Managing risks with green roofs

26 June 2023

The risks associated with green roofs and how to reduce them.

Living roof with a variety of different plants covering the entire space.

Carbon Zero and sustainability are at the top of the agenda for many organisations and improving the energy efficiency of buildings is necessary to achieve this stretching target.

This has seen a number of innovative construction methods and techniques emerge in recent times. While the insurance industry supports the drive to carbon zero, there are also risks involved with some of these modern methods of construction.

What is a green roof?

Green roofs, living roofs or sedum roofs as they are commonly referred to, are a popular choice for a sustainable building design. The sight of a roof covered in vegetation (for the right reasons) is a clear indicator that the building owner is environmentally aware.

A green roof is a roof which has been deliberately constructed to support plants and other growing media on its top or outer surface. It is not a flat roof which has been merely decorated with planted flower pots, plant troughs and flower baskets etc.

Living roofs are divided into three main types – "extensive", "intensive" and “biodiverse".

Extensive living roof systems provide a visual or biodiversity interest and are designed to support plants with a lower maintenance requirement.

Intensive living roofs (often called roof gardens) are designed to create recreational and amenity spaces for people to enjoy. They are generally accessible and require a higher level of maintenance, including regular irrigation.

Biodiverse roofs attract wildlife and aim to recreate the habitat that was lost when the building was erected, or even enhance it.

Green roof construction

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

The supporting frame and deck can be constructed from concrete, steel or timber. Many different types of insulation material including combustible insulation can also be used.

Growing media may be organic or inorganic in type. Inorganic materials can include materials such as shale, clay, mineral wool, and pumice.

Layers can also be provided for other purposes to act as filters, for drainage, moisture retention or vapour control purposes, as a root barrier, to provide thermal insulation, or as part of the structure.

Fire and insurance considerations

The flammability and combustibility of a living roof can depend upon the supporting roof frame and deck, the insulation and membrane layers used, and also the plant types growing on it. The combination of a timber roof deck and foam-based insulation presents a significant combustible element to the roof system. There have also been concerns that non-irrigated green roofs may, in periods of dry weather provide an expanse of dry combustible vegetation that could spread a fire. Where this combination exists there is potential for rapid fire spread across the roof of a building.

From an insurer’s perspective, our preference is for the roof to be supported on a non-combustible deck such as steel or concrete and for the insulation layer to also be of non-combustible material such as 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 (e.g. around roof lights) with a shingle perimeter to act as a fire break. Larger living roof buildings may have a wider width shingle perimeter or concrete fire breaks at designated intervals designed to stop the spread of fire across the roof. Regular inspections of the roof should be undertaken to prevent weed growth over the fire break area.

‘Extensive’ roofs are generally not irrigated and the fire risk must therefore be mitigated by the specification of the roof build-up and the incorporation of fire breaks. Ongoing maintenance should account for any dry spells (especially for wildflower systems), and appropriate action taken to ensure no significant volume of dry material is left on the roof, particularly if overlooked where a carelessly discarded cigarette could cause a fire.

Water damage and insurance considerations

Unless properly designed using the right materials, roots can, over time, cause damage by penetrating the water-resisting layer. In some living roofs, an additional root barrier is installed for protective purposes.

Damage to the waterproofing layer will require removal and replacement of the damaged section. This will involve tracing the source of any leaks, the location of any damaged areas, and their replacement which can be labour-intensive and costly, leading to a sizeable claim cost for a modest leak.

To reduce the risk of water damage, green roofs should only be installed and maintained by contractors with the appropriate qualification, knowledge and experience, e.g. LANTRA qualified, and who follow the Gro Green Roof Code of Practice.

Regular visual inspection of the roof and its underside will help to detect water leaks as soon as possible. Water leak detection devices can be used as part of this process.