Intumescent coatings are the standard fire protection system for exposed structural steel in commercial buildings across Ireland. They appear in office interiors, retail centres, sports facilities, public buildings, and any construction type where the architect has specified exposed steelwork as part of the design language of the space. Despite their widespread use, the specification, application, and quality assurance process for intumescent coatings is less well understood than it should be by the main contractors and developers who are responsible for procuring them.
This article is written for the construction industry in Ireland: main contractors, project managers, developers, and procurement teams who need a practical understanding of intumescent coatings without the complexity of a detailed fire engineering study. It covers what intumescent coatings are, how they work, how they are specified, the application process, and what documentation is required for building control sign-off in Ireland and Northern Ireland.
FireShield Ireland carries out intumescent coating work on structural steel across Ireland and Northern Ireland. Full details of our intumescent coating service are at https://fireshieldireland.com/services/intumescent-coatings/.
What Intumescent Coatings Are and How They Work
An intumescent coating is a fire protective coating applied to structural steel that behaves in a fundamentally different way to conventional paint when exposed to the temperatures generated in a building fire. At ambient temperatures and under normal building conditions, an intumescent coating applied to a steel beam or column looks very similar to a conventional paint finish. It is smooth, can be specified in any colour, and provides a finish quality that is appropriate for architectural applications where the steel is part of the visual design of the space.
The difference between an intumescent coating and conventional paint becomes apparent only when the coating is exposed to heat. At temperatures typically above 200 degrees Celsius, the chemical components of the intumescent coating undergo a reaction that causes the material to expand rapidly to many times its original thickness. A coating applied at a dry film thickness of perhaps two millimetres will expand to form a carbonaceous char layer of 50 millimetres or more in thickness within minutes of being exposed to fire temperatures. This char layer is highly insulating and acts as a thermal barrier between the fire and the steel surface beneath it.
The purpose of this thermal barrier is to slow the rate at which heat is conducted from the fire to the steel. Structural steel loses its load-bearing capacity progressively as its temperature rises, and most structural steel grades reach their critical temperature, the point at which they cannot support their design load, at around 550 degrees Celsius. By slowing the rate of temperature rise in the steel, the intumescent char layer extends the time it takes for the steel to reach this critical temperature. This extension of time is the fire resistance period achieved by the coating, expressed in minutes: 30 minutes, 60 minutes, or 90 minutes are the most common requirements for commercial buildings in Ireland.
Thin Film vs Thick Film Intumescent Systems
Two categories of intumescent coating are used on structural steel in Ireland: thin film and thick film systems. Understanding the difference is important for correct specification.
Thin Film Intumescent Coatings
Thin film intumescent coatings are applied at dry film thicknesses typically ranging from around 0.5 millimetres to 5 millimetres, depending on the section being protected and the required fire resistance period. At these thicknesses the coating provides a finish quality comparable to a conventional paint and is appropriate for all architectural and exposed steel applications. Thin film systems are the standard choice for offices, retail spaces, public buildings, sports facilities, and any building type where exposed structural steel is part of the design intent.
The required dry film thickness for a thin film intumescent system on any given structural member is not a single value that applies to all steel sections. It is calculated for each section type using the product’s certified fire resistance data. The calculation takes into account the section factor of the steel member, which is the ratio of its heated perimeter to its cross-sectional area, and the required fire resistance period. A large, heavy universal column with a low section factor requires less intumescent thickness than a small, light universal beam with a high section factor for the same fire resistance period. This is why a project-specific thickness schedule, produced from the structural drawings and the product’s fire resistance data, is a necessary part of any intumescent coating specification.
Thick Film Intumescent Systems
Thick film intumescent systems are applied at considerably greater thicknesses than thin film systems, typically 5 millimetres or more. They are used where higher fire resistance periods are required than thin film systems can achieve at a practical application thickness, or where the structural sections are very large. Thick film systems have a distinctly different surface texture from thin film systems and are generally not appropriate for architectural applications where finish quality is important. They are more commonly used on concealed steelwork where fire resistance requirements are demanding.
Where Intumescent Coatings Are Specified
The fundamental criterion for specifying an intumescent coating rather than a spray-applied system such as Monokote is whether the structural steel is visible in the finished building. Concealed steelwork, above suspended ceilings, within service voids, or in areas not accessible to building occupants, is typically protected with Monokote spray fireproofing, which is more efficient on large concealed areas. Exposed and architectural steelwork is protected with intumescent coatings to achieve the required fire resistance while maintaining an acceptable surface finish.
