A green roof, also called a living roof, is a roof surface that is partially or fully covered with growing vegetation over a suitable substrate. Green roofs provide thermal insulation, reduce surface water run-off, support biodiversity, and improve the visual appearance of flat or shallow-pitched roof structures. Installing one is increasingly popular for garden outbuildings, shed roofs, and flat garage roof extensions.

Types of Green Roof

There are two main categories. An extensive green roof uses a thin substrate of 50 to 150mm and supports low-growing, drought-tolerant plants such as sedum, mosses, and alpines. It is lightweight, requires minimal maintenance, and is suitable for most flat or shallowly pitched surfaces. An intensive green roof uses a deeper growing medium of 150mm or more and can support a wider range of plants, shrubs, and even small trees, but is heavier and requires a structure designed to carry the additional load. For most residential garden building applications, an extensive green roof is the appropriate choice.

Structural Assessment

Before installing a green roof, confirm that the existing structure can carry the additional load. An extensive green roof when saturated with water weighs approximately 60 to 150 kg per square metre depending on the substrate depth. A structural engineer or competent builder should assess the roof structure's load-bearing capacity before proceeding, particularly for larger areas or existing structures not originally designed for roof loading.

The Layers of a Green Roof

From bottom to top, a green roof typically consists of the structural deck, a root-resistant waterproof membrane, a drainage layer of dimpled sheet or gravel to allow excess water to escape, a filter fleece to prevent the substrate migrating into the drainage layer, a growing medium or substrate of the specified depth, and the planted vegetation. The waterproof membrane must be root-resistant; standard EPDM or bituminous membranes without root resistance may be penetrated by plant roots over time.

Installing a Sedum Mat

The simplest approach to establishing an extensive green roof is to use pre-grown sedum matting, which consists of a layer of growing medium with established sedum plants already rooted into it. The mat is rolled out onto the prepared roof surface and the plants begin growing immediately. Sedum mats are available in standard sizes and can be cut to fit the roof shape. They are laid from the parapet or edge inward and the joints between rolls are butted together tightly. Establish the installation with an initial watering and allow the sedum to grow together over the following season.

Summary

A green roof consists of a root-resistant waterproof membrane, drainage layer, filter fleece, growing medium, and planted vegetation. Extensive green roofs with sedum are the simplest and most appropriate type for most residential outbuildings. Structural load must be assessed before installation. Pre-grown sedum mats provide the fastest and most reliable establishment. Ensure adequate drainage gradient and edge retention for long-term performance.