Welcome back to “Lessons in Firestop.” Today’s topic is “The Unexpected Danger of Softer Metal Pipes.”
In our last segment, we unpacked the challenges of firestopping plastic pipes—how the timing between pipe softening and intumescent firestop expansion is key to performance. We looked at how different plastics behave under fire conditions and how that timing can mean the difference between a passed or failed fire test.
Now let’s pivot to metal pipes. These should be easier, right?
Yes—if we’re talking about steel, cast iron, or copper. These materials have high melting points and expand only moderately under heat. Most 1000-series firestop systems handle them just fine, and when they are properly installed, fire doesn’t stand a chance of sneaking around the pipe.
But what happens when your pipe isn’t steel or copper? What if it’s brass, bronze, or aluminum?
Suddenly, you’re in a different world—a riskier one.
Why Softer Metals Aren’t So Safe
Let’s get one thing straight: just because it’s metal doesn’t mean it’s easy to firestop.
Brass, bronze, and aluminum are softer, lower-melting-point metals. They behave much more like some of the tougher plastics when exposed to fire. They may not burn, but they do deform, transfer heat rapidly, and fail faster than you would expect if treated like traditional pipe materials.
Let’s break it down.
Back to the Buckets
Remember the buckets we talked about in the last discussion? We discussed how more-robust plastic pipes such as CPVC and FRPP are more challenging to firestop because they don’t soften until the intumescent/expanding material has already done a lot of its expanding. Because of this, more intumescent material is needed to properly protect these applications. Well, when it comes to our subject today—brass, bronze, and aluminum—these pipes are stronger than plastic, but they still deform in ways the firestop is challenged to handle.
That “just right” zone is very narrow—and these softer metals are outside of it.
Here’s Another Problem: Lack of Listed Systems
There are very few tested firestop systems for these softer metals. Most UL 1000-series systems assume steel, cast iron, or copper. They rarely list aluminum, brass, or bronze.
So what happens?
- Installers treat them like copper or steel.
- Firestop is applied incorrectly.
- The system potentially fails in a real fire—because it was never designed or tested for the correct material.
This isn’t theoretical. It’s a growing issue on jobsites where aluminum is being introduced for light piping or where older buildings still use bronze/brass drain lines. You must check if the firestop system is tested with the exact material you’re using.
If not, you’ll need an engineering judgment (EJ) or a manufacturer-approved alternate solution. Otherwise, you’re just rolling the dice.
Fire Dynamics Matter
Let’s take a second to think about how these metals behave in a fire:
- Aluminum has a melting point around 1,200°F. It also expands twice as fast as steel and transfers heat fast enough to bridge fire to the other side.
- Brass and bronze may hold up better structurally, but they still deform, expand, and lose stability earlier than harder metals.
This isn’t about combustion—it’s about how the pipe’s changing shape and heat flow can displace or compromise your firestop materials. And if the intumescent sealant can’t keep up, you’re looking at a failed test—or worse, a failed real-world barrier.
Field Installation Red Flags
Here’s what to watch for:
- Aluminum fire sprinkler lines passing through rated walls
- PEX systems with brass connectors terminating in fire-rated assemblies
- Utility piping in kitchens or labs using bronze or brass
None of these should be firestopped using standard 1000-series systems unless the system explicitly includes that material. If it doesn’t? Stop and reassess.
What Should You Do?
- Pause and Check the Specs
If you’re working with any pipe that isn’t steel, cast iron, or copper, verify the UL listing before firestopping. - Get Manufacturer Input
Don’t guess. Contact your favorite firestop rep. Ask for a tested system or a proper EJ. - Train Your Team
Make sure installers, inspectors, and designers know that “metal” doesn’t always mean “safe.” - Don’t Mix Materials
Be cautious when terminating plastic pipes with brass or bronze fittings within rated assemblies. If no tested system exists for the hybrid setup, it’s not compliant.
Final Word: Do Better, Because You Know Better
Einstein once said, “If you can’t explain it simply, you don’t understand it well enough.” Here’s the simple version:
- Softer metals like brass, bronze, and aluminum behave more like robust plastics than like steel.
- They transfer heat fast, deform early, and fall outside standard firestop system assumptions.
- Firestopping them the wrong way means a failed system—even if it looks fine on the surface.
So next time you spec, install, or inspect one of these pipes in a rated assembly, stop and ask: “Is this detail designed for this metal?” If not, you’ve got work to do.
If you’re facing this situation in the field—or if you just want to be ready when it shows up—reach out. I’m happy to walk you through it or connect you with the right resources.
Until next time, stay safe and smart. This has been another Lesson in Firestop.
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 sharron@halpertlifesafety.com 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.
