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:

The height of the green roof system build-up.
The height of the landscape finish (hard or soft landscaping).
The maximum saturated load of the green roof.
The sheer force the green roof will exert when on a pitched roof.
Safe access for maintenance.

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.

Bauder waterproofing membrane systems and liquid systems are manufactured in accordance with FLL guidelines for root resistance and carry BBA certification for green roof applications.

When designing the detailing (e.g. upstands) it is important to meet Building Regulations for the height of the waterproofing above the landscaping finish.

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.

Roof drainage designs should comply with the requirements of BS EN 12056-3:2000 Gravity Drainage Systems inside buildings and all rainwater outlets should be protected by an inspection chamber with removable covers to allow access for maintenance and be surrounded by a pebble vegetation barrier to prevent encroachment.

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.

Extensive green roofs – all compliant roof deck types
Biodiverse green roofs – all compliant roof deck types
Intensive green roofs – should only be constructed on concrete decks

Our technical team will be able to assist with advice on deck construction types. Get in touch so we can assist you with indicative weight loadings to start your design process.

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.

Sheer forces - need to be taken into consideration generally on roof pitches more than 10 degrees, though this depends on the levels of substrate being installed and the vegetation used. Anti-shear measures should suit the individual system and on a Bauder sedum blanket system, this comprises a simple mechanical retention strip with spikes that hold the blanket onto the waterproofing.

Our technical team can provide information and advice so relevant data can be passed to the structural engineer. Get in touch.

Wind uplift

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

Intensive green roofs consist of heavier garden style planting schemes often incorporating trees, large shrubs and turfed areas making them less likely to be affected by wind forces.

Extensive green roofs are much lighter and so wind forces can lead to erosion problems on exposed sites, especially if plant establishment is in the early stages. Substrate is lighter and prone to displacement, particularly at corners and edges of the roof where wind forces are stronger.

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.

Low growing sedum will need a shallow, low nutrient, free draining substrate. Wildflowers and biodiverse vegetation will require deeper substrate (100-150 mm). Herbaceous plants, shrubs and trees on intensive green roofs will require greater depths (300-450 mm+). As planting becomes more substantial it is likely to require a higher organic content within the substrate. There need be no limit to the complexity of the landscape or recreational space.

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.

The BauderGREEN FV 125 100 is a fine polypropylene geotextile fleece with a pore size of around 0.13 mm to separate and contain substrate and sediment.

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.

The water storage cells can be filled with gravel, type 1 or concrete to increase the compressive capabilities or as a bed for paving or other surface finishes. The components can be manufactured from a variety of materials such as high-density polyethylene (HDPE) or polystyrene; each having their own reservoir capacity.

Our technical team can provide information and advice on the required product for the vegetation it is required to support. Get in touch.

Protection Layer

This layer delivers protection against mechanical damage. Generally, the more the roof is trafficked the higher the level of protection required.

Extensive green roofs would generally experience much lower levels of traffic damage during maintenance than publicly accessed areas and so the BauderGREEN SV 600 mat is generally specified as a lightweight protection mat.

Intensive green roofs are designed for high levels of access, both by people and sometimes vehicles if on a podium roof, and therefore require high levels of protection. We have three products that will deliver this level of protection for the waterproofing: the BauderGREEN FSM 600 or 1100 and the BauderGREEN GGM 6.

Our technical team can provide information and advice on the required product for the access levels planned. Get in touch.

Separation layer

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

The layer separates the waterproofing system from the green roof so that any movement in the green roof due to frost heave etc. will not damage the waterproofing membranes.

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.

Pre-grown vegetation mats - these sedum and wildflower blankets will establish quickly and give high aesthetic finish immediately after installation.

Plug plants - enable certain species to be planted in particular locations. They require careful aftercare during establishment.

Seed - offers a lower establishment cost but will normally take a couple of growing seasons to fully establish.

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.

The only exceptions are:

At perimeter kerbs where the waterproofing extends fully to the outer edge finishing in a traditional welted drip, or edge trim.
Flush threshold access doorways can normally be accommodated with a method to suit the components to be sealed to.

Vegetation barriers

These are created with round washed pebbles or paving slabs and provide several important functions on extensive green roofs.

A vegetation barrier

Prevents vegetation from encroaching on roof elements such as rainwater outlets.
Protects the vegetation from water running down vertical surfaces or rainwater downpipes.
Provides rapid surface drainage during heavy rainfall.
Protects the waterproofing from mechanical damage during maintenance.
Acts as a fire break, where the pebbles are installed in accordance with GRO guidelines.
Increases resistance to wind uplift at roof perimeters.

Establishment watering

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

It is important to make provision for establishment watering with an effective method for delivering a reliable water supply to the roof, even if the installation takes place in the spring or autumn.

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.

For extensive and biodiverse roofs, the ongoing watering needs are dictated by the type of vegetation, roof slope and exposure of the roof.

The more intensive a roof finish, the more likely irrigation will be required. The plants must have sufficient levels of water to ensure they remain healthy during prolonged dry periods.

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.

An intensive green roof will require the regular maintenance associated with the planting scheme and landscape design, whereas an extensive green roof will need only minimal maintenance (maybe one or two visits a year) to ensure that any unwanted species do not become established.