Navigating Fire Safety Challenges with Green Wall Postmark Phase 2 with SOCOTEC's Specialised Expertise

We carry out fire engineering analysis for every uncommon situation including living wall / green roof systems. Our main objective is that the installed system can adequately resist the risk of rapid fire spread and then to support other environmental objectives which includes biodiversity support, air quality improvement, Carbon Sequestration, and mitigation of Urban Heat Island Effect.

In 2020, SOCOTEC UK was appointed by both McAleer & Rushe (Main Contractor) to develop the fire safety strategy for three residential blocks in Mount Pleasant phase 2 and by Taylor Wimpey Central London for Postmark (PMC) to oversee the external wall design and construction, to ensure that the design aligns with the requirements stipulated in regulation 7 and part B4 of Building Regulations, and ultimately supporting the issuance of EWS1 forms upon completion of the project.

The design incorporated a modular green wall system on the northwest elevation of Block D. Since this block is less than 18 meters in height, strict compliance with regulation 7(2) wasn't mandatory, permitting the use of combustible materials on the external wall, provided that a thorough assessment of fire spread risk on the external wall confirms that the requirements of Part B4 of the Building Regulations can be met.

The analysis was based on DCLG guidance “Fire Performance of Green Roofs and Green Walls” published in August 2013, the latest research studies and common good practice.

The first step in designing a green wall fire strategy is to carry out a SWIFT fire risk analysis to ensure that the strategy considers all credible scenarios and can meet the functional requirements of Parts B1 – B5 of the Building Regulations. The main factors that should be considered are:

• How hard it is to ignite the organic material?
• What is the likelihood and speed of fire spread through the organic material?
• What is the likelihood, speed and consequences of fire spread back into the building, or to other buildings?
• What are the available mitigative measures should a fire grow rapidly, and can a typical fire engine provide adequate access and water supply to conduct necessary external firefighting operations?

Upon completion of the SWIFT analysis, the final fire strategy will usually consist of multiple layers of protection that can reduce the likelihood of fire by limiting ignition sources, reduce the consequences of fire by controlling the fuel load and providing adequate mitigative measures to support external firefighting operations.

The below Swiss cheese model shows an example of the main layers of protection used in the fire strategy on the Mount Pleasant modular green wall system.

The above layers of protection and fire strategy design can be divided into three main steps:

Reducing Fuel Load

At the time of publication of this case study, there is still no modular green wall system in the market that is non-combustible (class A1 to EN 13501-1) or limited combustible (class A2-s3,d2). This is mainly due to the presence of HDPE modules as well as other plastic components and accessories.

However, the type of vegetation, leaves, growing medium and humidity play a critical factor in understanding the combustibility of the system where in some cases it can still achieve a class B classification.

Nevertheless, even if a system was developed using only metallic components and non-combustible vegetation, such fire performance cannot be guaranteed during the system life span unless regular irrigation and maintenance is carried to limit dead leaves and twigs.

Therefore, it is important to document in the fire strategy:

• The type of vegetation, the fire reaction, the relevant tests and installation guidelines to ensure that it can comply with the fire reaction requirements under Diagram 40 of Approved Document B based on the building type and height.
• The Irrigation and Maintenance schedule to clarify that such control measures need to be incorporated in the Building Management Strategy.

Reducing Ignition Risks

To limit the ignition risks, ignition sources should be identified in the SWIFT Fire Hazard Analysis which may include electric installations, lightning, arson and openings as flame can emit from windows when fire develops to flashover. 

Having identified the ignition sources, the strategy should aim to eliminate or reduce the ignition sources. This can be achieved by:

• Removal of electrical installations.
• Provision of a vertical setback distance between the grade level and the living wall in order to prevent arson activities.
• Provision of a fuel break between the windows/doors and the living wall to limit the risk of fire spread from inside the building.

As an example, the below picture on the left for Mount Pleasant depicts the fire engineer’s requirements during the design stage for providing a fuel break between the vertically aligned windows and the living wall, as well as between the door on the grade level and the adjoining living wall, which was a restricted access area and therefore deemed to be acceptable in terms of arson fire risk.  The picture on the right shows the installed green wall in line with the fire strategy requirements.

Providing Mitigative Measures

In the field of Fire Engineering, it's essential to recognise that no fire strategy can be entirely devoid of risk. Even with effective management of fuel load and ignition risks, it remains necessary to incorporate additional provisions to address unforeseen fire events. The primary mitigative barriers that warrant careful consideration in the context of green wall systems include:

• Adequate Fire and Rescue Access to the building and façade holding the green wall to allow external firefighting operations. While evaluating the fire and rescue access, the height of the building should be considered as it will be important to understand whether a high reach appliance with larger pumping capacity and aerial platform will be needed, or a typical fire engine can be used.
• Fire water supply should be made available for the fire and rescue services where at least one hydrant should be located within 90 m from the building. If the hydrant is existing, the designer should make sure that the necessary inspections and maintenance are conducted.
• Proximity of neighbouring buildings can also have an impact on the fire strategy. When a building is located in very close proximity, a fire in a large compartment could lead to high radiant heat exposure and therefore an increased risk of fire spread to other buildings which is against the functional requirements of the Building Regulations (B4).

Conclusion

In conclusion, the integration of sustainable design elements, such as living walls or green roofs, in buildings mirrors the challenges faced in Block D of Mount Pleasant. To ensure the successful implementation of green building practices, meticulous attention must be given to addressing potential fire risks associated with these eco-friendly features. The imperative lies in the careful planning and execution of these designs to strike a balance between environmental sustainability and fire safety.