Managing risks with green roofs

Green roofs, otherwise known as living roofs or sedum roofs, are a popular choice for a sustainable building design. They can help improve your sustainability credentials and boost energy efficiency.

Green roofs are deliberately constructed to support plants and other growing media. They are not just a flat roof decorated with planted flowerpots.

Living roofs are divided into three main types:

• Extensive living roof systems offer visual or biodiversity benefits while supporting plants with lower maintenance requirements.
• Intensive living roofs (often called roof gardens) are spaces for people to enjoy. They are generally accessible and need a higher level of maintenance, including regular watering.
• Biodiverse roofs attract wildlife and aim to recreate or enhance a habitat lost by building work.

All living roofs are built-up roof systems that comprise of different layers, including a frame and deck, insulation, water-resistant layers, and a media in which the plants can grow.

The supporting frame and deck can be made from concrete, steel, or timber. Many different materials, including combustible insulation, can also be used.

The materials used to hold and grow the plants may be organic or inorganic in type. Inorganic materials can include shale, clay, mineral wool, and pumice.

Layers may also serve other purposes, acting as filters, drainage, moisture retention use a vapour control, root barriers, thermal insulation, or as part of the structure.

The level of fire risk can depend on the supporting roof frame and deck, and the materials used in each layer. The type of plant you grow will also be a factor.

The combination of a timber roof deck and foam-based insulation material presents a significant fire risk. In periods of dry weather, non-irrigated green roofs may dry out, and this combination has the potential to cause rapid fire spread across the roof.

From an insurer’s perspective, non-combustible materials are preferred, for example:

• A deck constructed of steel or concrete
• An insulation layer using non-combustible mineral wool or cellular glass

The current UK Code of Practice for green roofs: The Gro Green Roof Code by The Green Roof Organisation (GRO), advises that living roofs have borders and upstands (for example, around roof lights) with a shingle perimeter to act as a fire break.

Larger living roofs may have a wider shingle perimeter or concrete fire breaks at designated intervals to stop the spread of fire. You should carry out regular inspections of the roof to prevent weed growth over the fire break area.

Generally, ‘Extensive’ roofs are not irrigated, and the fire risk must be managed by the specification of the roof build-up and the inclusion of fire breaks. Maintenance should account for any dry spells (especially for wildflower systems) and prevent large volumes of dry material from building up, particularly in areas where a carelessly discarded cigarette could cause a fire.

Damage to the waterproof layer will require the removal and replacement of the damaged section. Tracing the source and replacing the damaged area can be labour-intensive and costly, so a small leak can lead to a big claim.

• Always use contractors with the appropriate qualifications, knowledge, and experience, for example, LANTRA qualified, and who follow the Gro Green Roof Code of Practice.
• Install a root barrier to protect the waterproof layer from damage over time.
• Regular visual inspection of the roof and its underside will help you find water leaks sooner. Water leak detection devices can be used as part of this process.