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Structural Design

You may have a grand vision for your green roof, maybe as a roof garden to host summer parties or a place to express your creativity and green fingers. Before you go ahead and invest a great deal of time and resource into the design phase, it’s critical that you begin the process of understanding the structural capacity of the existing roof and the structural requirements for any green roof ideas that you may have. This is a process which will require the involvement of a structural engineer.

If you’re working on a new build project, the subject of structural design becomes much simpler as you can accommodate green roof designs and structural requirements into the plans for construction. For green roof design on an existing building, it’s essential to get a thorough understanding of the structural load capacity early on as cost implications will usually mean that you’ll need to limit your design ambitions and ideas according to the limitations of the existing structure.

The simplest way to get a structural evaluation is by checking the blueprints of the construction. However, these are rarely available for older buildings, so it’s highly likely that you’ll need to include a structural engineer on the project. This is the stage where many projects breakdown and plans get shelved, and those that do survive usually require a degree of compromise between design aspirations, structural limitations and the cost of decreasing these limitations and increasing load capacity.

There are various components that makeup the structural construction of a green roof, and for several of these components the design stage needs to consider the dry weight and the saturated weight of materials and elements. A structural engineer will consider the dead load and estimated live load of a green roof project, and prescribe the structural work required to support the load. This will take into account water saturation and snow.

Components of a green roof structure

There are several components in a green roof structure which are common for many other types of roof, such as insulation and membranes, liquid solutions and metal roofs. This section will look at materials which provide specific value to green roof construction.

Moisture retention products

Gel packs and particles – These packets contain starch-based elements and are laminated to a geo-textile. The packs are placed around plant roots and can blend into the growth media. The product is effective in establishing green roof plants. However, they can absorb a few hundred times their volume in water and can be detrimental to the irrigation strategy for the roof. Also, as the pack can expand to several hundred times its original size, the material can displace growth media. When the pack displaces growth media, large craters or voids can appear in the growth media. These packs can have a big impact on the live load of the green roof.

Dimple membranes and mats – Dimple membranes provide passages beneath the growth media to direct water across the roof towards drainage points. The dimples may also act to store water and feed plant roofs. If this material is used on a green roof, the structural design needs to consider the load capacity of the material with the volume of water retained in each square metre. Some manufacturers provide this information.

Root barriers

Filter fabrics – Filter fabrics can be used to prevent media particles from exiting the green roof with storm water run-off via the drainage system and from causing a drainage blockage. Filter fabrics often contain chemicals that prevent root growth and act as a barrier to protect media particles from water run-off. The fabrics tend to be lightweight and don’t have a major impact on the roof’s load capacity.

Thermal plastic – Thermal plastics are a more expensive alternative to fabrics. However, thermal plastics provide extra root protection from large plants such as trees and bushes. These plastics are installed in a similar way to many single ply membranes. Once the seams have been welded, the material offers complete water resistance and root protection.

Drainage layer materials

Aggregate – Aggregates are an option for creating drainage layers on a green roof. However, whilst the material itself is low cost, spreading the aggregate across the roof is labour intensive and the additional weight of aggregate usually make it impractical for many green roof projects.

Geotextiles – A geotextile is a lightweight rolled product. Products vary, but many contain a formation of cups, embedded on a plastic sheet. The cups serve to store and direct water towards drainage points. The live weight of these materials are often provided by the manufacturer.

Drain/core root barrier combination – This drainage layer option is the most commonly used on green roof projects. The product combines a soft filter fabric laminated to the surface of a drain core. The drain core provides the cup formation to direct water to drainage points whilst the fabric provides the root barrier to repel root growth. This option provides one solution for both root barriers and drainage layer.

Growth Media and Plant material

There are several options for growth media and plant material, and the choice is often a combination of aesthetic and practicality. Whilst we won’t list the options here (it’d be a long list!) it’s important to remember that for both growth media and plants, you need to calculate the dead and live load per metre square. Commercial blends of growth media will often provide saturation testing on the saturated weight of their product. With plants, sedums and succulents are often a popular choice as they require minimum maintenance and are one of the lightweight plant options.