When Two Different Fire Ratings Meet

Donald E. Smith, CCS

December 2006

On a current project I have two fire-rated walls intersecting each other. The walls have two different ratings, one has a two-hour rating and the other is rated at one hour. Can you direct me to a source of a detail showing how to install the drywall at the intersection?

I also took a long look at all of the sources I have available in the John H. Hampshire Memorial Library, the largest of its kind in the industry, and found the same thing you did—nothing. So I placed a couple of phone calls to some of the experts in the field. Most of the leads were dead ends.

Finally, Bob Wessel at the Gypsum Association verified what we already had discovered: Details of the condition you have to deal with do not exist in an industry standard or industry publication.

So where do we go from here?

First, Underwriters Laboratories tells us that the gypsum panels used in fire-rated assemblies must be continuous and uninterrupted. So if we apply some simple reasoning to your problem, we arrive at the following solution: The two-hour wall would be framed and hung before the one-hour wall. The one-hour wall would then be framed and hung with the framing for the termination of the one-hour wall being placed on top of the gypsum panel of the two-hour wall.

Simple, right?

Not really, when you consider how most framing is sequenced for interior partitions commonly installed on most projects—generally, all of the framing is installed prior to the hanging of gypsum panels.

Another common practice is to hang gypsum panels on one side of the partition. After the mechanical and electrical items inside the partition are inspected, the open side of the partition is then hung and the taping and finishing of the drywall begins. The solution is to proceed with framing as you normally would, but do not install the last stud for the one-hour partition at the intersection with the two-hour partition. After completion of the framing, cut the floor and ceiling runner where the one-hour rated partition intersects the two-hour partition to allow for the thickness of the drywall on the two-hour partition to pass through uninterrupted from floor to ceiling. Hang the drywall on the two-hour partition and then install the last stud for the one-hour partition. Then fasten the stud through the drywall on the two-hour partition and hang the drywall.

The Gypsum Association’s 18th edition of the Fire Resistance Design Manual includes a detail of how to seal the end of a fire-rated partition to ensure that smoke or heat do not penetrate the junction of the two walls at the intersection.

The only problem you may encounter is when you use wood studs as the detail for intersecting partitions, and this detail has the stud offset from the framing in the two-hour partition toward the one-hour partition. This would create an interrupted condition for the drywall panels.

Also, additional planning during layout is required to ensure the framers are aware of the special condition. In reality, the designer should have included a detail on the contract drawing to indicate how this condition is to be handled.

About the Author
Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to him at smith@awci.org, or give him a call on (703) 538.1611.