Green Roof Fire Safety: What Specifiers Need to Know

Green roofs can support sustainable drainage, biodiversity and building performance, but fire safety must form part of the design and specification process.

Fire performance depends on the complete roof construction and its surface finish. The waterproofing system, growing medium, vegetation and other roof components all need to be considered as part of the finished build-up.

This guidance focuses primarily on England, where Approved Document B supports the fire safety requirements of the Building Regulations. Different requirements apply across Scotland, Wales and Northern Ireland, so designers should check the regulations and guidance relevant to each project.

Understanding Green Roof Fire Performance

External roof fire performance concerns the ability of a roof construction to resist fire spread across its surface and fire penetration from an external source.

The ABG technical note explains that the performance of the finished roof build-up is generally more relevant than assessing the fire rating of every individual component or layer. The focus is on the fire spread and penetration resistance of the completed roof surface.

This distinction is important for green roofs because vegetation and growing media sit above the waterproofing system. Their condition, composition and arrangement can affect how the finished roof performs.

Dry or dead vegetation can introduce additional combustible material, so planting design and long-term maintenance also need to form part of the fire safety strategy.

External Fire Performance and BROOF (t4)

BS EN 13501-5 is used to classify the external fire performance of roofs. It considers fire penetration and the spread of flame caused by an external fire source rather than fire originating inside the building.

BROOF(t4) is the highest classification within this system. In England, a roof construction achieving BROOF(t4) is regarded as unrestricted in relation to distance from a relevant boundary.

The “(t4)” element refers to Test 4 under the external fire test methods referenced by EN 13501-5. The ABG technical note states that this is the test used to demonstrate compliance with Approved Document B requirements for external roof fire performance.

Specifiers should obtain fire performance information that relates to the roof construction being proposed rather than relying on the classification of one component in isolation.

Growing Media and Fire Safety

Growing media forms an important part of green roof fire design.

The ABG technical note summarises Green Roof Organisation guidance stating that, where growing media is installed on a green roof without permanent irrigation, its organic content should be below 20% and the substrate should be peat free.

Controlling organic content helps reduce the amount of combustible material within the roof surface while still providing suitable conditions for planting.

The depth, composition and specification of the growing medium should therefore be confirmed as part of the wider roof design.

Fire Breaks and Vegetation-Free Zones

Green roofs can require vegetation-free areas that help restrict fire spread and separate planting from roof edges, penetrations and other features.

The GRO guidance summarised within the ABG technical note specifies fire breaks of at least:

• 500 mm wide and 50 mm deep, using 20 to 40 mm rounded pebbles, or
• 40 mm thick concrete or stone paving slabs around roof perimeters and penetrations.

It also states that an additional 1 metre wide fire break should be installed every 40 metres across the roof.

These figures provide much clearer specification guidance than simply stating that fire breaks “may be required”. However, designers should still refer to the current GRO Code, relevant Building Regulations guidance and system-specific information before finalising a roof design.

Vegetation Selection and Dry Weather

Plant selection can also influence fire risk.

Vegetation that becomes dry or dies back can increase combustible material at roof level. Species should therefore be selected with the growing conditions, exposure, irrigation strategy and maintenance programme in mind.

Sedum and other plants commonly used on extensive green roofs still require periodic inspection. Dead material should be removed, unwanted growth controlled and vegetation kept away from designated fire breaks.

Periods of prolonged dry weather also need consideration. Some roof systems may require supplementary irrigation to maintain healthy planting and prevent excessive drying.

The need for irrigation will depend on the vegetation type, substrate, roof design, location and anticipated weather conditions.

Maintenance and Fire Break Management

Fire safety does not end once the roof has been installed.

The ABG technical note highlights ongoing maintenance as an important part of fire performance. Vegetation should be prevented from growing across fire breaks, while plant material should be removed or cut back at suitable times of year.

A green roof maintenance programme should therefore include checks for:

• Dead or dry vegetation.
• Vegetation growing into fire breaks.
• Accumulated organic material.
• Unwanted plant growth.
• Condition of pebble or paved fire breaks.
• General condition of the growing medium and planting.

Access for inspection and maintenance should also be considered during the design stage.

Assessing the Complete Green Roof System

Green roof systems can vary considerably in their waterproofing, drainage, substrate depth, vegetation and surface arrangement.

For this reason, external fire performance should be considered at the system level. The ABG technical note makes the same point for blue roof constructions, explaining that it is the performance of the completed build-up rather than individual intermediate layers that is usually relevant.

Specifiers should obtain technical information covering the proposed roof construction and confirm that the system is suitable for the building, location and intended use.

Factors such as building height, boundary distance, roof layout, penetrations and adjoining construction may all affect the required approach.

At ABG Building Solutions, we provide green roof systems and technical support for a range of roofing applications. Our team can help designers and contractors select suitable green roof components and assess the requirements of the proposed roof build-up.

Contact ABG Building Solutions to discuss your green roof specification.

Green Roof Fire Safety FAQs

Do green roofs need fire breaks?

Green roofs can require fire breaks or vegetation-free zones as part of their fire design.

GRO guidance summarised by ABG includes 500 mm wide pebble fire breaks, 40 mm thick paving around perimeters and penetrations, and additional 1 metre wide fire breaks every 40 metres across the roof. The exact arrangement should be confirmed against current guidance and the proposed roof system.

What is BROOF(t4) for green roofs?

BROOF(t4) is a classification under BS EN 13501-5 that relates to the external fire performance of a roof construction.

It considers fire spread across the roof surface and fire penetration caused by an external fire source. In England, constructions achieving BROOF(t4) can be used without a minimum boundary distance restriction under the classification table reproduced in ABG’s technical note.

What organic content should green roof substrate contain?

The GRO guidance summarised by ABG states that where a green roof has no permanent irrigation, the growing medium should contain less than 20% organic material and should be peat free.

What happens to green roofs during prolonged dry weather?

Vegetation and growing media can dry during extended periods with little rainfall. Dry or dead vegetation can increase the amount of combustible material present.

Plant selection, maintenance and an appropriate irrigation strategy should therefore form part of the green roof design.

How does green roof maintenance affect fire safety?

Maintenance helps prevent dead vegetation and organic material from building up. It also keeps vegetation away from designated fire breaks.

The GRO guidance referenced by ABG specifically identifies maintenance as important for preventing vegetation growing over fire breaks and for cutting back vegetation at suitable times.

Are extensive green roofs fire safe?

Their fire performance depends on the complete roof construction, vegetation, substrate, moisture conditions and maintenance regime.

Specifiers should therefore assess the proposed system against the fire safety requirements for the particular building rather than assuming that all extensive green roofs will perform in the same way.