Ontario Building Code Paint and Coating Requirements: Fire-Rated Assemblies and Intumescent Coatings

A commercial painting specification may contain a line that sounds simple: “Apply fire-rated paint to exposed steel.” For a painting contractor, however, that description leaves several important questions unanswered. Which structural members need protection? Is the requirement related to fire resistance or surface flame spread? What fire-resistance duration is required? Which primer, coating thickness, and topcoat are approved?

Under the Ontario Building Code, fire-protection coatings must be considered as part of a complete building design. A bucket labelled “fire-retardant” or “intumescent” does not automatically give a beam, column, wall, or ceiling a one-hour or two-hour rating. Before pricing or applying the coating, contractors need the approved project documents, the specified fire-protection system, and a clear quality-control plan.

Which Ontario Building Code Applies?

Ontario’s current code is the 2024 Ontario Building Code, which came into effect on January 1, 2025. A transition period allowed certain projects already in design to continue under the previous code until March 31, 2025. As a result, most new permit applications now fall under the 2024 Code, although older permitted projects may still be governed by earlier requirements. Contractors should confirm the applicable code edition rather than relying on the project’s construction date alone.

The 2024 Ontario Building Code adopts the National Building Code of Canada 2020 together with Ontario-specific amendments. It establishes the required performance of building components and assemblies, but it does not provide a universal paint product or thickness that works for every project.

Before submitting a quote, ask for:

• The building permit number and applicable code edition
• Approved architectural and structural drawings
• Fire-resistance schedules
• Structural steel member schedules
• The required fire-resistance duration
• The tested or listed fire-protection system
• Approved primers and topcoats
• Intumescent coating thickness schedules
• Inspection and closeout requirements

This is especially important in restaurants, warehouses, mixed-use buildings, commercial renovations, and change-of-use projects. Contractors handling hospitality work should also review the broader coating considerations covered in this guide to commercial paint for Toronto and Montreal restaurant build-outs.

Fire Resistance and Flame Spread Are Different Requirements

“Fire-rated paint” is often used as a catch-all phrase, but it can refer to products that perform very different jobs.

Fire-Resistance Rating

A fire-resistance rating describes how long a building assembly or structural element can perform under specified fire-test conditions. Depending on the building design, a wall, floor, ceiling, beam, or column may require a rating such as:

• 45 minutes
• One hour
• Ninety minutes
• Two hours
• Another project-specific duration

CAN/ULC-S101 is a Canadian standard used to evaluate the fire endurance of building construction and materials. For structural steel protection, testing and certification can be used to determine the coating thickness needed to keep the protected substrate below a defined temperature for a particular period.

Flame-Spread Rating

Flame spread evaluates how quickly fire travels across the exposed surface of a material. It is commonly considered for interior finishes and combustible materials.

CAN/ULC-S102 surface-burning testing exposes a material to controlled flame conditions and measures both flame propagation and smoke development. A coating may improve the flame-spread classification of a surface without providing a one-hour structural fire-resistance rating.

Requirement	What it evaluates	Typical coating application
Fire resistance	How long an assembly or structural member performs during fire exposure	Intumescent or other tested fire-resistive systems
Flame spread	How quickly flame travels across an exposed surface	Fire-retardant surface coatings
Smoke development	The amount of smoke generated during surface-burning testing	Tested interior-finish systems

Contractors should never substitute a coating with a favourable flame-spread result for a system that is required to provide structural fire resistance.

What Is an Intumescent Coating?

An intumescent coating reacts when exposed to high heat. The coating expands and forms a thick, insulating char that slows heat transfer into the material underneath.

On structural steel, this matters because steel loses load-carrying capacity as its temperature rises. The intumescent layer delays that temperature increase, helping the beam or column perform for the period required by the building design.

