What Is the Fire Rating of Fiberglass Doors vs. Steel Doors?

Fiberglass and steel doors can both provide meaningful fire protection, but steel assemblies typically achieve higher and longer fire ratings, while fiberglass offers more design flexibility and corrosion resistance.

Most fiberglass doors are fire-rated between 20 and 90 minutes depending on the specific product, while steel doors can be rated from 20 minutes up to 3 hours when they are built and labeled as a tested fire door assembly. You want a front door that does more than look good in real estate photos; it needs to hold back flames and smoke long enough for people to get out and firefighters to get in. On projects where the right door bought an extra 20 to 90 minutes of protection, the difference was literally which side of the smoke line families ended up on. This guide explains how fiberglass and steel doors really compare on fire rating, where each one makes sense, and how to read ratings so you are not guessing with life safety.

Fire Ratings 101: Time on the Clock, Not “Fireproof”

Modern fire-rated door assemblies are designed to inhibit the spread of smoke and flames by compartmentalizing a fire within a building, working alongside sprinklers and alarms rather than replacing them as a stand‑alone “fireproof” fix, as described for fire-rated door assemblies. A rating like 20, 45, 60, or 90 minutes, or 3 hours, is simply the amount of time a complete door system—leaf, frame, and hardware—survived a standardized furnace test without letting flames break through.

In these tests, the door assembly is mounted in a furnace that follows a prescribed time–temperature curve, with temperatures climbing well above 1,700°F in longer tests, and many doors then face a high‑pressure hose stream to prove they can handle both heat and impact without failing, as outlined in how fire ratings are determined. The criteria are stability (the door stays in the opening), integrity (no openings that admit flames), and insulation (limited temperature rise on the non‑fire side in some cases).

It also matters what kind of performance is being claimed. A fire-protection rating focuses on stopping flames and smoke for the stated time, while a fire-resistance rating also limits how much heat passes through, which is why temperature‑rise doors are used in stairwells and tight egress paths to keep escape routes survivable. Separate from time ratings, material classifications such as Class A under ASTM E‑84 or A1/A2 in European systems describe how a surface contributes to fire and smoke, not how long a door holds a fire back.

The key point is that “20 minutes” or “90 minutes” belongs to the tested assembly, not to “steel” or “fiberglass” in the abstract, and not to one decorative slab hung in a random frame.

Typical Fire Ratings: Fiberglass Doors vs. Steel Doors

Across real products, the two materials occupy different parts of the fire‑rating spectrum.

In the commercial world, hollow metal (steel) doors are the only common door material regularly granted a 3‑hour fire label, depending on the exact door, frame, and opening size, as shown in industry data on steel door fire ratings. Typical fire ratings for steel door assemblies range from 20 minutes to 3 hours, and the same steel platforms can be configured as temperature‑rise doors that cap the temperature increase on the safe side—often to 250°F above ambient—for critical egress routes.

Fiberglass and fiberglass‑reinforced plastic (FRP) doors occupy a slightly different band. Manufacturers of commercial fiberglass and FRP doors highlight their products as excellent fire door materials that can withstand extreme furnace conditions up to around 1,925°F when built as fire door assemblies, combining fire resistance with long service life and minimal maintenance, as discussed for FRP and fiberglass fire doors. Industry comparisons note that fiberglass doors generally share the same typical maximum 90‑minute ratings seen with wood and aluminum doors, often relying on intumescent seals to reach those higher times while still being harder to trim or modify in the field.

On the residential side, many solid‑panel fiberglass doors from mainstream brands can be ordered as 20‑minute fire‑rated units that withstand at least 20 minutes of exposure at temperatures above 1,400°F without splitting, cracking, or rotting, while keeping styling aligned with the rest of the entry door line. Those lighter ratings are commonly used at attached garage entries or other lower‑risk locations where code allows a 20‑minute door.

To make the contrast more concrete, a 3‑hour rated steel door provides up to nine times the tested fire‑protection time of a 20‑minute fiberglass entry door, while a 90‑minute fiberglass or FRP door sits in the middle and is often sufficient where the surrounding wall is rated for 1 hour or similar.

