November 2022 Lessons in Firestop: Hambro Floor-Ceiling Assemblies and Mickey Mouse Ears

Hey there, firestop friends. I promised in the last Lessons in Firestop to talk to you about some of the issues with the Hambro floor-ceiling assemblies, or really any of the G500 series floor-ceiling assemblies. First, I want to be sure we are on the same page with what I mean by that. If you are standing on the floor looking down, it will look just like a typical concrete floor, but don’t let that fool you because it is anything but that. If you are on one of these projects and you look up and see metal trusses, or at the top of the metal-framed walls you see rippers of drywall, those are two red flags that you should take a look at your plans—specifically the area that shows the floor assembly—before you start throwing firestop sealant in any holes.

If your plans look like the image to the right that is pulled from UL’s G752 assembly, then you will need firestop details that are very different from a concrete floor and more similar to the ones used in wood-framed construction. Here is what the assembly will actually look like: You have a concrete floor poured over a metal deck. You have steel trusses, and under the trusses is hat track used to fasten the type C drywall that will make up the ceiling. I see people doing a million things wrong on this construction type, but this discussion is for plumbing engineers so we will stick to this scope. If you are a GC and you want to know more, please reach out to me; I will gladly help anyone who wants to build better. But for now, let’s stick to the plumbing scope!

Here are a few things people tend to overlook:

  • 2½ inches of concrete alone is not going to give you a fire rating.
  • Steel loses 50 percent of its structural capacity around 1,000°F.
  • The gypsum board on the underside that becomes the ceiling is a critical element of the fire protection of the entire floor-ceiling assembly. That means proper construction of the ceiling is important and so is proper firestopping of the penetrations.

Similar to wood-framed construction, the plumbing penetrations that go through this assembly will go through the floor, the ceiling, or both. Obviously, firestop will be required when the penetration goes through either the floor or the ceiling, or both.

The image to the right is PEX pipes that are surrounded by a plastic bracket. Can you see that the top track of the wall is metal framing? The PEX pipe runs through the metal framing. There is a risk of movement causing abrasion, which may cause a leak, so the installers need to have something protecting this, but that means the firestop system now must be able to accommodate both the PEX pipe as well as the bracket. Do you know what annular space is allowed or how much sealant is needed? Without a firestop detail from the firestop manufacturer that provides guidance, you don’t really know what you should do. I can tell you that this application would require an intumescent firestop material that would need to be  installed 5/8 inch (because that is the depth allowed by the 5/8-inch drywall). What I can’t tell you is the annular space that is allowed. I did a search of the available UL details and was not able to find a single one that allowed both the PEX pipe and the plastic grommet, so the challenge becomes this: What is an installer supposed to do if they want to properly firestop this application and protect the PEX pipe from abrasion? Quite frankly, there is no good answer. Plumbers are not the only ones with this issue; electricians have a similar issue.

These G500 assemblies and wood-framed construction have several similarities when it comes to firestop requirements. They both have a floor surface and a ceiling surface, both of which need to have firestop installed on any through penetration. Both also have a ceiling assembly, and keeping fire out of the space in between is critical to the structural integrity and fire performance of these assemblies. Typically, they require a different type of drywall than what is used in wall assemblies, but you folks are plumbing engineers, not drywall installers, so we won’t dive into that here. I only mention it so you know the two materials are not the same, but back to the plumbing scope, if you see something like this, where the drywall is all broken up, do not assume that the metal framing member is going to be able to stop fire, especially if you have something like this shown to the right.

Remember, our lazy firestop needs as much direction as you can give it, which means the drywall needs to be parallel to the plastic. If the drywall is not intact or the plastic pipe is at an angle other than 90 degrees to the cut edge of the drywall, you are going to have a problem that is unique to plastic pipes. The lazy intumescent or expanding material will expand into the area of least resistance. If there is a sharp 90-degree edge like a well-cut drywall or a concrete or wood assembly, then the sharp edge will provide the direction. If the opening is hatcheted or the pipe is at an angle, the expanding material will almost slide off the plastic pipe and out of the opening. This means it will not be able to close down the opening the way the system is designed. It also often means your annular space is too big, and if you remember from our Goldilocks story, that is a problem.

From time to time you might bring this up to someone who will give you some pushback, so I want to show you where to find the official documentation on which you can hang your hat. On UL’s website are tons of free resources. You will have to register, but you don’t have to pay to get access. At this link, you will see the XHEZ document. If you deal with firestop a lot, you might want to read through the whole thing. It’s a great resource.

When you read through it, you will find a section that says:

“Where the uninsulated penetrating item in the individual design is indicated as a metallic pipe, conduit, tube, duct, or cable, and the firestop system consists at minimum of a fill material (such as sealants, putty, or mortar), the penetrant may pass through the opening in the wall or floor assembly at any angle, provided the annular space is maintained on both sides of the wall or floor assembly and all other specifications in the design are satisfied. In all other cases, except where otherwise indicated in the system, the penetrating item should penetrate the wall or floor assembly at a 90-degree angle.”

Keep in mind this is referencing metal pipes with no insulation. If your application involves insulation or plastic pipes, the fire dynamics are different.

With that, I hope this Lesson in Firestop helps you better understand firestop and in general helps you build better.

In our next discussion we will talk more about this XHEZ document and specifically what you should pay attention to if you are working in hollow-core concrete projects. If you work in hollow-core concrete, DO NOT MISS THIS ONE! See you there!

Connect with Sharron

A former kindergarten teacher turned firestop expert, Sharron is President of Halpert Life Safety Consulting LLC, a leading provider of firestop-related life-safety and passive fire protection solutions.

If you like what you read here and want to know more, email [email protected] or connect with her on LinkedIn or Twitter to tell her what else you want her to cover in this column. You can also follow her on Instagram. If you find this information valuable, please like, share, comment, repost, retweet, and throw it on IG to help people build better.

The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.

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