Fire Resistant Plaster: Enhancing Safety in Building Design

19th Jun 2024 -

Builders are increasingly turning to innovative materials to enhance fire resistance. A material gaining prominence is fire resistant plaster. For this feature, we discover how this plaster provides a robust defence against the devastating effects of fire.

What is Fire Resistant Plaster? 

Fire resistant plaster is engineered to withstand high temperatures and resist the spread of flames. Unlike traditional lime plastering, which can contribute to fire propagation, it maintains integrity and provides a protective barrier. There are 5 different types of fire resistant plaster. Each type is engineered with specific additives and compositions to optimise its performance.

1. Gypsum-based plaster

• These are the most common and versatile fire resistant plasters. They are made primarily of gypsum and lightweight aggregates like vermiculite or perlite.
• Suitable for interior wall and plaster ceiling applications, as well as for encasing structural steel elements like beams and columns.
• Provides fire resistance ratings ranging from 1-4 hours, depending on formulation and thickness.
• They are often used in commercial, residential and educational spaces because of their ease of installation.

2. Cement-based plaster

• Cement-based or cementitious plasters are based on a blend of Portland cement, sand and water.
• Known for their exceptional durability, impact resistance and ability to withstand harsh environments.
• Commonly used in industrial settings, tunnels, parking garages and areas exposed to high moisture or abrasion.
• Offers fire resistance ratings of up to 4 hours. 

3. Intumescent plaster

• These specialised plasters contain a high concentration of intumescent additives, which swell upon exposure to heat. This forms a thick carbonaceous char (a carbon-rich solid residue) insulating barrier.
• Particularly effective for protecting steel structures. The char layer prevents the rapid temperature rise that may compromise structural integrity.
• Often used in high-rise buildings and other large-span structures. 
• Provides fire resistance ratings up to 4 hours, depending on the thickness and specific formulation.

4. Lightweight plaster

• Lightweight plasters incorporate aggregates like perlite or vermiculite, resulting in a low-density material with exceptional insulating properties.
• Suitable for applications where weight is a concern, such as high-rise construction or roof assemblies.
• Commonly used for fire resistant shaft walls, enclosures for ducts and chase ways, and as a thermal barrier in roofing systems.
• Offers up to 3 hours of fire resistance. 

5. Sprayed fire resistant materials (SFRMs)

• These are cementitious or gypsum plasters designed for spray application. This allows for rapid and efficient coverage of hard-to-reach areas.
• Ideal for retrofitting existing structures, as well as protecting structural steel elements in new construction.
• Used in various settings, including high-rise buildings, industrial facilities and infrastructure projects like bridges and tunnels.
• Provides fire resistance ratings up to 4 hours, depending on the thickness of the applied material.

What are the Benefits of Fire Resistant Plaster?

The performance characteristics extend far beyond thermal insulation. With its advanced formulations and unique properties, fire resistant plastering actively participates in mitigating various fire hazards. Here, we detail the key benefits. 

Extended evacuation time

By slowing the spread of fire and heat, fire proof plaster extends the time available for people to evacuate safely. In standard fire tests, a fire plaster wall can resist fire penetration for up to 4 hours. This is compared to less than 1 hour for lime plaster.

Thermal shock resistance

Lime or gypsum traditional plaster can crack or spall when exposed to rapid temperature changes. Fire resistant plasters, especially those with glass or cellulose fibres, maintain cohesion under thermal shock, preventing dangerous debris.

Low thermal conductivity

The porous structure created by expanded vermiculite or perlite offers low thermal conductivity (typically 0.2-0.3 W/mK). It slows heat transfer through ceiling and wall plaster, keeping adjacent rooms cooler.

Smoke mitigation

Some formulations include additives that absorb or neutralise smoke particles. This can improve visibility during evacuation and reduce smoke-related damage to contents. It is particularly valuable in areas like server rooms where smoke can be as damaging as the fire.

Self-healing properties

Certain formulations have a degree of self-healing. With slightly damaged plaster spots, the heat causes the intumescent additives in the area to expand and fill small cracks. This helps to maintain the fire barrier.