For fire safety alone, a properly specified steel door assembly will usually outperform fiberglass on maximum rating and heat‑control options; fiberglass earns its place when design, corrosion resistance, or consistent finish are equally high priorities.

Where Fiberglass Fire-Rated Doors Make Sense

Fiberglass and FRP doors earned their place in commercial projects because they are durable, low maintenance, and easy to customize, and code treats them as interior finishes that must demonstrate good surface burning performance under ASTM E‑84 testing, per ASTM E‑84 testing requirements for fiberglass doors and frames. When a fiberglass door and frame achieve a Class A result in that test—a flame spread index of 0–25 and a smoke‑developed index of 0–450—you are getting a surface that contributes very little fuel and smoke even before you look at the time rating of the assembly.

Pultruded fiberglass and FRP bring another advantage: dimensional stability. Under severe heat, these materials are engineered to maintain their shape instead of bowing or twisting, which keeps fire doors operable and helps seals stay engaged during a fire. They also do not contribute fuel to the fire and are non‑conductive, which is helpful around electrical rooms and similar spaces where both fire and electrical safety are in play.

Design flexibility is where fiberglass really speaks to curb‑appeal projects. Seamless FRP door and frame assemblies with concealed Category A intumescent seals can deliver clean, monolithic faces and color‑matched frames, with standard and custom color options that blend into modern facades or bold accent entries, as shown in UL fire-rated fiberglass-reinforced plastic doors. Residential fiberglass fire doors from mainstream brands mirror their non‑rated siblings, so you can run one language of panels, glass lites, and finishes across the front elevation and still meet a required 20‑minute rating at a garage entry or side door.

A practical example is a contemporary home in a wildfire‑prone area that needs a 20‑minute fire‑rated door between an attached garage and a primary hallway while keeping a warm, wood‑look aesthetic. A solid‑panel fiberglass fire door with the right label meets the time rating, resists splitting and rot under heat, and visually ties into the rest of the entry system. For many homeowners, that balance of code compliance, low maintenance, and visual continuity is exactly the sweet spot.

One nuance worth stressing: a “Class A” fiberglass door per ASTM E‑84 is about surface flame spread and smoke, not about being a 1‑hour or 90‑minute fire door. One manufacturer explicitly warns that some manufacturers publish E‑84 component test results without a full Class rating, which can be misleading for specifiers, as noted in ASTM E‑84 testing requirements for fiberglass doors and frames. Always look for the separate time‑based fire label on the door and frame when life safety or code compliance is on the line.

When You Should Step Up to a Steel Fire Door

Steel shines when the fire scenario is less “protect the garage door for a short burst” and more “hold the line between people and a serious fire for as long as possible.” Hollow metal doors with the right core—whether steel‑stiffened, mineral, or temperature‑rise—are the only common door type regularly listed at 3 hours, and they are the default choice for many commercial stairwells, high‑hazard rooms, and major occupancy separations, according to comparisons of steel door fire ratings. Those higher ratings come from surviving the same severe furnace and hose‑stream regime that all fire doors face, simply for longer durations, and doing so as a complete, labeled assembly, as described in guidance on fire-rated door assemblies.

Temperature‑rise steel doors add another layer of protection by limiting how hot the safe side of the door can get—often to a 250°F rise—during the first part of the test, which matters in tight stair towers where people may have to shelter next to the door while others evacuate, as documented in industry data on steel door fire ratings. This is the kind of nuance that usually does not appear in glossy brochures but has real consequences when you are choosing doors for multistory escape routes.

Security and durability also skew toward steel. Residential and light‑commercial steel entry doors built from robust steel skins with reinforced lock areas are highly resistant to kicking, prying, and other forced‑entry attempts, while also providing notable fire resistance that helps contain a fire inside the home, as explored in resources on the effectiveness of steel doors against burglaries. For owners who care as much about secure living as they do about aesthetics, a carefully detailed steel entry—paired with modern hardware and glazing—can be the backbone of both burglary resistance and fire strategy.

