Caring for a green roof

An extensive green roof enhances the look of a building, extends the life of the roof waterproofing and, depending on the region, retains around 40–80% of annual rainfall. For a green roof to work well in the long term, it needs regular maintenance and – after the establishment phase – one or two care visits a year. This page covers extensive green roofs; intensive green roofs have higher demands for water, nutrients and care.

Maintenance: keeping drains and gravel strips clear

Roof drains, gutters and inspection chambers must be kept free of vegetation so that excess water can drain away freely. Check that they work properly at least once a year and clear away dirt and debris. The gravel strips must also be kept clear: they protect the waterproofing connections at rooflights, roof edge profiles and gravel guard strips from roots, and serve as safety strips for preventive fire protection – so remove dried-out plant material there too. If this maintenance is neglected, the building may be damaged.

Checking for unwanted growth

The surface must be kept free of unwanted growth. Particularly important: at least once a year, check for woody seedlings such as birch and willow and remove them – their strong roots can damage the roof waterproofing. Aggressively growing herbs and grasses such as orache, couch grass, clover or vetch can also crowd out the existing vegetation and should be weeded; this is most effective after longer dry periods or before they set seed. Collect litter and leaves. Since an extensive roof is a natural-style planting, a certain amount of clover and vetch is acceptable, though – in the flower-poor height of summer they are an important food source for bees and bumblebees.

Watering and fertilising

In the first weeks after sowing or planting, the layered build-up must stay sufficiently moist. Once the vegetation has rooted, watering is reduced – the plants of an extensive green roof cope with drought and heat. Watering only makes sense during extremely long dry spells (around four weeks), for thin build-ups under 7 cm, and for south-facing pitched roofs with a build-up under 10 cm.

Depending on how the vegetation develops, an application of fertiliser makes sense after one to two years. Signs are heavy moss growth and sedum species that turn noticeably red. An organic-mineral slow-release fertiliser is suitable (coated NPK controlled-release fertiliser, 5 g nitrogen/m², effective for 8–9 months), spread evenly between March and early June. Avoid fast-acting fertilisers – they are easily washed out of the thin build-up. After a few years, because of the nutrient-poor build-up, maintenance fertilising or liming is advisable to preserve the flowering display.

Cutting back and replanting

In autumn, cut back the perennials and remove the clippings to keep the surface lean and the nutrient supply low. Cut grasses and herbs back to a height of 7 cm, ideally before the seeds ripen (end of May to early June), as they otherwise self-seed heavily. When working with a brushcutter, keep a distance from the substrate build-up and the roof waterproofing. If individual species die off due to competition, frost or long wet or dry periods, push back dominant species and replant or resow in a targeted way – the landscape gardener can source suitable plants.

Take care with the roof waterproofing

The greatest risk of damage concerns the roof waterproofing – damage to it can cause immense costs. So: do not mix the vegetation layer and the drainage layer during care, do not drive sharp objects (rotary washing lines, tent pegs, nails) into the planting, avoid pointed garden tools, no open fire on the substrate or vegetation, and avoid large point loads (e.g. relocated substrate or gravel), as, depending on the structural design, extensive roofs have only limited load reserves.