High pH stability

Fire resistant plasters often have a high pH (around 9-10), which contributes to their fire resistance. This alkalinity prevents combustion and neutralises acidic combustion byproducts that might otherwise cause material degradation.

Chemical stability

The mineral-based components are chemically stable up to very high temperatures. They do not melt or produce harmful gases, maintaining their protective properties longer than many synthetic materials.

Water resistance post-fire

After fire exposure, the charred layer of intumescent plaster provides temporary water resistance. It is important in protecting water-sensitive materials or electronics from firefighting efforts or sprinkler activation.

Radiant heat reflection

Some formulations include reflective additives (like mica) that increase the plaster's ability to reflect radiant heat. This is particularly effective in protecting combustible materials that might ignite from radiant heat without direct flame contact.

Thermal mass

The density of fire resistant plaster provides significant thermal mass. It helps moderate temperature swings, reducing the frequency of reaching critical temperatures that could compromise structural elements.

Hygroscopic properties

Many fire resistant plasters are hygroscopic, absorbing excess moisture from the air and releasing it when conditions are dry. This helps regulate humidity and prevents the risk of electrical fires because of condensation on wiring or equipment.

Fire vs Heat Resistant Plaster: What is the Difference?

While both offer protection against high temperatures, their intended purposes and performance capabilities differ significantly. The following table highlights the key differences between these 2 plaster types.

Factor	Heat resistant plaster	Fire resistant plaster
Purpose	Thermal insulation and preventing heat transfer.	Fire protection and structural integrity during fires.
Specialised additives	Generally, heat resistant plaster lacks fire-retardant additives.	Contains vermiculite, perlite, fibres and intumescent compounds.
Fire performance	Provides thermal insulation but has limited fire resistance.	Meets fire resistance ratings (1-4 hours); forms insulating char layer.
Applications	Overall, it is used for projects like industrial ovens, furnaces and fireplaces.	Load-bearing structures, fire-rated assemblies, structural fire protection.
Compliance	Does not meet fire standards and building codes for fire resistance. Not suitable for various applications.	Designed to meet building regulations like Approved Document B for fire-rated construction.

How to Boost Plaster Fire Resistance

Various techniques can be introduced to further boost the performance of fire resistant plasters. These methods utilise plaster characteristics and involve optimising the application process. Below, we discover how to optimise fire resistant plasters. 

1. Increased thickness application

A straightforward technique is to apply a thicker layer of the plaster. The thicker the plaster, the longer it takes for heat to penetrate and reach the underlying surface. This extended protection time allows for safer evacuation and firefighting efforts. Thicker applications also provide a more robust char layer.

2. Incorporation of additional fibres

Reinforcing the plaster with added fibres like polypropylene, glass or steel greatly improves its fire resistance. These fibres help to maintain the plaster's structural integrity at high temperatures, preventing cracking and spalling. Plus, they enhance the bonding of the char layer, ensuring it remains intact and provides thermal insulation.

3. Intumescent coating or topcoat

Applying an intumescent coating or topcoat over the fire resistant plaster enhances fire protection. These coatings contain intumescent additives that swell and form a thick, insulating char layer when exposed to heat. This char layer acts as an extra barrier, increasing the overall fire resistance rating of the plaster system.

4. Layering with other materials

Combining fire resistant plaster with other materials such as mineral wool or ceramic fibre blankets creates a multi-layered system. Each layer contributes its unique fire-resistant properties, providing even greater thermal insulation and structural integrity during a fire event.

5. Optimised curing process

Proper curing is crucial for ensuring their optimal performance. Manufacturers usually detail specific curing instructions, including maintaining temperature and humidity levels during the process. Following these guidelines ensures that the plaster achieves its designed density, strength and fire resistance properties.

6. Phase change materials (PCMs)

Some advanced formulations incorporate PCMs like paraffin, which absorb and release thermal energy during phase transitions. When exposed to heat, PCMs absorb energy as they melt, acting as a heat sink and delaying the temperature rise within the plaster. This extends the fire resistance duration and offers added protection to the underlying substrate.