The required coating thickness is not determined by floor area alone. It may change according to:

• The size and shape of the steel member
• The ratio of exposed surface area to steel mass
• Whether three or four sides are exposed
• The required fire-resistance duration
• The steel design temperature
• Whether the member is a beam, column, hollow section, or open section
• The tested coating system
• Interior or exterior exposure conditions

UL explains that fire-test data can be analyzed to identify the coating thickness necessary for a specific substrate, design temperature, and protection period. That is why one coating thickness should not automatically be applied to every member on a project.

Normal architectural metal painting, such as the procedures discussed in how to paint metal railings, is not equivalent to applying a tested structural fire-protection system.

Intumescent Paint Is Only One Part of the System

A compliant intumescent installation typically involves several coordinated components. Changing one component without written approval can affect adhesion, durability, and whether the installation matches the tested or listed system.

The Substrate

The specification should identify the material being protected, such as:

• Structural steel beams
• Steel columns
• Hollow structural sections
• Metal deck components
• Timber or mass-timber elements
• Another approved substrate

The contractor must also understand the required surface-preparation standard. Applying an intumescent product over oil, mill scale, rust, loose paint, condensation, or an unknown coating can lead to premature failure.

The Primer

Not every rust-inhibiting or bonding primer is approved for use beneath every intumescent coating.

Primer compatibility can depend on:

• Primer chemistry
• Dry film thickness
• Cure time
• Adhesion
• Environmental exposure
• The intumescent manufacturer’s tested system
• Project-specific engineering approval

A general commercial primer may perform well in an ordinary painting system without being approved beneath the specified intumescent material. Contractors can review the basic selection principles in choosing primers for Canadian commercial projects, but the fire-protection specification must take priority.

The same principle applies to bonding primers. Strong adhesion does not, by itself, prove that a primer belongs in a tested fire-protection assembly.

The Intumescent Layer

The intumescent product must be applied to the required dry film thickness for each member. The specification may divide the project into multiple thickness zones because different steel sections need different levels of protection.

One coat may not provide the required thickness. Several controlled passes may be necessary, with minimum and maximum recoat times between applications.

The Topcoat

A decorative or protective topcoat may be used for:

• Colour
• Washability
• Moisture resistance
• Chemical resistance
• Exterior weather protection
• Improved durability in occupied spaces

However, the topcoat must be compatible with and permitted by the specified system. Applying an unapproved architectural paint over an intumescent coating can create adhesion problems or place the installation outside its tested configuration.

Fire-Retardant Paint Has a Different Purpose

Fire-retardant paints are generally intended to reduce flame spread across a material’s surface. They may be specified for certain wood, wall, ceiling, or interior-finish applications when supported by appropriate testing.

They should not automatically be presented as:

• Structural steel fireproofing
• A one-hour wall assembly
• A substitute for layers of fire-rated gypsum board
• A firestop for service penetrations
• A repair material for damaged spray-applied fireproofing
• A way to convert combustible construction into noncombustible construction

For example, contractors may encounter products categorized as INSL-X fire-retardant paint. A product page can help identify what is available, but it does not prove that the coating satisfies the approved design for a particular building.

The contractor still needs to review the current technical data, test results, permitted substrates, application rate, preparation instructions, and project specification.

How Fire-Rated Assemblies Affect Painting Work

Fire-rated construction is not limited to exposed structural steel. A painting contractor may work around:

• Fire separations
• Gypsum wall assemblies
• Floor and ceiling assemblies
• Shaft walls
• Rated doors and frames
• Structural steel fireproofing
• Mass-timber encapsulation
• Firestop systems
• Fire-rated joints
• Mechanical and electrical penetrations

The fire-resistance rating usually belongs to the complete assembly—not to the visible finish coat.

For example, a rated gypsum wall may depend on the number and type of boards, fastener spacing, stud dimensions, insulation, joint treatment, and construction details. Applying ordinary paint does not create the rating. Likewise, adding fire-retardant paint does not compensate for a missing gypsum layer.