In practice, most commercial construction projects in Ireland involve a combination of both systems, with Monokote on the concealed structural frame and intumescent coatings on any exposed steel features or structural elements that form part of the architectural design. The interface between the two systems must be carefully co-ordinated at design stage and managed on site to avoid gaps, incompatibility issues, or double-counting of fire protection.
Office Buildings
Contemporary commercial office design frequently features exposed structural elements, particularly in reception areas, atria, and large open-plan floor plates where architectural steel trusses, transfer beams, or feature columns are visible. These elements require intumescent coating to achieve their fire resistance rating while maintaining the visual quality that the architect has designed.
Retail Centres and Mixed-Use Developments
Retail centres and mixed-use developments often incorporate large exposed steel structures at entrances, canopies, and in main concourse areas. The scale of these structures and their visibility to large numbers of people makes both the aesthetic quality and the fire resistance of the coating critical.
Sports Facilities and Leisure Buildings
Sports facilities and leisure buildings regularly use exposed roof structures with long-span steel trusses and arched frames that are a deliberate architectural feature of the building. Intumescent coating of these structures allows them to achieve their required fire resistance without concealment.
Schools and Educational Buildings
Schools and educational buildings increasingly incorporate exposed structural elements as part of their design, particularly in newer buildings where the structure is expressed as an educational feature or where the budget for ceiling finishes is limited by keeping the structure visible. Intumescent coating allows the steel to remain exposed while meeting building regulations requirements.
Public Buildings
Public buildings including libraries, museums, civic buildings, and transport infrastructure often feature architectural steelwork as a deliberate design statement. The precision of intumescent coating application and the quality of the finished surface are particularly important in these high-profile applications.
Specifying Intumescent Coatings: What the Structural Engineer and Architect Need to Determine
Specifying intumescent coatings correctly requires input from both the structural engineer and the architect, and ideally early engagement with the specialist fire protection contractor.
The structural engineer determines the required fire resistance period for each structural element based on the building regulations applicable to the project. In the Republic of Ireland this is in accordance with Technical Guidance Document B. In Northern Ireland it is in accordance with Technical Booklet E. The required fire resistance period is typically 30, 60, or 90 minutes for most commercial building types, though some building types require longer periods.
The structural engineer also provides the structural section information required to calculate the intumescent coating thickness. This includes the section designation and weight for every exposed structural member that requires fire protection, the section factor for each member, and the design load ratio where this is used to reduce the required fire resistance period through a fire engineering calculation. We prepare the project-specific thickness schedule from this information using the certified fire resistance data for the specified intumescent product.
The architect determines the aesthetic requirements for the coating. This includes the required surface finish, whether the coating should be smooth or whether a textured finish is acceptable, the specified colour, whether the topcoat will be applied by the intumescent coating contractor or by the main decorative contractor, and the standard of finish quality required at inspection. Clear communication of these requirements at the specification stage avoids misunderstandings and disputes about finish quality at the end of the project.
Product selection should take into account the intumescent system’s track record, the availability of certified fire resistance data for the section sizes on the project, the compatibility of the system with the substrate primer and the specified topcoat, and the contractor’s experience with the system. We advise on product selection as part of our pre-contract support service.
Surface Preparation Before Intumescent Coating
The performance and durability of an intumescent coating system depends critically on the quality of the surface preparation and the primer applied before the intumescent material. This is an area where corners are sometimes cut on commercial projects in Ireland, with consequences for both the durability of the coating and potentially its fire resistance performance.
Structural steel that will receive intumescent coating should be blast cleaned to a minimum cleanliness standard of Sa 2.5 in accordance with ISO 8501-1, which is near-white metal blast cleaning. This standard of preparation removes all mill scale, rust, and contamination from the steel surface and creates a surface profile that allows the primer to achieve good mechanical adhesion. Lower standards of surface preparation, such as wire brushing or mechanical grinding, are not adequate for intumescent coating applications.
The primer applied over the blast-cleaned steel must be compatible with the intumescent system. Not all primers are compatible with all intumescent products. The primer and intumescent must be from a compatible system, meaning they have been tested together and the combination is covered by the fire resistance certification. Using an incompatible primer can result in adhesion failure of the intumescent coating and delamination, which in a fire event means the char layer does not form properly on the steel surface.
