A multifunctional roof is a flat roof that, besides keeping the building watertight, performs one or more additional functions: storing water, carrying planting, generating energy or serving as usable outdoor space. The term comes from climate adaptation in cities, where roofs represent a large and largely unused area.
The usual combinations
- Blue roof (retention roof). Beneath the planting or paving sits a storage layer that holds rainwater temporarily and releases it slowly through a flow restrictor. The water authority or local council often sets a required emptying time, for example 24 or 48 hours
- Green roof. A substrate layer with sedum, herbs or grass; it retains water, lowers the roof temperature and extends the life of the waterproofing
- Blue-green roof. The combination: storage below, planting above, with the plants drawing from the buffer through capillary cones
- Solar green roof. Photovoltaic panels above the planting; the lower roof temperature improves panel output
- Roof terrace or roof garden. A usable function, with the corresponding requirements for edge protection and access
Application
Before anything else, one question decides the matter: can the roof carry it?
- Structure. Water weighs 1 kg per litre, so 100 mm of storage is already 100 kg/m². A saturated extensive green roof typically weighs in the order of 50 to 150 kg/m², and an intensive roof garden a great deal more. Always have the roof structure checked by a structural engineer first
- Waterproofing. Use a root-resistant membrane or lay a root barrier over it. A leak under a planted or filled roof is hard to find and expensive to put right — carry out a flood test before building up the layers
- Overflow. An emergency overflow is essential on any flat roof and doubly so on a retention roof: it must always be able to discharge freely, even if the restrictor blocks
- Falls and drainage. Make sure the storage layer does not leave standing water on the membrane where that is not intended
- Maintenance. Allow at least two inspections a year: clear outlets and overflows, remove self-seeded woody growth, and on a blue roof check the flow restrictor
- Grants. Many councils and water authorities contribute per square metre of green or water-storing roof, or offer a reduced surface water charge. Amounts and conditions vary widely by area, so check locally before you commit
Related terms
- Green roof
- Roof covering
- Flat roof
- Rainwater downpipe
- Roof terrace
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