Structural steel is often hidden by the finished building. It sits above ceilings, within partitions, behind casings and across roof spaces, carrying loads without drawing much attention to itself.
In an occupied building, protecting that steel is rarely a simple coating exercise.
The work has to take place around people, existing finishes, active services, restricted access and the daily operation of the building. Surfaces may be old, previously painted or partly concealed. Ventilation may be limited. Temperatures may vary. Dust, noise and odour have to be controlled. The intumescent coating must still be applied to the specified thickness and allowed to cure under suitable conditions.
The required fire resistance does not change because the building is in use. What changes is the route to achieving it.
Why Structural Steel Needs Fire Protection
Steel is strong, efficient and predictable under normal conditions. Under sustained heat, however, it begins to lose strength and stiffness. If the temperature rises far enough, a loadbearing section may no longer perform as designed.
Fire protection slows that temperature rise.
An intumescent coating reacts when exposed to heat, expanding to form an insulating char around the steel. This delays heat transfer and helps the section retain its loadbearing capacity for the required resistance period.
The performance depends on several factors, including the steel section, section factor, exposure condition, required fire resistance period and the tested coating system. It also depends on the quality of the site application.
The coating is thin when compared with fire boarding or other forms of encasement. That apparent simplicity can be misleading. Its performance relies on preparation, compatibility, thickness, curing and inspection being controlled properly.
Occupied Buildings Change the Working Conditions
In a new or empty building, access can often be organised around the fire protection package. In an occupied building, the work has to fit around existing use.
Offices may remain open. Residents may stay in place. Schools, hospitals, shops or public buildings may continue operating in adjacent areas. Plant, alarms, lighting and ventilation systems may need to remain active. Routes through the building may have to be preserved.
This requires a phased approach.
Work zones need to be clearly separated. Access and escape routes must remain usable. Noise, dust and odour need to be managed. Application times may have to sit outside normal working hours. Areas may need to be handed back in stages rather than treated as one continuous site.
The technical standard remains the same, but the sequencing becomes more exact.
Begin with the Steel, Not the Coating
Before any coating is specified or applied, the steelwork needs to be understood.
The project team should identify the sections requiring protection, their required resistance period and the number of exposed faces. The steel profile and section factor influence the coating thickness. A beam exposed on three sides may require a different build-up from a column exposed on four.
Existing records are not always reliable in occupied or refurbished buildings. Steel may be concealed, altered or partly inaccessible. Connection plates, brackets and secondary members may not appear on old drawings. Previous coatings may be present without clear documentation.
A survey should establish what is there before the application begins.
The design should be based on the steel as built, not the steel as assumed.
Access Is Part of the Fire Protection Detail
On-site intumescent work needs physical access to the steel surface.
That sounds obvious, but occupied buildings are often crowded with ceilings, ducts, cables, pipework, partitions, furniture, plant and decorative finishes. Some areas may be reachable only from one side. Others may require temporary removal of ceilings, boxing or adjacent finishes.
Poor access affects more than productivity. It can affect preparation, film thickness and inspection.
If an applicator cannot reach the back of a flange, the underside of a beam or the area around a connection plate, the coating may be incomplete. If the inspector cannot see or measure the surface, compliance becomes difficult to verify.
Access should therefore be planned as part of the work, not left to the applicator to resolve with whatever space remains.
Temporary platforms, towers, protection systems and opening-up works may be required. These should be coordinated before the programme begins.
Surface Preparation Determines Adhesion
Intumescent coating must bond properly to the steel or to a compatible primer system. Existing contamination, loose paint, corrosion, dust, grease or incompatible coatings can undermine adhesion.
Occupied buildings often present uncertain surfaces.
Steel may carry old decorative paint, previous primers, patch repairs or historic coatings with no clear product record. Some areas may have been exposed to moisture. Others may sit above kitchens, plant rooms or industrial areas where grease and deposits have accumulated.
The surface needs to be inspected and prepared in accordance with the selected system.
