Components of a sedum green roof
A sedum green roof is a build-up of component layers that sit on top of a conventional roof surface. The layers work in partnership with each other to provide a whole host of benefits for the building, its occupier and the environment.
Protecting the waterproofing
On a green roof shed, the first layer of the green roof system (the one that sits closest to the waterproofing) is normally a sheet of strong polythene or butyl pond liner. This layer ensures that plant roots cannot damage the waterproof layer. It also gives protection from boots, dropped tools and other hazards associated with building work or DIY.
A quick tip here from David Fisher, one of our most experienced green roof installers; polythene can be slippery to work on, which is not ideal when working on a roof. If you douse the poly with water before starting to install the next layer, you’ll find it much easier to get a good grip on the surface.
Green roof drainage layer
On a flat roof – that is to say one with a slope of 3 degrees or less – the next layer needs to be a drainage mat to make sure that excess rainwater is channelled swiftly away from the plants’ roots. Sedum plants are particularly well adapted to living in dry soils and they simply cannot cope with soggy soil. The drainage layer helps keep them comfortable and it also has a bonus function; few plant species other than Sedums, saxifrages and sempervivums can survive for long in shallow soils unless they have a really good water supply. The drainage layer on an extensive sedum roof actually helps prevent weed infestations by ‘droughting out ‘ most of the impostors that manage to germinate in the inhospitable rooftop conditions.
On a pitched roof there is no need for drainage mat- rainwater will run off naturally.
Water retention fleece
Before installing your sedum matting, you must spread out a layer of water retention fleece. This is a lightweight blanket that can absorb up to 9 litres of water per square metre. It ensures that the plants will have enough water available to see them through the average UK summer.
The sedum matting sits on top of the water retention fleece. An edging of your choice will ensure that everything stays in place and there you have it………a living sedum green roof.