Understanding UL Nomenclature for Firestop-Rated Joints

Welcome back! I’m excited that you believe knowing the UL nomenclature for firestop is important enough to learn about! If you are a firestop inspectors, general contractor or first tier subcontractors who have hired firestoppers, know that by the end of this series you will have a few tips on how to make your job easier and command better quality from your firestop installers. If you are a firestop installer this blog series will help you prepare for an educated inspector or general contractor and help raise you head and shoulders above your competition. These tips we will share are only effective once you know how to identify the details. Let’s get started making your life easier.

I will take it easy on you for the first bit and we will start with the rated joints.


When you are looking at the UL details for rated joints, there will be a combination of two letters, then a third letter followed by a series of four numbers. Once you are finished reading this blog you will understand the nomenclature or naming system for the UL listed firestop systems for rated firestop joints. You will know what all of the letters and numbers mean, and you will be able to thumb through a firestop submittal easily and find the detail you need for the application you are about to install or inspect (this last step may take a little practice, but if you care enough to read this far then you are smart enough to figure it out quickly.)

Understanding the UL nomenclature for rated joints is easy once someone explains it to you. When you see F think floor, when you see W think wall. This means that an FF system is a floor to floor-joint application, a WW system is for a wall to wall joint application and an FW is for a floor to wall application. HW is a head of wall application and if you need it BW is for a bottom of wall joint. There are also CW or CEJ for Curtain Wall joints but they are tested to a different standard so we won’t really go into it here. Link to category 7-firestopping for glazing industry

Then, there will be a third letter; either a D or an S. D is for a dynamic system; which means the system was tested to allow for movement. We will get into the details of how it is tested, why it is important and much more in a later blog. So lets keep going. If the third letter is an S, then it is a system tested for a static joint system; which means it was not tested for movement. Most all buildings will experience movement, whether it is from live load, dead load, wind load, thermal load, seismic load or who knows what else. Therefore, it is in the best interests of the team to be very careful about when and where they allow the use of static systems.

Next will be a series of numbers, but the first digit will be the most important. If the number series starts with a 1 then the joint will be 2” or less, if it starts with a 1 then it will be more than 2” but less than 6”, if it starts with a 2 then the joint will be more than 6” and less than 12” and so on.

When you understand the UL nomenclature you can easily rifle through the firestop submittal to find the right group of firestop details that should be used to install or inspect the firestop application. Someone well versed in the nomenclature can tell you that a duct through a rated gypsum wall should be installed according to a WL 7000 series detail. They can’t tell you what material, what size duct, if it is insulated or bare, what annular space is allowed or even if retaining angle will be required; without finding the actual detail. However, this basic information will get you to the right group of details much faster and from there you can go over the specifics to make sure the field condition appropriately complies with the UL listed assembly. In a later blog we will actually dissect a few details to help you better understand the layout.

Next we will go over the UL nomenclature for through penetrations.

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