Fireproofing of steel structures: intumescent paints vs spray-applied mortars

Steel is the most used structural material in industrial buildings, office buildings and parkings in Catalonia. Light, strong and versatile, it nevertheless has a critical weakness against fire: it rapidly loses rigidity with temperature. At 300 °C it already begins to deform under load; at 550 °C it has lost half its strength; at 700 °C, most of its load-bearing capacity.

A real fire can reach 900 °C in the first ten minutes. Without fireproofing, a steel structure can collapse before occupants evacuate and firefighters intervene.

Fireproofing is the solution that allows these structures to maintain their stability long enough. In this guide we explain how the two main systems work and when each is appropriate. You can complement it with our guide on fire doors in residential buildings, another essential element of passive fire protection.

Why steel needs specific protection

Steel is not combustible, but it is an excellent heat conductor. In a fire, the temperature of a steel beam or column rises more quickly than that of other materials because it has no thermal mass to absorb the heat. The result is that the structure fails before the enclosing elements of brick, concrete or solid timber around it.

Fireproofing creates a thermal barrier between the flame and the steel. The objective is not to prevent the steel from heating up forever, but to delay that heating long enough for occupants to evacuate and emergency teams to act safely. This time is defined by regulation based on the building use.

Intumescent paints: how they work and when to use them

Action mechanism

Intumescent paints are organic-based coatings that at room temperature look like normal paint. When exposed to heat (between 150 and 250 °C, depending on the formulation), they undergo a chemical reaction that triggers three simultaneous processes:

1. Melting of the foaming agent: the paint binder melts and releases gases that inflate the layer.
2. Carbon layer expansion: the paint can increase its volume 30–50 times, forming a dense and insulating carbonaceous foam.
3. Thermal insulation: the carbonaceous foam has very low thermal conductivity; it acts as a barrier that slows heat transmission to the steel.

The result is that the steel, protected by this expanded carbon layer, takes much longer to reach the critical 550 °C.

Achievable resistances and thicknesses

| Target resistance | Intumescent paint thickness (approx.) | |---|---| | R-30 | 0.4 – 0.8 mm | | R-45 | 0.7 – 1.5 mm | | R-60 | 1.0 – 2.5 mm | | R-90 | 2.0 – 5.0 mm | | R-120 | 4.0 – 9.0 mm (depends on the section of the profile) |

The necessary thickness does not depend only on the required R: also on the profile section factor (ratio between the fire-exposed perimeter and the cross-section of the steel, A/V or Hp/A). Thin-rolled profiles need more protection than solid profiles.

When to choose intumescent paint

• Buildings with aesthetic requirements: offices, hotels, high-end areas of commercial buildings. Intumescent paint can be finished with coloured paint or varnish.
• Accessible structures: easy to inspect visually and to repair in spot-damage areas.
• Elements with complex geometry: bracing, joints, purlins. Paint is applied by spraying and covers any geometry.
• Resistances up to R-90: for higher Rs, the required layer is so thick that mortar may be more cost-effective.
• Renovations of buildings in use: lower impact on building activity during works.

Limitations of intumescent paints

• Sensitivity to humidity: in environments with permanent relative humidity > 85% (some parkings, coastal areas), the paint can degrade prematurely. Formulations for humid environments exist, but at higher cost.
• R-120 and higher: required thicknesses are large and cost can equal or exceed that of mortar. For R-180 and R-240, intumescent paint is practically unviable.
• Service life: requires periodic inspection and possible reapplication every 10–15 years depending on environmental conditions.

"Many new-build projects correctly define intumescent paint thickness in the project, but in real execution less material is applied to save cost. Without micrometer measurement at the end of the work, it is not possible to verify that the required R is actually guaranteed. The end-of-work certificate without thickness measurements is worthless before an inspection by the Departament d'Empresa i Treball."

— Technical manager, Igniescut

Spray-applied mortars: when they are the right alternative

Types of fireproof mortar

Spray mortars are inorganic, non-combustible materials applied by pneumatic spraying onto the steel profile. The most used in Catalonia are:

Vermiculite and expanded perlite mortar: a mixture of lightweight mineral aggregates with a binder. Economical, lightweight and easy to apply. Offers R-60 to R-120 with thicknesses of 20 to 50 mm. Does not tolerate impacts or vibration well.

Spray-applied rock wool mortar: rock wool (basaltic mineral fibre) mixed with binder offers excellent thermal and acoustic insulation properties. Can reach R-240 with thicknesses of 40–90 mm. It is the most effective system for high-risk industrial structures.