That may involve cleaning, abrasion, removal of loose material, local repair, treatment of corrosion or application of a compatible primer. Where an existing coating is to remain, compatibility must be established rather than assumed.
Applying intumescent material over a weak or unsuitable surface only hides the problem for a while.
Existing Coatings Need Careful Review
A sound-looking painted surface may not be a suitable base for intumescent protection.
The existing product may be unknown. It may soften under the new coating, prevent adhesion or react during curing. Its thickness may also affect the total build-up and final finish.
Where product information is unavailable, testing or technical guidance may be required. Small trial areas can help assess adhesion and compatibility, but they should form part of a controlled process rather than informal reassurance.
If the existing coating cannot be relied upon, it may need to be removed.
That decision can affect cost, programme, dust control and building use. It is better resolved during survey and planning than discovered after the application has begun.
Protecting the Occupied Environment
Intumescent work takes place on the steel, but the surrounding building must also be protected.
Spray application can create overspray. Brush and roller work can still produce drips, splashes and handling marks. Preparation can release dust and debris. Access equipment can damage floors, walls and fittings. Adjacent services may need masking or temporary protection.
The work zone should be contained appropriately.
Floors, glazing, walls, ceilings, plant, furniture and finished surfaces may require sheeting, masking or temporary screens. Ventilation routes should be considered so that dust and airborne material do not spread into occupied areas. Fire alarms or smoke detection systems may require controlled isolation and reinstatement under the site’s fire safety procedures.
The purpose is not simply to keep the area tidy. It is to protect the building while the protection work is taking place.
Curing Conditions Cannot Be Ignored
Intumescent coatings need suitable conditions during application and curing.
Temperature, humidity, ventilation and substrate condition all influence the result. In occupied buildings, these conditions may be affected by heating systems, open doors, overnight shutdowns, local plant or restricted airflow.
A coating applied in poor conditions may cure slowly, remain soft, trap moisture or fail to develop the expected finish. Applying further coats too soon can create defects within the film build.
The manufacturer’s requirements should guide the process.
Environmental conditions should be checked and recorded. Drying times should be allowed for within the programme. Temporary heating or ventilation may be needed, but it must be used without creating condensation, contamination or unsafe conditions.
The programme must follow the material. The material will not adapt itself to a handover date.
Dry Film Thickness Is the Measured Evidence
The fire resistance of an intumescent system depends on achieving the specified dry film thickness.
This thickness is calculated for the steel section, section factor and required resistance period. It is not judged by colour, appearance or the number of coats alone.
Wet film readings can help the applicator control each coat during installation. Final dry film thickness measurements provide the evidence that the required build-up has been achieved after curing.
Measurements should be taken systematically across the protected section, including flanges, webs, corners and less accessible faces. Readings should be recorded by location and compared with the project specification.
Where the coating is too thin, further material may be required. Excessive thickness can also create problems, including slow curing, surface defects and poor finish.
The aim is controlled thickness, not generous application.
Connection Details Need Attention
Structural steel is not made only of clear, uninterrupted lengths.
Bolts, plates, brackets, stiffeners and connections create changes in geometry. These areas can be harder to prepare, coat and measure. They may also have different exposure conditions from the main section.
The fire protection system needs to continue around these details.
Edges and corners should receive enough material. Tight spaces around bolts and plates should not be left untreated. Where connections are concealed or inaccessible, the project team must resolve how protection and inspection will be achieved.
A well-coated beam with incomplete protection at the connection is not a complete system.
The load path continues through the joint. The protection should do the same.
Phasing and Temporary Fire Safety
Occupied-building projects are often carried out in stages. One area may be opened, prepared, coated and returned to use while another remains active.
This phasing must consider temporary fire safety.
Existing fire-resisting construction may be opened to reach the steel. Ceilings or encasements may be removed. Detection systems may be isolated temporarily. Escape routes may be affected by access equipment or work zones.
These changes need to be managed through the site fire safety plan.
