Over the past 10 years, head of wall performance expectations have grown considerably, as well as overall fire ratings. Today’s engineers and architects are designing buildings that can move under seismic activity as well as structural conditions that allow the building to deflect under various load conditions. In the past, standard rigid construction was the norm, but it is now being replaced with buildings that incorporate anticipated movement into each and every floor of multistoried buildings.
The challenge for the engineer and architect is how to allow the building to move but still provide adequate fire rated assemblies. Fortunately, for these engineers and architects they do not have to look far for this information. The Underwriters Laboratories online directory provides all of this information free to the end user. However, finding the right assembly or understanding how to read the report is sometimes challenging the first time through. For the sake of this article, we will touch on Head of Wall assemblies only.
The vast majority of construction requires HW-D-systems that have movement capabilities. The movement capabilities are broken down into three class levels I, II, and III:
Class I-1 cycle per minute for 500 cycles (Thermal)
Class II-10 cycles per minute for 500 cycles (Wind sway)
Class III-30 cycles per minute for 500 cycles (Seismic)
The first page of the UL report will give the joint performance criteria (see illustration).
Assembly Rating: This indicates the hourly fire rating.
Class Movement Capabilities: This indicates what class of movement (I, II or III) the system will be able to handle as stated above.
Nominal Joint Width: This indicates the distance between the top of the installed drywall and the lowest point of the overhead structure.
L-rating: The L-rating is the amount of air (or cold smoke) that can leak through a penetration and the number indicates the amount of air in cubic feet per minute.
Manufacturers of these products listed in the UL report may re-test the assembly to improve their joint system performance. Once the test is completed, the UL report will be updated. Therefore, before starting your project, it is important to refer back to the most current report which can be found on the UL online directory. The date listed on the front page will indicate the reports last revision.
Currently, there are hundreds of head of wall systems to choose from with new systems or revised systems being added monthly. These systems can be broken into two general types, encumbered and unencumbered systems.
Encumbered systems require the fire stopping material to be installed in the gap between the top of the installed drywall and the overhead structure. The material in this gap will restrict the amount of potential movement. Encumbered systems generally rely on the adhesive strength of fire caulking or elastomeric sprays given by overlapping the material onto the drywall and overhead structure.
Unencumbered systems, on the other hand, are systems that have no movement-restricting fire stopping material in the gap between the top of the drywall and overhead structure. Unencumbered systems generally rely on thin intumescent tape that is installed on the leg of the header track and concealed behind the drywall allowing the drywall free range of motion within the nominal gap. Still other unencumbered systems rely on drywall rips that protect the movement gap.
WHAT TO LOOK FOR WHEN CHOOSING A SYSTEM
How many different UL HW-D-System reports does the manufacturer list, and just as important how many different wall type assemblies do they list with UL. With many manufactures they will provide a base line UL HW-D-system and then provide engineering judgments for other wall assembly variations. Engineering judgments do not always get approval from the IOR.
Do the manufacturer’s UL assemblies cover both standard walls and shaft walls? If not, the contractor may be forced to use different manufacturers for different assemblies. Thereby mixing systems that may need to be signed off by the IOR or fire marshall.
What is the joint system performance? Not all systems are created equal even though they may look alike. Carefully review the expansion/compressing percentage of the nominal gap to ensure the proper movement is provided.
What kind of labor is involved with the particular UL system? Will access be an issue and if it is how can you ensure that it is being properly installed according to the UL report?
What is the longevity of the UL system? Is there maintenance required? If so, how will access be provided once the building is occupied?
As stated earlier, the performance expectations of joint systems have greatly increased over the last 10 years and many manufacturers have stood up to the challenge. A decade ago, a 1/2-inch total deflection movement joint would have been all that was needed to accommodate the building’s structural anticipated movement. But now, 10 years later in today’s construction, a 1-inch movement joint is becoming the norm, and many multistory buildings are requiring up to a 2-inch movement joint to accommodate the structural deflection.
Two-inch movement joints are no easy task and typically very limited in UL fire rated assemblies. It is important to make sure up front that there are UL assemblies for the structures anticipated movement or else an engineering judgment may have to be considered.
Engineer judgments can be provided by the manufacturer or a third party engineer. Engineering judgments are based on relatable test data, but are not actual tests performed according to ANSI/UL 2079 and therefore sometimes difficult to get approved by architects, IOR or fire marshalls. W&C