Painting contractors should avoid:

• Sanding deeply into a rated gypsum membrane
• Filling joints with unapproved materials
• Covering firestop identification labels
• Painting over inspection openings before approval
• Damaging spray-applied fire-resistive materials
• Applying coatings over fireproofing without confirmation
• Filling penetrations with ordinary acrylic or silicone caulk
• Concealing incomplete fire-protection work

CAN/ULC-S115 is a separate testing standard used for firestop systems. An intumescent coating intended for structural protection should not be assumed to function as a tested penetration firestop.

What Contractors Should Confirm Before Pricing

The most expensive mistakes usually begin before anyone opens a paint container. A vague scope can lead to an inaccurate material allowance, unrealistic labour estimate, or non-compliant substitution.

Request the Complete Fire-Protection Specification

Do not price solely from a note that says “fire-rated paint.”

Ask for:

• The manufacturer and exact product
• The listed or certified system
• The required rating
• Steel member sizes
• Coating thickness by member
• Approved surface preparation
• Approved primer
• Approved topcoat
• Required application method
• Environmental limitations
• Inspection procedures

ULC-certified fire-resistance-rated assemblies can be located through certification directories that identify the assembly construction and approved materials. Contractors should work from the design selected for the project rather than choosing a vaguely similar system themselves.

Clarify Scope Responsibility

Determine who is responsible for:

• Abrasive blasting or mechanical preparation
• Shop primer assessment
• Primer application
• Intumescent coating
• Decorative topcoat
• Masking connections and labels
• Repairing damaged fireproofing
• Moving lifts and access equipment
• Heating and ventilation
• Thickness testing
• Third-party inspection
• Deficiency repairs
• Closeout documentation

Price the Steel, Not Just the Floor Area

Ordinary wall painting is frequently priced by square footage. Structural intumescent work may require a more detailed takeoff.

A reliable estimate should consider:

• Member type
• Member length
• Exposed sides
• Surface area
• Required thickness
• Number of application passes
• Access difficulty
• Masking requirements
• Recoat intervals
• Inspection and repair time

Contractors purchasing large volumes may benefit from planning through a bulk paint and coating supply program. However, every container supplied must match the approved product, system, and project documentation.

Application Conditions and Surface Preparation

Intumescent coatings can be sensitive to the conditions under which they are installed.

Before application, record:

• Air temperature
• Substrate temperature
• Relative humidity
• Dew point
• Ventilation
• Surface cleanliness
• Primer condition
• Primer thickness
• Time since primer application

The steel surface must remain sufficiently above the dew point to reduce the risk of condensation. Conditions should be checked throughout the shift, particularly in unheated buildings or when weather changes quickly.

Existing coatings require special attention. The contractor should not apply intumescent material over an unidentified shop primer or previously painted steel without confirmation that the existing coating is compatible, sound, and permitted by the specified system.

Wet Film and Dry Film Thickness

Thickness control is one of the most important parts of intumescent coating work.

Wet Film Thickness

Wet film thickness is measured while the coating is still wet. It helps the applicator determine whether enough material has been deposited during a pass.

Wet-film readings allow the crew to correct its spray technique before the coating cures. They do not replace final dry-film measurements.

Dry Film Thickness

Dry film thickness is measured after the coating has dried or cured sufficiently. The required thickness should be checked across the entire member, including:

• Flanges
• Webs
• Edges
• Corners
• Connections
• Difficult access areas
• Previously repaired locations

Too little material may leave the member without the required protection. Applying excessive material in a single pass can also contribute to sagging, slow curing, cracking, or adhesion problems.

The contractor should follow the measurement method, tolerances, and acceptance criteria in the project specification and manufacturer’s instructions.

Inspection and Documentation

Fire-protection work may be inspected by the consultant, fire-protection engineer, coating inspector, manufacturer’s representative, building official, or another designated party.

