Intumescent coatings: how thickness and fire rating are determined

The thickness of an intumescent coating follows from three things set before the job: the fire-resistance rating the code requires for the member, the listed design selected (tested to CAN/ULC-S101, UL 263 or, for hydrocarbon fires, UL 1709) and the massiveness of each steel section. The designer and the manufacturer produce a thickness schedule; the applicator applies and measures the prescribed thickness, but does not calculate it.

An intumescent coating swells under heat and forms an insulating char that slows the heating of steel. It allows exposed steel framing while meeting fire-resistance requirements. Every beam and column still has to receive the right thickness, over the right primer, with a compatible topcoat. This guide is for project managers, industrial building managers and maintenance teams in Quebec and Ontario preparing an intumescent coating project.

Who sets the fire-resistance rating?

The fire-resistance rating is set by the applicable building code, based on the building's occupancy, height and area, then assigned to each member by the project's designers (architect and structural designer). It is expressed as the duration during which the member must keep its function in a standardized test.

In Quebec, the requirement comes from the Construction Code; in Ontario, from the Ontario Building Code. Drawings and specifications state which members must be protected (columns, beams, bracing, floors) and to what rating. The authority having jurisdiction confirms those choices at permit and inspection.

This division of roles matters: the applicator decides neither the rating, nor which members to protect, nor the listed product. It carries out protection defined by the project documents. When drawings are silent or ambiguous, the question goes back to the designer before work starts.

What is a listed design?

A listed design is a specific assembly (section type, intumescent product, primer, thicknesses, conditions) that has been subjected to a standardized fire-resistance test and published in a listing by a testing organization. It is what gives the achieved rating its validity.

Three test standards come up in North America:

The choice of standard depends on the hazard. A warehouse or manufacturing plant generally falls under a cellulosic fire; a refinery, fuel depot or process unit may require protection tested for hydrocarbon fire. Products are not interchangeable: a product listed for cellulosic fire is not automatically valid to UL 1709. The listed design has to match the product actually applied, and substitutions go through the designer.

How does section massiveness affect thickness?

Massiveness directly affects thickness: a heavy, compact section heats more slowly than a light, thin section exposed over the same surface, so it needs less product to reach the same rating. A very light section may need a large thickness, or may not be able to reach certain ratings with a given product.

This is expressed as a ratio between the steel mass and the surface exposed to fire: the W/D ratio in North America (weight per unit length over heated perimeter), or the Hp/A section factor in European references, which varies the opposite way. The more massive the section, the higher the W/D and the lower the Hp/A.

In practice:

• Each section has its own thickness. One structure often has several thicknesses depending on beam and column sizes.
• Exposure matters. A beam exposed on three sides (under a slab) and a column exposed on four sides do not have the same heated perimeter.
• The thickness schedule governs. The manufacturer, from the listed design and the project's sections, produces a schedule; the designer approves it. That document is what the application crew follows.

How is thickness applied and checked?

The prescribed thickness is applied in one or more coats per the data sheet, then checked with wet film thickness readings during application and dry film thickness readings after drying. Readings are taken member by member, following the measurement plan in the specification or from the manufacturer.

A few practical rules:

• Measure during application. A wet film gauge allows immediate correction; insufficient thickness found once dry requires an extra coat and another wait.
• Deduct the primer. A reading on steel includes the primer. The primer is measured before the intumescent is applied so it can be deducted; otherwise the actual intumescent thickness is overestimated.
• Cover every face. Flanges, web, edges and hard-to-reach areas (near connections, under decks) receive the same thickness as the rest.
• Record. Each member has its log: mark, prescribed thickness, measured values, product batches, ambient conditions. See the dry film thickness entry.

Ambient conditions (temperature, relative humidity, dew point) strongly affect drying, especially for water-based products. A winter job is planned with heating and ventilation.

Why is primer and topcoat compatibility critical?

It is critical because the listed design was tested with certain primers and topcoats: an incompatible primer can release under heat and drop the expanded char, and an unsuitable topcoat can restrict swelling. The complete system has to be approved by the intumescent manufacturer.

• Primer. It protects steel from corrosion and provides adhesion. Its type and thickness have to be among the accepted primers. Existing shop primers and galvanized steel need specific checks, sometimes a tie coat.
• Surface preparation. It follows the primer data sheet, generally abrasive blasting or power tool cleaning depending on the steel condition; it falls under our steel structure painting service.
• Topcoat. It protects the intumescent from moisture, impact and UV, and provides colour. It is required in exterior or humid exposure for most products, and its thickness is limited by the data sheet.

The exposure environment (dry interior, humid interior, exterior) guides the choice of product itself. A product designed for dry interiors does not suit steel exposed to the weather. The complete system falls under our protective coatings service.

How is inspection carried out?

Inspection checks that what was applied matches the listed design and the thickness schedule: right product, right primer, compliant thickness on every member, specified topcoat, no damage. It is done by the project inspector or a designated third party, at the request of the owner or the authority having jurisdiction.

The handover file usually includes:

• data sheets and the listed design reference;
• the thickness schedule approved by the designer;
• thickness logs per member, with primer deducted;
• product batches and ambient conditions;
• photos of repaired areas.

After handover, other trades can damage the protection (fasteners, anchors, welding, cable runs). Any repair is made with the same system and thickness, then recorded. In maintenance, periodic inspection spots damaged or delaminated areas.

How to prepare an intumescent coating project

You prepare the project by gathering the drawings showing required ratings, the selected listed design, the list of sections and their exposure, the service environment and the steel condition. With those documents, the manufacturer produces the thickness schedule and the application crew plans coats, preparation and access.

Our crews and qualified subcontractors apply intumescent systems in industrial and commercial buildings, working from LaSalle, including Montreal, Montérégie and Ontario. The service is described on the intumescent coatings page. Send drawings, the specification and photos through the request a quote form: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can the applicator calculate the thickness?

No. Thickness follows from the listed design, the required rating and the massiveness of each section. The manufacturer produces the thickness schedule and the designer approves it; the applicator applies and measures the prescribed thickness.

Is an intumescent coating suitable outdoors?

Some products are, with a suitable topcoat; others are limited to dry interiors. The choice depends on the exposure environment stated in the data sheet and the listed design.

What is the difference between UL 263 and UL 1709?

UL 263, like CAN/ULC-S101, simulates a building fire fuelled by cellulosic materials. UL 1709 simulates a hydrocarbon fire whose temperature rises much faster. Sites with hydrocarbon hazards often require protection tested to UL 1709.

Can an intumescent be applied over old paint?

Only if that paint is among the primers accepted by the manufacturer and is sound and well bonded. When in doubt, run an adhesion test and ask the manufacturer; otherwise, strip it and apply an approved primer.