Extensive green roofing

Installing a green roof on a DESIGA® garden shed requires precise coordination of the functional layers. The primary focus is on protecting the supporting structure and ensuring waterproofing. The visual finish is provided by the 5 cm high perimeter roof upstand, which allows for a flat and controlled installation of the green roof system.

Technical layer structure: seven layers, each with a specific function

To ensure healthy plant growth whilst protecting the roof membrane, a multi-layer system is used. Each layer fulfils a specific function – none is optional, none is redundant. The structure is organised from bottom to top:

1. 1. EPDM roofing membrane – The primary waterproofing layer, as described in Chapter 2.
   
   
2. 2. Root barrier fleece/membrane – Absolutely essential. Over time, plant roots can penetrate even the most durable materials. A certified root barrier layer prevents sedum roots from damaging the EPDM membrane and ensures long-term waterproofing.
   
   
3. 3. Protective layer – A storage fleece that protects the root barrier membrane from mechanical damage and also stores water.
   
   
4. 4. Drainage and storage element – Usually dimpled sheets. These drain excess rainwater away whilst simultaneously storing a water reserve for dry periods.
   
   
5. 5. Filter fleece – Prevents fine particles from the substrate being washed into the drainage layer.
   
   
6. 6. Substrate – A mineral soil layer specially tailored to the needs of flat-roof plants. With a 5–8 cm upstand, the substrate height is adjusted to create a flush finish.
   
   
7. 7. Vegetation layer – Various sedum species (stonecrop), which are sown as seedlings or laid as ready-made mats.

Only the extensive variant is suitable for greening: low-growing, undemanding sedum species that require minimal substrate depth and no regular watering. These plants are hardy, heat-resistant and regenerate themselves after dry spells. Intensive greening with perennials, grasses or even turf is not envisaged given the height of the kerb and the structural requirements – and would contradict the minimalist design principle.

Functional benefits: protection, climate, water management

In addition to the ecological and aesthetic benefits, the layered structure offers concrete technical advantages for the building. The combination of root protection, substrate and plants completely shields the EPDM membrane from ultraviolet radiation – thereby completely eliminating the most significant factor in the ageing of flat roof waterproofing. Extreme temperature peaks on the roof surface are buffered: whereas a bare black membrane can reach temperatures of over 70 °C in midsummer, the surface beneath the substrate remains at 25–30 °C. This drastically reduces material fatigue in the roof membrane.

In summer, the green roof has a cooling effect on the interior of the garden shed through evaporative cooling – an effect that is noticeable when used as a workshop, office or retreat. In winter, the entire structure acts as additional thermal mass, dampening rapid temperature changes and protecting the structure from freeze-thaw cycles.

An often underestimated benefit is water management: a large proportion of the rainwater is retained within the layers and evaporates directly on site. During a typical heavy rainfall event, the green roof delays runoff by 30–60 minutes and reduces the volume of runoff by up to 50%. This relieves the burden on the drainage system and, depending on the local authority, may even reduce the rainwater charge.