Contractors should keep organized records of:

• Product names
• Batch or lot numbers
• Technical data sheets
• Safety data sheets
• Delivery and storage conditions
• Surface-preparation results
• Ambient conditions
• Substrate temperatures
• Dew-point readings
• Wet-film readings
• Dry-film readings
• Recoat times
• Repair locations
• Photographs
• Inspection reports

Do not conceal the work before the required inspection has been completed. Installing ceilings, drywall, cladding, or decorative finishes too early can prevent inspectors from confirming the condition and thickness of the coating.

Appropriate gauges, spray equipment, masking materials, and access tools should be included in the project plan. These can be sourced alongside other professional paint supplies and tools, provided that the selected equipment meets the coating manufacturer’s application requirements.

Common Intumescent Coating Mistakes

Contractors should watch for these recurring problems:

• Accepting “fire-rated paint” as a complete specification
• Confusing flame spread with fire resistance
• Selecting a product based only on its marketing name
• Substituting an unapproved primer
• Applying an unapproved decorative topcoat
• Using one thickness for every steel member
• Pricing only by building floor area
• Ignoring difficult edges, connections, and concealed surfaces
• Applying over rust, grease, dust, or condensation
• Failing to record environmental conditions
• Relying on wet-film readings without final dry-film checks
• Applying overly thick passes to reduce labour
• Covering the coating before inspection
• Repairing damaged areas with ordinary paint
• Assuming a coating can replace firestop or missing assembly components

Why Trust This Guide?

This guide was developed using Ontario’s current Building Code framework, the National Building Code of Canada 2020, and official UL and ULC information concerning fire-endurance, surface-burning, structural-steel protection, and certified fire-rated assemblies.

It is intended to help painting contractors identify the information they need before quoting and applying a fire-protection coating. The approved permit documents, design professional, listed system, manufacturer’s instructions, and local authority having jurisdiction remain the controlling sources for an individual project.

Frequently Asked Questions

Does Intumescent Paint Automatically Give Steel a One-Hour Rating?

No. The achieved rating depends on the tested system, member dimensions, exposure, design temperature, required coating thickness, substrate preparation, and complete installation.

Is Intumescent Paint Required on All Exposed Structural Steel?

No. The building design determines whether the steel requires a fire-resistance rating and how that protection will be achieved. Other methods may include gypsum enclosure, concrete protection, board systems, or spray-applied fire-resistive materials.

Is Fire-Retardant Paint the Same as Intumescent Paint?

Not necessarily. A fire-retardant paint may improve surface flame-spread performance. An intumescent system may be designed to protect structural steel or another element for a specified fire-resistance period.

Can Any Primer Be Used Under an Intumescent Coating?

No. The primer must be permitted by the manufacturer, tested or listed system, engineering specification, and approved project documents.

Can Regular Interior Paint Be Applied Over Intumescent Paint?

Only when the decorative topcoat is compatible and permitted by the specified system. The allowed product, preparation, application rate, and maximum thickness should be confirmed before use.

Who Determines the Required Fire-Resistance Rating?

The building designer establishes the requirement through the code analysis, drawings, specifications, and permit process. A painting contractor should not independently decide whether a beam needs a one-hour or two-hour rating.

Does Applying More Intumescent Paint Produce a Higher Rating?

Not automatically. The installation must match the specified system and thickness schedule. Extra material cannot be assumed to create a longer rating, particularly when other system variables remain unchanged.

Treat Fire-Protection Coatings as Engineered Systems

An intumescent coating project should never be estimated or completed like an ordinary commercial repaint. Before ordering materials, confirm the applicable Ontario Building Code edition, required fire-resistance rating, certified system, substrate, primer, thickness schedule, topcoat, application conditions, inspection process, and closeout documentation.

When a tender simply says “apply fire-rated paint,” pause before assigning coverage rates or labour hours. Request the complete fire-protection design. That information protects the contractor from inaccurate pricing, unsuitable substitutions, failed inspections, and responsibility for a coating system that was never properly defined.