If you’ve ever walked a job site, stood under a wood roof that’s just waiting for the last piece of fascia to go up, or had a client call at 9 PM panicking about a wildfire approaching their new home, you know trusses aren’t just framing pieces—they’re the skeleton of a building, and when fire hits, they’re often the first line of defense (or failure). For the last 12 years, I’ve run a truss supply business, and let me tell you, the question I get asked more than “when can you deliver?” is this: “Are there actual fire-resistant trusses out there, or am I just supposed to cross my fingers and hope the building code holds?” Trusses
A few years back, we had a project out in southern Oregon where a client refused to use anything “regular” for a 12-unit apartment complex—their community is in a very high fire risk zone, and their previous building had survived a wildfire only because a neighbor’s irrigation system doused embers an hour before the flames hit. They wanted trusses that wouldn’t just burn slow, but wouldn’t collapse long enough for firefighters to get through, or for people to get out. At the time, I told them “we can do fire-retardant treated (FRT) wood trusses, but they’re not truly fire-resistant.” They pushed back, so I dug in and spent three months testing samples, talking to code officials, and sitting in on ASTM meetings, because I didn’t want to sell them something that would fall short. What I found is that the line between “fire-resistant” and “fire-retardant” is way more blurred than most truss suppliers will admit, and there are actually products that meet the strict definitions of fire resistance—you just have to know where to look.
First, let’s get the terms straight, because this is where half the confusion starts. The International Building Code (IBC) doesn’t use the word “fire-resistant” for trusses by default—it uses two key ratings: fire resistance rating (FRR) and flame spread index (FSI). Flame spread index is how fast fire travels across the surface of a material; wood has an FSI of 100, per ASTM E84, so anything below that is considered slower-burning. But fire resistance rating is the big one: that’s the time (in hours) a material or assembly can withstand a standard fire test without failing—meaning no collapse, no structural loss, no flame penetration. A 1-hour FRR means the truss can hold up for 60 minutes under controlled fire conditions, which is what most areas require for residential garages or commercial buildings in high-risk zones.
For decades, the only “fire-resistant” trusses people knew about were FRT wood trusses, which are pressure-treated with chemicals (usually ammonium phosphate or boric acid) that slow flame spread. But here’s the catch: FRT trusses only have an FRR of about 15 to 30 minutes, max, under small-scale tests. That’s enough to delay a small embers fire, but if you have a structural fire or a large wildfire with direct flame contact, those trusses burn through fast—we saw that firsthand in the 2020 Oregon fires, where three apartment complexes with FRT trusses collapsed 22 minutes after flames reached the roof. So FRT is fire-retardant, not fire-resistant, and that’s a critical distinction for anyone building in a high-risk area.
Now, the actual fire-resistant trusses? They fall into three main categories, and we stock all three now because after that 2020 project, I realized clients need options that match their budget and risk level. Let’s break them down, because not all fire-resistant trusses are the same, and you can’t just order “fire-resistant” online—you have to specify the rating you need.
First, mass timber fire-resistive trusses. Wait, I know what you’re thinking: mass timber is big cross-laminated timber (CLT) or glulam, right? Glulam has come a long way when it comes to fire resistance. When wood burns, it creates a char layer—about 1.5 inches per hour, per NFPA 251, the standard fire resistance test. So if you make a glulam truss big enough, the char layer actually protects the inner, unburned wood for the FRR you need. For example, a glulam truss with a 12-inch deep bottom chord and 8-inch top chord will have a 1-hour FRR, because the char takes an hour to eat through the outer layer, and the inner wood is still carrying the load. The trick here is that you can’t just use regular glulam—you have to specify fire-rated glulam, which is engineered to have consistent density so the char rate is predictable. We worked with a local glulam mill to engineer these for that 12-unit apartment in Oregon, and they passed the IBC’s 1-hour test with zero issues. The downside? They’re about 20% more expensive than regular wood trusses, and they weigh more, so you might need a slightly heavier load-bearing wall, but most clients in high-risk areas see that as a worthwhile investment.
Second, steel fire-resistant trusses. A lot of people assume steel is fully fire-resistant, but that’s another myth. Steel loses 50% of its strength at 1,100 degrees Fahrenheit, which is the temperature of a typical structural fire, so unprotected steel trusses will buckle and collapse in 10 to 15 minutes. To make them fire-resistant, you have to add fire-resistant coating (intumescent or cementitious) or encase them in concrete or gypsum board. Intumescent coatings are the most common for trusses—they’re a thin paint that swells when exposed to heat, creating a insulation layer that keeps the steel below 1,100 degrees for the required time. We’ve installed steel trusses with intumescent coating that have a 2-hour FRR, which is overkill for most residential projects, but perfect for schools or hospitals in fire-prone areas. The upside of steel is it doesn’t rot or attract termites, and intumescent coating is easy to touch up if a piece gets damaged during installation. The downside? They’re heavier than wood, more expensive, and can be noisy during wind or hail storms, which is a turn-off for some residential clients.
