The Green Roof Challenge

By Fraser Brown at Benchmark.

In a time when we are urgently seeking solutions to global warming, one of the most promising construction opportunities is the growth in green roofs.  Seen by many as an innovation, the idea of course is far from new, with evidence of turf roofs on buildings going back as far as 2500 BC in the Orkney Isles of Scotland.

The current interest surrounding green roof construction is understandable, because the benefits are multiple and significant.  Whether you want to breathe a bit of green into a city development, or need to blend in with surrounding countryside, a green roof enhances the thermal performance of the roof, helps to retain stormwater, cleans the air, reduces dust and smog levels and visually enhances quality of life for the occupants of the building and those surrounding it.

Extensive green roofs are constructed using low maintenance planting such as succulents, grasses and herbs.  It is not suitable for recreational purposes, but requires very little maintenance.

Intensive green roofs can be simple gardens or fully landscaped recreational areas with all the features of a green space at ground level.  There is the added advantage of a useable outdoor living space.  Naturally, this also carries high maintenance with it, as any cultivated garden or green space does.

The potential downside of all of these projects is that a green roof can dramatically increase the loading on a building, so a primary factor is to determine whether a building structure can take the additional weight that the green roof imposes.  Where an existing structure is being retro-fitted with a green roof, it is essential to obtain professional advice from an architect or structural engineer before such an undertaking.

Clearly this is less of an issue where the green roof is part of a new design, as the loading will be taken into account when specifying the supporting structure.

The additional loading of a green roof varies depending on the type. Although they all consist of the same basic series of layers, the depth of the growing medium and type of vegetation and features will produce significant differences.

Perhaps the hardest aspect to assess is the vegetation, certainly for an intensive green roof which may incorporate trees or larger shrubs that will grow to maturity, necessitating a long term projection of weight.  Any calculations should of course be based on the growing medium and substrate being saturated.

So what are the likely impacts of this additional loading on a commercial building?  Naturally the primary structure must be designed to withstand the weight, but there would also be an expectation of increased secondary steelwork, which would have cost and possibly space implications.  An innovative way round this is to use a structural insulated panel that has the strength to reduce or even eliminate secondary steelwork.

There is one structural composite insulated panel system available that can provide double spans of up to 6 metres depending upon saturated green roof weight.  The potential for such a system to revolutionise the market for green roofs is great, as it simplifies the design process, speeds up construction, lends excellent weather tightness and added quality assurance to projects.  The elimination of secondary steelwork also helps to keep costs down whilst maximising space.  Even more importantly it reduces the overall loading on the primary structure. Speak to Kingspan Benchmark about its Envirodek product to understand the benefits in more detail.

At the end of the day the real challenge lies, not in the design feasibility of green roofs, but in changing the attitudes that would dismiss this exciting and beneficial possibility as too difficult or costly.