less than 8 in. OC, studs 3/8 in. less in length than assembly height.”

So what is a contractor to do in a case like this? Have your estimators look at the specified UL design to determine what stud was used in the assembly and if the stud is in fact available—a cumbersome task but the only way you can protect yourself is to verify the availability of the stud. If the stud is not available, request that the architect select a design with an available stud. From personal experience I know that most architects select the first design that meets the rating requirement and do not look into the details of the design. Make sure the architect gives you written documentation of the change, and keep the letter in your files with the project just in case a question arises later in the job. In fact, it could come back to haunt you many years after the project has been closed out.

Personally I believe that when laboratory personnel measured the stud used in the UL405 assembly, they made a mistake in measuring the lip flange. UL admitted during our meeting that they had problems accurately measuring the lip flange.

While the studs and drywall are important, review the entire material list included in the test. You may discover other items that are not apparent at first glance. In some cases the extent of joint finishing may be specified as well. Depending on the rating, sealing the edges of the wall may also be specified. Also included in this category are head-of-wall and edge-of-wall details. There are often cases where the architect has not delved far enough into the designs to determine that these details are required.

Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to or call him directly at (703) 538.1611.

Browse Similar Articles

You May Also Like

As we start a new year with the 2024 International Building Code now available, we also start a new code development cycle. Every three years, the International Code Council starts

I often get calls about specific fire rating details: Do I have to install this backer piece behind my control joint? Do butt joints have to be staggered? Is it