7. Vapour barriers or sealants

Applying a vapour barrier or plaster sealant over the plaster prevents moisture intrusion. This can compromise the plaster's fire resistance over time. These barriers prevent the release of potentially harmful gases or fumes from the plaster during a fire event.

8. Proper substrate preparation

Ensuring that the substrate surface is properly prepared before applying plaster is essential for optimal performance. Techniques such as roughening the surface, applying bonding agents, or using mechanical anchors enhance the plaster's bond strength. Adopting these techniques reduces the risk of delamination or spalling during a fire.

Is Clay Plaster Fire Resistant?

Clay plaster is known for its breathable, durable and aesthetic benefits. However, when it comes to fire resistance, the performance is limited compared to the above plaster types. In its natural state, clay plaster does not have significant fire-resistant properties. 

The primary components of clay plaster including sand and fibrous additives are combustible materials. These materials can contribute to the spread of fire. When exposed to high temperatures, the organic fibres act as fuel, potentially compromising the integrity of the plaster system.

However, it is possible to enhance the fire resistance of clay plaster through various methods and additives. 

• Mineral additives: Incorporating non-combustible mineral additives like perlite or vermiculite into the clay plaster mix may improve its fire resistance. These lightweight aggregates create an insulating barrier that slows heat transfer and reduces the plaster's combustibility.
• Fire retardant additives: Certain chemical compounds, such as borates or phosphates, can be added to clay plaster to create fire-retardant properties. These additives help stop combustion and reduce the spread of flames.
• Intumescent coatings: Using an intumescent coating over the clay plaster builds an additional layer of fire protection. These coatings swell and form an insulating char layer when exposed to heat, creating a barrier against flames. 
• Increased thickness: Applying a thicker layer of clay plaster improves its fire resistance. It increases the distance that heat must travel to reach the underlying substrate or structural elements.

Can You Repair Fire Resistant Plaster?

Fire resistant plaster can be susceptible to wear, damage or degradation over time. Proper repair techniques are key to ensuring they continue to perform as intended, safeguarding the structural integrity of buildings. Here, we explore repair techniques to consider.

Patching and resurfacing

Small cracks or surface defects can often be repaired by patching plaster walls or ceilings with a compatible compound. You can also resurface with a new layer of the same material for a clean finish.

It is important to properly prepare the damaged area. This ensures a secure bond between the existing plaster and the repair material. Manufacturer-approved patching compounds or plaster mixes should be used to maintain the same fire resistance rating.

Segment replacement

In cases of more extensive damage like large cracks or spalling, it may be necessary to remove and replace sections. This typically involves carefully cutting out the damaged area, preparing the substrate and applying new layers of plaster.

The new plaster will need to match the original thickness and composition. Proper curing and finishing techniques should be followed to ensure a seamless integration with the existing plaster. 

Intumescent coatings

For localised areas of damage, applying an intumescent coating over the affected area provides an added fire protection layer. The insulating char layer temporarily restores the fire resistance until a more comprehensive repair is performed.

How Much Maintenance Does Fire Rated Plaster Need?

Regular maintenance and inspections identify potential issues with fire resistant plaster systems before they become more severe. Below are some recommended maintenance practices.

Visual inspections

Conduct regular visual inspections to check for signs of cracking, spalling delamination or other visible damage. Pay particular attention to high-traffic areas, corners and areas prone to impact or moisture exposure. Visual inspections should be conducted annually or more frequently in high-traffic areas.

Environmental monitoring

Monitor environmental conditions like humidity levels and temperature fluctuations. These can impact the long-term performance of fire resistant plasters. Address any issues related to moisture intrusion, condensation or excessive drying, which can compromise the plaster's integrity.

Adhesion testing

Carry out adhesion tests (e.g. pull-off tests) to evaluate the bond strength between the plaster and the substrate. Poor adhesion leads to delamination and reduced fire resistance. Adhesion testing is recommended every 3-5 years.