01473 257671Contact usMembers Area

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

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

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

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

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

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

Wind uplift

The weight of the green roof system can counteract the negative pressure (suction) imposed on the waterproofing.

Substrates for vegetation

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

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

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

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

Separation layer

This layer is installed directly on to the completed waterproofing system in an intensive green roof solution.

Vegetation

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

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

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

Establishment watering is particularly important during the first 6-12 weeks following installation. Differing system finishes will require different watering practices. 

Irrigation

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

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.