Lightweight sprayed concrete: less common; used in singular elements where high mechanical and fire resistance are simultaneously required.

Indicative resistances and thicknesses (rock wool mortar)

| Target resistance | Approximate thickness | |---|---| | R-60 | 15 – 25 mm | | R-90 | 25 – 35 mm | | R-120 | 35 – 50 mm | | R-180 | 50 – 65 mm | | R-240 | 65 – 90 mm |

When to choose spray mortar

• Industrial buildings and warehouses: where aesthetics are secondary and the surface to protect is large. Mortar is significantly more economical per m² in large volumes.
• R-120 and higher: mortar is the standard solution for high resistances in RSCIEI high-risk.
• Parkings and basements: mortar is more resistant to ambient humidity than organic intumescent paints.
• Aggressive environments (chemical, ATEX): there are mortar formulations resistant to acids and solvents, while intumescent paints can degrade in those environments.

Mortar limitations

• Aesthetics: the finish is rough and grey. Not suitable for visible areas in public-use or office buildings.
• Impact sensitivity: mortar is mechanically fragile. In areas with forklift or vehicle traffic, it must be protected with edge guards or metal casings.
• Added weight: in structures with tight load margins, the weight of mortar must be considered in structural calculation.

What resistance does regulation require in Catalonia

| Building use | R required by CTE DB-SI | |---|---| | Residential housing ≤ 28 m evacuation | R-60 | | Residential housing > 28 m evacuation | R-90 | | Hospital / healthcare | R-120 | | Educational, administrative | R-60 | | Commercial, public attendance | R-90 | | Parking | R-90 | | Industrial low risk (RSCIEI) | R-30 | | Industrial medium risk (RSCIEI) | R-60 | | Industrial high risk (RSCIEI) | R-120 |

For industrial establishments in the industrial parks of Tarragona and Barcelona, the RSCIEI (RD 2267/2004) classifies the intrinsic risk based on the weighted fire load. Buildings storing flammable or high calorific products may require R-120, which in practice means sprayed rock wool mortar. To dive deeper into risk classification and sectorisation required by RSCIEI in industrial environments, see our guide on fire sectors in industrial buildings in Tarragona and Camp de Tarragona.

Certified application process

Fireproofing work must be carried out by companies authorised by the Departament d'Empresa i Treball of the Generalitat de Catalunya, with technicians certified by the manufacturer of the applied system. The process includes:

1. Technical calculation: determination of the section factor of each profile and the necessary thickness for the required R.
2. Surface preparation: shot blasting or sanding of the steel to remove rust, dirt and previous paint. System adhesion depends on the quality of this preparation.
3. Primer (intumescent paints): application of an anti-corrosion primer coat compatible with the intumescent system.
4. System application: spraying or layered application according to the manufacturer's technical specifications.
5. Thickness control tests: dry thickness measurement with magnetic micrometer (paints) or depth gauge (mortars) to verify compliance with the calculation.
6. Certification: issue of the end-of-work certificate by the installer company, with reference to the system applied, the protected profiles and the results of thickness measurements.

Frequently asked questions

How long does intumescent paint protection last?

Indoors, protected from humidity and without impacts, service life is 15 to 25 years depending on the manufacturer. Outdoors or in aggressive environments, it can be reduced to 10-15 years. Annual visual inspection and spot repainting is recommended where cracks, peeling or insufficient thickness are detected.

What is the difference between R-30, R-60 and R-90?

The R indicates the minutes during which the structure maintains its load-bearing capacity against the standardised ISO 834 fire. CTE DB-SI requires R-60 in residential and office buildings, R-90 in buildings over 28 metres, and R-120 in hospitals and high-risk industrial establishments.

Who can certify fireproofing in Catalonia?

Only companies authorised in the Register of Installation and Maintenance Companies of the Departament d'Empresa i Treball of the Generalitat. The end-of-work certificate is essential for building legalisation, periodic inspections and insurance coverage.

How often should fireproofing be inspected?

The RIPCI requires that the state of passive protection elements be part of the annual PCI review. A professional inspection every 5 years is also recommended with thickness measurement to verify that there have been no losses of protection due to works, impacts or degradation.

Fireproofing service in Catalonia

At Igniescut we carry out fireproofing work on steel structures in Barcelona, Tarragona and throughout Catalonia with certified intumescent paints and spray-applied mortars. Our team is authorised by the Generalitat and we apply the approved systems of the main European manufacturers.

Contact us for a no-obligation fireproofing quote. We inspect the structure, calculate the thickness required for the demanded R and deliver all the certified documentation.