Third, composite fire-resistant trusses. This is the newest category, and it’s blown up in the last five years for mid-sized projects. Composite trusses mix wood (top and bottom chords) with a lightweight, fire-resistant core made of mineral wool or gypsum, or they use a structural polymer composite for the web members. Wait, no—let’s get specific: our go-to composite trusses have wood top and bottom chords that are pressure-impregnated with a fire-retardant, and the webs are made of a high-strength mineral wool composite that’s non-combustible. These trusses have a 1-hour FRR, same as glulam, but they’re 10% cheaper than glulam, and 15% lighter, so they work with standard load-bearing walls. We used these for a residential development in northern California last year, and the fire marshal signed off on them immediately because they have a flame spread index of 15, which is way below the IBC limit of 25 for high-risk zones. The best part? They don’t have the charring issue of glulam, so if you ever need to modify the truss later (like adding a skylight or a new duct), you don’t have to replace the entire member—you can cut into it and patch it with the same material, no structural loss.
But here’s the thing: none of these trusses work if you install them wrong, and that’s a mistake we see all the time, not just by our competitors, but by some of our subcontractors. For example, if you use FRT trusses and then staple plastic vapor barriers directly to the wood, you’re trapping moisture, which reduces the effectiveness of the fire retardant. Or if you use steel trusses and skimp on the intumescent coating, missing a spot at the joints, that spot will fail in a fire. Last year, we had a client who tried to save $2,000 on a 50-truss job by having his crew skip coating the web joints on steel trusses. We caught it during a pre-delivery inspection, and he had to pay an extra $1,800 to re-do it—way more than he saved. So when you’re ordering fire-resistant trusses, don’t just ask “what’s the FRR?” ask for the assembly details: what coating is used, how the joints are treated, what tests the trusses passed. All our fire-resistant trusses come with a third-party test report, and we keep a copy on file for every project, because code officials will ask for it during inspection.
Another common question: do fire-resistant trusses need special building permits? The short answer is yes, but only if you’re in a high fire risk zone, which most areas west of the Rockies, parts of the Southeast, and even some suburbs east of the Mississippi are now classified as. The IBC updated its high-risk zone guidelines in 2021, requiring all commercial buildings over 10,000 square feet and residential buildings over two stories in those zones to have at least a 1-hour FRR framing. Fire-resistant trusses are the easiest way to meet that requirement, because solid masonry or concrete framing is way more expensive. We also work with a fire code consultant who will meet with our clients and walk them through exactly what’s needed for their area—no guesswork, no surprise fees.
Let me circle back to that 12-unit apartment project in Oregon. We installed the glulam fire-resistant trusses, and a year later, the area had a 1,200-acre wildfire come within 2 miles of the property. The flames reached the roof for about 45 minutes, and the trusses didn’t char through. The firefighters told the client later that the truss structure let them set up a fire line right under the roof, which saved the entire complex. That’s the difference between a truss that delays fire and one that actually resists it. Since then, we’ve supplied fire-resistant trusses to over 80 projects in high-risk zones, and I haven’t had a single client come back with a complaint about performance—they all come back for more when they expand.
Now, I know what you’re thinking: this all sounds great, but how do I know which fire-resistant truss is right for my project? Let’s break it down by budget and use case, because that’s what matters most. If you’re building a single-family home in a moderate risk zone, a composite truss is probably your best bet—cost-effective, easy to install, 1-hour FRR, meets most code requirements. If you’re building a commercial building, school, or apartment complex in a high risk zone, glulam or steel intumescent trusses are better—they have higher FRR options, more durability for commercial use. If you’re on an extremely tight budget, FRT trusses work for low-risk areas, but only if you understand their limitations and pair them with other fire safety measures like ember-resistant soffits and fire-resistant sheathing.
At the end of the day, fire-resistant trusses aren’t a “magic bullet” that will protect your building from every wildfire or structural fire. But they’re the single most effective structural investment you can make in a fire-prone area, because they buy time—time for people to evacuate, time for firefighters to act, time to save the building. For 12 years, we’ve supplied trusses to job sites across the West, and our number one priority isn’t just delivering on time—it’s making sure our clients have the right trusses for their risk, not just the cheapest ones.
If you’re researching fire-resistant trusses for your next project, and you want to talk through what’s needed for your area, your budget, and your building specs, reach out to our team. We’ll send over the third-party test reports, walk you through pricing, and even connect you with local code officials to make sure everything checks out. Fire safety isn’t something to cut corners on, and the right trusses are the foundation of that safety.
References
Columns and Beams National Fire Protection Association. (2022). NFPA 251: Standard Methods of Fire Tests of Building Construction and Materials. International Code Council. (2021). International Building Code, Chapter 7: Fire and Smoke Protection Features. ASTM International. (2020). ASTM E84: Standard Test Method for Surface Burning Characteristics of Building Materials. American Wood Council. (2021). Fire-Resistive Mass Timber Construction Guidelines. Steel Construction Institute. (2022). Fire-Resistant Steel Truss Design and Installation Manual.
GNEE Steel Structure (Tianjin) Co., Ltd.
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