Intumescent or Cementitious? How to Choose the Right Fireproofing for Structural Steel

If the steel will be seen, intumescent fireproofing is usually the right call. If it will be hidden above a ceiling or behind a wall, cementitious fireproofing (often called SFRM, for sprayed fire-resistive material) usually makes more sense. Both do the same job. They keep structural steel cool enough, for long enough, that the building holds its fire rating while people get out and the fire department gets to work. That's the short version. The longer version depends on your building, your spec, and how the space will be used for the next few decades, so let's walk through it the way we would with a GC or a building owner across the table.

What does fireproofing actually do for steel?

Steel doesn't burn, but it softens as it heats. Somewhere around 1,100°F it has given up roughly half its strength, which is why building codes require structural steel in many buildings to be protected for a set number of hours. You'll see those ratings written as 1-hour, 2-hour, or 3-hour.

Fireproofing buys that time. It's insulation, applied directly to the beams, columns, and decking, that slows how fast heat reaches the steel. The two systems just go about it in very different ways.

How does intumescent fireproofing work?

Intumescent coatings look and go on a lot like heavy paint. They're sprayed, brushed, or rolled in layers measured in mils (thousandths of an inch), and when finished they can be topcoated in almost any color.

The clever part happens in a fire. As the coating heats up, it swells to many times its original thickness and forms a dense, foamy char. That char is what insulates the steel. Until then, it sits there looking like a clean, smooth painted finish.

There are two broad families. Thin-film intumescents, usually water or solvent based, are the standard for commercial buildings, lobbies, schools, and offices. Epoxy intumescents are tougher and built for exterior steel, industrial plants, and energy facilities, where the coating also has to handle weather and chemical exposure.

How does cementitious fireproofing (SFRM) work?

SFRM is a gypsum or cement based material sprayed onto steel at a much greater thickness, often a half inch to well over an inch depending on the rating. It insulates simply by being there: a thick, lightweight blanket between the fire and the steel. It has a rough, textured look, which is why you'll find it above drop ceilings, in mechanical spaces, and on steel that nobody will ever see once the building is finished. It's typically the more economical choice per square foot, and it goes on fast across large areas of repetitive framing.

Which one is right for your building?

Here's how the decision usually breaks down on real projects.

Intumescent tends to win when the steel is architecturally exposed, like an atrium, a lobby with open trusses, a modern office with the structure on display, or a canopy the public walks under. It also makes sense on retrofit and renovation work in occupied buildings, where a thin, clean application is easier to manage than a heavy spray operation, and on exterior or industrial steel where durability counts as much as fire protection.

SFRM tends to win on new construction with large areas of concealed steel, such as warehouses, big box retail, office floors with finished ceilings, and parking structures where appearance isn't the priority. When budget per square foot is driving the decision and nobody will see the finish, cementitious is hard to beat.

Plenty of buildings use both. Intumescent on the exposed steel in the lobby, SFRM on everything above the ceilings upstairs. That's often the smartest use of the budget.

Why the UL design number matters more than the product name

Here's something that surprises a lot of owners: fireproofing isn't specified by product alone. It's specified by a tested assembly, usually a UL design number, that combines a specific material, a specific steel shape, and a specific thickness to reach a specific rating.

That's why two beams in the same building can need two different thicknesses of the same product. A heavy, thick-walled column heats more slowly than a light beam, so it needs less protection to reach the same hour rating. Your architect's drawings or the fire protection engineer's schedule should list the design numbers. If they don't, that's the first question to ask, before anyone orders material.

It also means you can't swap products casually midway through a job. A substitution has to fit a listed design that matches your steel and your rating.

What happens at fireproofing inspection?

Fireproofing is almost always subject to special inspection under the building code. An independent inspector will check the work before it's covered up or closed in, and they're looking at a few things.

For intumescent, they'll measure dry film thickness with a gauge at set intervals across the steel and compare it to what the UL design requires. For SFRM, they'll check thickness, and they'll also test density and bond strength, which confirms the material is adhering and was mixed and applied correctly. They'll also confirm the surface prep and primer match what the listed design allows.

A good fireproofing contractor keeps its own thickness readings as the work goes, so inspection day is a confirmation rather than a surprise.

Keeping fireproofing in good shape after the ribbon cutting

The most common reason fireproofing needs attention in an existing building has nothing to do with fire. It's other trades. Electricians, plumbers, and HVAC crews working overhead can knock SFRM off beams while hanging hangers or running new lines, and it doesn't always get patched.

If you manage a building, it's worth having your fireproofing walked during major tenant fit-outs or renovations. Touch-ups are straightforward when they're caught during the work, and fire marshals and insurance inspectors notice when they aren't.

Fireproofing with Alpine

Alpine Painting & Sandblasting Contractors has been coating steel across New Jersey, New York, Connecticut, and Pennsylvania since 1975, and fireproofing is a natural extension of that work. If you're looking at a set of drawings and trying to figure out which system fits, send them over. We're happy to talk it through. Call (973) 279-3200 or email info@alpinepainting.com.

Frequently asked questions

What is intumescent paint?

It's a fire-resistive coating that looks like a heavy paint finish but expands into an insulating char when exposed to high heat, protecting the steel underneath for a rated amount of time.

Can you paint over intumescent fireproofing?

Yes, with a topcoat the manufacturer has approved for that system. Most projects add a decorative topcoat for color and durability. Random paint over intumescent isn't a good idea, since it can interfere with how the coating expands.

Can intumescent fireproofing be used outdoors?

Some systems are made for it, particularly epoxy intumescents with an appropriate topcoat. Standard thin-film products are generally designed for interior conditions.

How long does intumescent fireproofing last?

Properly applied and protected indoors, it's intended to last the life of the building. Exterior and industrial applications depend on the system and the topcoat maintenance.

Is cementitious fireproofing cheaper than intumescent?

Usually, per square foot. Intumescent costs more but delivers a finished appearance, which can save the cost of enclosing the steel.

What is SFRM?

Sprayed fire-resistive material, the industry term for cementitious and mineral fiber fireproofing sprayed onto structural steel.