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Green Roof Design Considerations

Considerations

Why are you creating the green roof?

Before we drill into the design options for a green roof, there’s a couple of questions you need to ask yourself which will set the direction for the design process. Firstly, what is the purpose of the green roof? Why are you or your client considering building one? For example, if you’re aiming to create an outdoor space for people to enjoy the outdoors and entertain guests, you’ll need to consider the structural requirements to support this type of use, including the safety elements for access (such as doors, stairs, elevators, etc.) and the requirements to allow safe assembly on the roof (i.e. railings and roof edge set backs). Alternatively, you may be creating the green roof for aesthetic purposes that can be seen from other parts of the building or ground. Or, perhaps the primary function of the roof will be to perform more practical functions, such as improving energy efficiency, to meet BREAM requirements or to reduce rainwater run off.

The vision of the green roof design and build may evolve over time, but it’s good to have a clear idea of the main purpose of the roof during the design phase and to clearly outline this purpose, together with any limitations to the scope of the project.

Access

With a rooftop garden, the design needs to consider how people will gain access to the roof. If there’s a door leading to the roof, the access to that door also needs to be considered and whether the location of the door will require integration into the property security policy. Access still needs to be addressed even if the roof isn’t designed to allow for communal gathering to allow for maintenance. This access may accommodate a roof hatch type construction to allow for interior access, or to plan for access via a ladder, therefore including ladder safety elements in the design.

Structural support

If you’re designing a rooftop garden, you need to consider the structural requirements to support the intended roof top activity. Everything that is being placed on the roof, including plants, soil and required water saturation needs to be entered into the load equation as well as the intended number of people that the roof is designed to support. Local requirements may vary so you need to consult with local regulatory requirements. Saturated weight data will be available from manufacturers of the green roof materials. Plans to create a rooftop garden on an existing roof can be challenging due to the structural requirements required and it can be significantly more expensive than creating one for a new build. For existing buildings, it’s often more realistic to create a green roof as oppose to a rooftop garden as the structural load bearing requirements are significantly reduced.

As with the rooftop garden, the saturated weight of a garden design must be calculated to determine whether the existing roof requires structural alternations. For any plants that require a deeper growth media (and therefore greater load bearing support), these can be strategically placed on structural support members. However, it’s important to calculate the growth media depth of the planned plants, as this can have an impact on the structural requirements and it may be better to select plants that require a minimal media depth, and therefore minimal structural alterations. Once you’ve determined the dead load of the green roof, you can design the structure to support that load.

Safety

For rooftop gardens, design must ensure that visitors are safe whilst on the roof. That could include perimeter railings, skid resistant surfaces, lighting, well defined boundaries, etc. Secure storage for maintenance tools should also be considered. For both rooftop gardens and green roofs, fall protection will be required for anyone working within 10 ft of the roof edge (including individuals involved with either construction or maintenance).

Liquid or roll materials

When considering waterproofing solutions in the design process, you’ll generally have two different options to apply on a green roof:

Liquid roofing solutions

Liquid applied roofing materials and procedures are often referred to as rubber roofing or liquid plastics (brand name). Liquid roofing involves the application of a monolithic, fully bonded, liquid base coating to the roof. Whilst the coating is still in liquid form, a secondary material such as glass-reinforced plastic is added to provide additional tensile strength. Once cured, the top coat is applied forming a rubber like elastomeric waterproof membrane. The system can be applied to felt, asphalt, bitumen, concrete and numerous other materials.

Features:

Depending on the system employed, a liquid roofing solution can provide a waterproof working life of 25 years

For refurbishment scenarios,  the cost of applying a liquid roof is an estimated 70% lower than a full roof replacement

A liquid roof provides a fully adhered membrane, removing any risk of lateral movement

If the membrane becomes damaged, water can only go through the concrete, making the damaged area easier to locate from inside the building

Roll materials

Roll materials, such as roofing felt, are the traditional roofing product used for waterproofing and can either be bonded or mechanically fixed.

Features:

The option to mechanically fix rolled material rather than fully bond them reduces labour costs, however it also comes with a risk of lateral movement and the migration of water beneath the membrane.

A mechanically fixed roll material also makes it harder to identify damaged areas should they arise.