Green Roof Design Considerations Overview
There are fundamental principles that apply to the design of a green roof, regardless of the proposed landscaping, location or climate. The specified solution must replicate nature within the build up and be able to support the vegetation.
The first step - What is the landscaping finish to look like?
The primary decision is the type of landscape required that best suits the rationale behind the development, whether it is an intensive, simple intensive, extensive or biodiversity roof. Whatever the landscape chosen, the plants will have some basic requirements to sustain them; nutrients, a balance between moisture and drainage to suit the vegetation, and aeration to the root systems.
The next step - Planning the green roof to meet the needs of the vegetation
Once the vegetation finish has been determined, the structural and design considerations and green roof components can be proposed.
Protection of the building fabric
It is vital that the protection of the building’s fabric and in particular its waterproofing is considered in the green roof design. There is important information that the building’s structural engineer will require:
Waterproofing
It is vital that the waterproofing is robust and proven to deliver long-term root resistance, preventing plant roots and rhizomes from growing in to or through it. Consideration must be given to the appropriate form of waterproofing so that it can meet the demands placed on it by the green roof and the access levels required for either maintenance or leisure activities.
Drainage
A green roof can retain varying levels of rainwater, depending on the depth of substrate required for the selected vegetation finish as well as needing falls to be created under Building Regulation Part H that requires adequate provision is made for rainwater to be carried from the roof of the building.
Deck construction for green roofs
Most roof deck constructions are suitable for some form of green roof if they can support the dead and imposed load. A structural engineer will confirm the suitability of the deck once the weight loading of the designed green roof build-up with its landscape finish is established.
Dead and imposed loads
Weight loadings should be calculated in accordance with BS EN 1991-1-1, BE EN 1991-1-3 and BS EN 1991-1 4 and should include the weight of the waterproofing system, saturated weight of the green roof components and substrate, vegetation, snow loads, and imposed loads such as access by people and vehicles as well as point loads for items such as water features and trees.
Wind uplift
The weight of the green roof system can counteract the negative pressure (suction) imposed on the waterproofing.
Substrates for vegetation
Generally, the depth of substrate is dependent on the type of vegetation being installed. Once saturated, the substrate contributes to the weight loading of the roof and so should always be factored in when designing the green roof.
Filter layer
This fine porous layer allows water to pass through whilst also preventing substrate fines from being washed into the water storage or drainage component.
Water storage / drainage layer
This layer helps to maintain the balance between the levels of water to be held on the roof to support the vegetation whilst allowing the surplus amount to drain away so that the substrate doesn’t become waterlogged.
Protection Layer
This layer delivers protection against mechanical damage. Generally, the more the roof is trafficked the higher the level of protection required.
Separation layer
This layer is installed directly on to the completed waterproofing system in an intensive green roof solution.
Vegetation
The selection of vegetation finish defines the plants that will be included in the visual finish. An intensive green roof planting scheme is a bespoke layout generally created by a landscape designer to meet the requirements of the client or building owner. For extensive and biodiverse finishes, the criteria will be quite different and range from blanket or swathes of pre-cultivated plants, plantlets or ‘plugs’, and seed mixes.
Perimeters
The minimum required upstand height for the waterproofing at abutments, parapets or pipe work penetrations etc, is 150 mm above the completed landscape surface.
Vegetation barriers
These are created with round washed pebbles or paving slabs and provide several important functions on extensive green roofs.
Establishment watering
Establishment watering is particularly important during the first 6-12 weeks following installation. Differing system finishes will require different watering practices.
Irrigation
Every roof is different, and levels of irrigation depend on the water demands of the vegetation, the levels of water storage on the roof and local microclimate.
General maintenance
All roofs require a minimum of two inspections a year to ensure that the outlets etc are maintained, regardless of the type of roof.