In a fire, unprotected steel loses much of its load-bearing capacity at around 500 to 600 degrees. To keep a steel structure standing for 30, 60, 90 or 120 minutes, there are two common routes: a fire-resistant coating that foams up when heated, or board cladding that encases the profile. Both work. The choice comes down to cost, appearance, maintenance and the situation on site.
How they work
Fire-resistant coating is an intumescent paint. At around 200 degrees the layer expands into an insulating foam, often many times its original thickness, which keeps the heat away from the steel. The dry film thickness is calculated per profile based on the section factor - the ratio between the heated surface area and the mass of the profile. A slender profile heats up faster and therefore needs a thicker layer than a heavy profile at the same fire resistance requirement.
Board cladding encases the profile with fire-resistant boards, usually based on calcium silicate or gypsum. The board physically insulates the steel. The required board thickness likewise follows from the section factor and the required duration.
The comparison
| Fire-resistant coating | Board cladding | |
|---|---|---|
| Appearance | Profile stays visible, suitable for exposed work | Profile becomes a rectangular column or beam |
| Cost at 30-60 minutes | Usually more economical | Usually more expensive |
| Cost at 90-120 minutes | Rises sharply due to thick layers | Often more favourable |
| Installation time | Longer: multiple layers with drying time in between | Shorter: dry installation, immediately serviceable |
| Weather sensitivity | Sensitive to temperature and humidity during application | Barely sensitive |
| Susceptibility to damage | Damaged by impact, can be repaired locally | More robust, but repair requires board replacement |
| Complex connections | Follows any shape effortlessly | Difficult and labour-intensive around connections |
| Maintenance | Periodic checks for damage, the layer can be repaired | Little maintenance, visual check of the joints |
When do you choose coating?
- The steel stays exposed and the architecture calls for the silhouette of the profile.
- The requirement is 30 or 60 minutes; there, coating is almost always the most economical route.
- The structure has many complex connections, lattice girders or angled connections that boards are hard to fit around.
- Protection must be applied retroactively in an existing building where space around the profile is limited.
When do you choose cladding?
- The requirement is 90 or 120 minutes; coating then becomes so thick that boarding often works out cheaper.
- The steel is out of sight, above a ceiling or in a shaft, where appearance plays no part.
- The schedule is tight: board installation has no drying times and is insensitive to site conditions.
- The environment is rough, for example a warehouse with forklift traffic, where a coating is quickly damaged.
- Work takes place in low temperatures or high humidity, where coating does not cure properly.
What is decisive for both
Whichever you choose, the protection stands or falls on three things. The section factor must be determined per profile - a uniform layer thickness across the entire structure is almost always wrong. The connections to walls and floors must be included, otherwise heat still reaches the steel via the flanks. And the execution must be documented : which system, which layer or board thickness, on which profile, measured and recorded. Without that record, an inspection cannot establish that the structure achieves the required time.
A combination is fine too
In practice a mixed solution is often the sharpest: coating on the exposed columns in the entrance hall, board cladding on the beams above the suspended ceiling. There is no reason at all to roll out one technique across an entire building when the situation differs per building section.
Would you like to know which route works out best for your structure? We assess the profiles, the requirement and the situation, and cost out both options. See fire-resistant steel cladding or request a quote.
Frequently asked questions
At 30 and 60 minutes, usually yes. At 90 and 120 minutes the required layer thickness becomes so great that board cladding often works out cheaper. The section factor and accessibility help determine exactly where the tipping point lies.
Only if the underlying layer is suitable and sound and falls within the tested build-up of the system. Blasting or a suitable primer is often required. Having this assessed beforehand prevents the entire layer from detaching later.
Indoors under normal conditions, a properly applied layer lasts decades. What matters is not its age but its integrity: damage from later construction traffic is the main reason protection locally no longer suffices. Periodic visual inspection is enough.
Yes. The required dry film thickness follows from the section factor of that specific profile and the required fire resistance duration. One uniform thickness across the whole structure means part of it is too thin, and that is exactly what gets rejected at inspection.
Want to know more about fire-resistant steel cladding?
Protect the load-bearing capacity of your steel structure for up to 120 minutes in a fire, without compromising the design.
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