Temporary measures may be needed until the permanent protection and surrounding construction are reinstated. Any alarm isolation should be controlled, recorded and reversed as soon as possible. Opened compartment lines should not be left unresolved because the coating beneath them is still curing.
Protecting the steel should not weaken the building elsewhere during the works.
Reinstating Surrounding Construction
Once the coating has cured and been inspected, ceilings, linings, casings and finishes may need to be reinstated.
This stage deserves care.
Fixings should not damage or scrape the intumescent coating. New brackets or services should not be attached through protected surfaces without approval. The required clearance around the steel should be maintained where specified. Any local damage caused during reinstatement should be repaired using the approved system.
The work should also remain inspectable until the coating records are complete.
There is little value in achieving the correct film thickness and then damaging it while closing the ceiling.
Finish Coats and Visible Steel
In some occupied buildings, the protected steel remains visible. The final appearance therefore matters.
Intumescent coatings can provide a clean finish, but the degree of smoothness depends on the product, application method, film build and condition of the steel. High-build coatings will not always resemble a thin decorative paint.
Where a decorative topcoat is required, it must be compatible with the tested system. Colour, sheen, maintenance requirements and environmental exposure should be agreed in advance.
The topcoat is not there to create fire resistance. It may provide colour, durability or environmental protection. It must not compromise the intumescent layer beneath it.
Architectural expectations should reflect the nature of the material. A protected steel section can be well finished without pretending it is ordinary painted metalwork.
Inspection and Documentation
Much of the protected steel may become hidden again after the project is complete. The evidence therefore needs to be assembled before concealment.
Records should identify the steel sections, required resistance periods, coating system, primer, intumescent product, topcoat where used, specified dry film thickness and measured results. Environmental readings, batch details, inspection reports and photographs should also be retained.
Photographs should show the wider location as well as the coated detail. Dry film thickness readings should be linked to clear section references. Any repairs, additional coats or local defects should be recorded and reinspected.
This information forms part of the building’s fire safety record.
The coating may disappear above a ceiling. The evidence should remain accessible.
Practical Checks for Occupied-Building Projects
Before on-site structural steel protection begins, the project team should confirm that:
• The steel sections and required resistance periods are known.
• The coating thickness has been calculated for the actual sections.
• Existing coatings and primers have been assessed for compatibility.
• Access is available to all required faces, connections and edges.
• The surface preparation method has been agreed.
• Occupied areas, finishes and services are properly protected.
• Ventilation, temperature and humidity can be controlled.
• Work zones and escape routes are coordinated.
• Temporary fire safety arrangements are in place.
• Wet and dry film thickness inspections are scheduled.
• Reinstatement work will not damage the completed coating.
• Product data, photographs and inspection records will be retained.
These checks create a workable process rather than a late attempt to fit coating work around a functioning building.
Why JW Simpkin Treats On-Site Intumescent Work as Controlled Construction
At JW Simpkin, on-site intumescent protection is treated as a measured fire protection system, not as ordinary painting.
The steel must be understood. The surface must be suitable. The coating build must match the required resistance. Site conditions must allow proper application and curing. The finished work must be measured, inspected and recorded.
Occupied buildings add constraints, but they do not lower the standard.
They require more careful phasing, better protection of surrounding finishes, clearer communication and closer attention to access. The purpose is to complete the work with as little disruption as reasonably possible while preserving the integrity of the tested system.
The finish may appear quiet once the work is complete. The process behind it should be exact.
Conclusion: Protecting the Structure Without Losing Control of the Building
Fire protection for structural steel in occupied buildings sits between engineering and practical site management.
The coating must protect the steel for the required period. The work must also respect the people, services, finishes and operations already within the building.
That depends on survey, access, preparation, compatible materials, controlled curing, measured dry film thickness and disciplined reinstatement. It depends on understanding that on-site intumescent work is not simply paint applied to visible steel.
It is a tested protection system installed under real conditions.
The building may remain in use throughout. The standard of the fire protection must remain intact.