Imagine a mid‑rise apartment building where the stair enclosure walls are required to be 2‑hour fire barriers. Code tables then call for 90‑minute, or 1½‑hour, fire door assemblies in those openings, sometimes with temperature‑rise requirements depending on use. In that context, a 20‑minute fiberglass door, no matter how good‑looking, simply is not an option; a properly rated steel door with the right hardware and closer is the responsible choice.

How to Choose: Match Rating to Location, Then Material

The most reliable way to decide between fiberglass and steel is to start with the opening, not the showroom sample.

Begin by confirming whether the wall or barrier around the opening is rated and for how long. Building and life‑safety codes set wall ratings first, then specify the required door ratings in coordinated tables so that exit enclosures, corridors, and hazardous rooms get appropriate opening protection. For example, a 2‑hour stair enclosure often requires a 90‑minute, or 1½‑hour, door, while some 1‑hour corridor walls are allowed to use 20‑minute doors because the primary goal there is smoke control rather than extreme heat resistance.

Once you know the required rating, you can decide whether fiberglass or steel (or in some cases wood or aluminum) can deliver that number in a listed assembly. Industry comparisons note that steel doors are commonly available from 20 minutes up to 3 hours, whereas fiberglass doors generally top out around 90 minutes in typical commercial offerings, based on steel door fire ratings. If you need 90 minutes or less in a non‑stairwell location, you often can choose either material and let design, environment, and maintenance considerations drive the decision.

Reading the actual label on the door is non‑negotiable. Proper fire‑rated doors carry a permanent label or metal tag—often along the hinge edge or top of the leaf—with the manufacturer, test standard, and time rating expressed in minutes or hours, and compatible frames carry their own labels from recognized agencies. If there is no credible label on the door or frame, you should assume it is not fire‑rated and bring in a qualified professional or the Authority Having Jurisdiction before relying on that opening for code compliance.

The same discipline applies at exterior openings. High‑performance homes increasingly treat exterior doors as part of a whole envelope that is insulated, impact‑rated, and fire‑rated where risk justifies it, so door systems must align with wall assemblies and nearby glazing to create a coherent fire and weather strategy, as described for exterior doors that are insulated, impact-rated, and fire-rated. From there, the material choice becomes an aesthetic and durability decision within the guardrails of those ratings.

A Crucial Nuance: Fire-Rated vs. “Fireproof”

Marketing language can muddy the water. Some homeowners hear “fireproof door” and assume unlimited protection, but manufacturers of fiberglass doors are clear that a fire-rated door is engineered to slow the spread of fire for a specific, limited period, not to stop it forever, as explained for fire-rated fiberglass doors. That distinction matters when you are comparing a 20‑minute fiberglass door at a garage entry with a 90‑minute or 3‑hour steel door guarding a stair tower or high‑hazard room.

In practice, every door is buying you time, not invincibility. The question is how much time each opening needs based on the people, paths, and hazards around it, and which material can deliver that rating with the look and performance you want.

Short FAQ

Q: Does every fiberglass door have a fire rating?

A: Most fiberglass doors are not automatically fire‑rated; only models that have been tested and labeled as part of a fire door assembly carry a time rating, and many residential fiberglass entries are non‑rated unless you deliberately order a fire‑rated version.

Q: Can fiberglass doors ever match steel on fire rating?

A: Fiberglass and FRP doors can be built as serious fire door assemblies and are commonly listed up to around 90 minutes in commercial applications, but hollow metal steel doors are the ones routinely granted 3‑hour ratings and more temperature‑rise options at the top end of the spectrum, according to steel door fire ratings.

Q: Is a Class A fiberglass door automatically a “1-hour” door?

A: No. A Class A result in ASTM E‑84 describes surface flame spread and smoke developed, not the time rating of the door assembly, and specifiers are advised to require both a Class A surface classification and a separate, clearly labeled fire‑protection time rating for the door and frame.

Closing

When you strip away the marketing gloss, the decision is straightforward: let the required fire rating for each opening set the floor, then use material to refine the balance between curb appeal, day‑to‑day durability, and secure living. Steel doors are the workhorses at the highest ratings and most demanding locations, while well‑specified fiberglass and FRP doors give you the freedom to draw cleaner lines and richer textures where design matters just as much as minutes on the clock.