On projects where the structural steel is shop-fabricated and shop-primed by the steel fabricator before delivery to site, the primer specification must be confirmed with the fire protection contractor before the fabricator applies the primer. This co-ordination is most effectively done at the design stage when the fabricator is being briefed, not on site after the steel has already been primed. We engage with project teams at this stage specifically to confirm primer compatibility and avoid the cost and programme impact of remedial primer work on site.
The Intumescent Coating Application Process
Intumescent coating application is carried out by airless spray in most commercial applications, which provides the fastest and most consistent application of the product over large areas of structural steel. Brush and roller application are also possible and are used in areas where spray application is not practical, for example in tight corners, connection details, or where masking requirements make spray application difficult to control.
The application process involves applying the intumescent material in one or more coats to build up to the required dry film thickness. Many thin film intumescent systems can achieve the required thickness in a single coat for lower section factors and shorter fire resistance periods. For higher section factors or 90-minute fire resistance requirements, multiple coats with drying intervals between them may be required.
Wet film thickness is monitored during application using wet film combs, which give an indication of the applied thickness in the wet state. Wet film measurements are used as a guide during application but the critical measurement is the dry film thickness after the material has cured, as the relationship between wet and dry film thickness varies with application conditions. Dry film thickness is measured using calibrated magnetic induction gauges after the coating has fully dried.
The specified minimum dry film thickness for each structural member type on the project must be achieved across the full surface of the coated steel. Local thin areas in areas of complex geometry, connection details, or web stiffeners are a common deficiency and must be addressed during the application before the next coat is applied or before the application of the topcoat.
Topcoat Application
Most intumescent coating specifications on commercial projects in Ireland include a decorative topcoat over the intumescent layer. The topcoat serves several purposes. It provides the specified colour finish. It seals the intumescent layer from moisture ingress, which is important for the long-term durability of the system particularly in internal environments with higher humidity. And it provides a smooth, consistent surface finish that meets the architectural quality requirements of the project.
The topcoat must be compatible with the intumescent system. Not all topcoat products are compatible with all intumescent systems. Incompatible topcoats can interfere with the expansion of the intumescent layer in a fire and may reduce the effective fire resistance of the system. We confirm topcoat compatibility as part of the specification confirmation process for every project.
Topcoat application can be carried out by the intumescent coating contractor or by a separate decorative contractor depending on the project arrangements. Where the topcoat is applied by a separate contractor, that contractor must use only the specified compatible topcoat products and must apply them in accordance with the intumescent system manufacturer’s requirements. We provide guidance notes for separate decorative contractors to ensure that the intumescent system is not compromised by inappropriate topcoat products or application methods.
Documentation for Building Control
Every intumescent coating installation we carry out in Ireland and Northern Ireland is supported by a complete documentation package for building control. The package includes the product data sheets and technical information for the intumescent system applied, the project-specific thickness schedule showing the required dry film thickness for each structural section type on the project, the dry film thickness records showing all measurements taken during the application, details of any remedial work carried out and the re-measurement records confirming the remedied areas meet the specification, confirmation of the primer and topcoat products used and their compatibility with the intumescent system, and a completion certificate confirming the intumescent coating has been applied in accordance with the system specification.
This documentation is required for building control sign-off in both Ireland and Northern Ireland and is provided as standard on every project. We liaise directly with building control officers where required and attend site inspections where the building control authority wishes to view the installed coating.
Getting Intumescent Coating Right from the Start
The most common problems with intumescent coating on commercial projects in Ireland arise from insufficient attention to specification, primer compatibility, and programme at the design and pre-construction stage. A clear, complete specification, early engagement with the fire protection contractor, and realistic programme planning for the application and drying time prevents the majority of these problems.
FireShield Ireland provides specification support, product selection advice, primer compatibility confirmation, preliminary pricing, and programme information at the design stage for any commercial project in Ireland or Northern Ireland involving intumescent coatings on structural steel. We work across all 32 counties and throughout Northern Ireland and are available to attend design team meetings to input into the specification at the appropriate stage.
Contact us to discuss your project at any stage of the design or construction process.
https://fireshieldireland.com/services/intumescent-coatings/
https://fireshieldireland.com/services/structural-steel-fireproofing/
North: +447340781054 South: +353871128539