Green roof essentials

ALL vegetated green roofs need appropriate levels of the following in order to flourish:

• Sunlight
• Moisture
• Drainage
• Aeration to plants / roots
• Nutrient
• Plants selection

Typically green roofs consist of the following build up layers:

Structural deck
The roof beneath the green roof elements needs to be flat, stable and strong enough to hold the weight of the plants and substrate but also wind uplift forces and snow load. The structural deck should be built to a slight fall to allow free flow of rainwater into the drains.

Root barriers
Often part of the waterproof membrane otherwise typically a sheet membrane that prevents penetration of plant roots and rhizomes into the waterproofing.

Protection fleece
Usually a permeable nonwoven geotextile fabric (sometimes a polyester sheet layer) loose laid onto the membrane beneath. It is important that the protection layer is non-abrasive and free from sharp edges and burs but should be strong enough to prevent damage from foot traffic and site works.
(Re-cycled polyester geotextile 300gsm GE-PE-PECT-0300-Z)

Drainage layer
Usually a loose laid plastic sheet or grid / box material with a three dimensional moulded shape to collect a certain amount of water within its shape and allow excess water to escape through drainage holes moulded within the sheet membrane / box. It is crucial that an air gap is created within the drainage layer to allow free flow of rainwater through the system.

Granular materials can also be used. It is essential to allow free drainage throughout the whole vegetated area to prevent water logging which in turn restricts aeration and turns substrates acidic killing off vegetation. (Protecto-drain).

Filtration fleece
A geotextile layer that prevents fines and sediments from being washed out of the substrate which could reduce or block the drainage layer (leading to waterlogging of the roots and killing the vegetation). Usually made form the same material as the protection fleece.
(Re-cycled polyester geotextile 300gsm GE-PE-PECT-0300-Z)

Substrate
An engineered growing medium specially mixed to provide a lightweight, free draining mass for green roof plants to root into and grow healthily in the long-term.
Typically containing a balanced mixt of lightweight aggregate, low levels of organic material and other ingredients; it will hold the correct level of moisture, nutrient and air, and the correct particle structure required for long term sustainable growth. It is made up from recycled and sustainable, mainly UK sourced materials and is often carbon neutral / negative.

Important aspects required are:

• Lightweight, so as not to overload the roof structure
• Free draining to avoid waterlogging / weight overloading
• An optimum level of nutrient and aggregate to suit the type of green roof being constructed; low organic content;
• Varied aggregate particle size; a balance between draining and moisture retention;
• Resistance to compaction and erosion;
• Resistance to wind and water erosion, chemically stable
• Avoidance of peat in the mix;
• Avoidance of weeds, impurities and contaminants within the mix;
• Adherence to green roof codes, standards and regulations (see section 6);
• Long-term sustainability & healthy growth of plants

3.7 Vegetation: Can be broken down into three main types:

• Extensive green roof plants – sedum / succulents, alpine
• Biodiverse roof systems – wildflowers, naturally germinating native plants
• Intensive green roof plants – shrubs, grass & trees

Many factors will influence the type of vegetation:

a) Design objectives – what is the roof to be used for?
b) Plant characteristics – shade /drought tolerance
c) Regional climate – certain plants will thrive in different geographic regions
d) Microclimate – the orientation of the building and aspect etc
e) Maintenance – what maintenance or aftercare regime is planned?

Vegetation can be delivered in many forms and these forms can be determined by cost, availability and speed of establishment.

Pre-grown sedum blankets and Wildflower blankets are delivered to site as semi mature plants that have a strong root and stem development. These blankets do not travel well. They are prone to overheating and ‘boiling’ while they are rolled up during transport and as such it is imperative that the minimal amount of time from the farm to being rolled out on site is given. In the summer this must be no more than 48 hours. Blankets should rest and cool before watering. Instant vegetation is provided with this method.

Plugs plants at around 16 – 20 per m2 is the next commonly used option. Being smaller plants, the supply cost is somewhat cheaper but this saving is reduced when labour costs are taken into account as plug planting is slow and laborious. The plants will take a significant time to fully mature on a roof space and are susceptible to being pulled up by birds after installation.
Cuttings and sedum chippings are a good source of vegetation as sedum especially will re-root itself very effectively and start to populate a green roof space in a matter of weeks. Cuttings and chips are literally sections of existing sedum which have been removed through the strimming process and thrown onto the green roof. They are a cheap and effective way to propagate sedum on green roofs.

Seeds are the slowest and sometimes riskiest method of establishing vegetation onto green roofs. Wallbarn recommends against growing sedum green roofs from seed on site as the fine seeds are often lost before they have the chance to germinate. For wildflower and biodiverse / brown roofs the seeds can be broadcast more easily by hand directly onto the substrate. Seeded roofs are slow to develop and can take up to 3 years to properly mature and establish.

Features
Particularly relevant for biodiverse roofs, a range of physical features should be included in the roofing scheme to mimic nature and encourage wildlife and biodiversity. Examples includes rock piles, rotting tree stumps, variations in substrate depth, bird boxes etc