Saturday, 13 November 2010

Case Study

I recently visited the award-winning centre at Kenfig Nature Reserve near Porthcawl in South Wales to see the green roof that was used on the building extension. This was installed in, I believe, 2007 and uses a mat of Sedum plants on a medium-pitch roof. As I noted in my previous post, this required significant additional strengthening to the roof structure compared to the other, tiled, roofs.
The installation contractor was a company called Bauder who used a variety of Sedum plants in the mats on the basis that the ones most suited to the prevailing site conditions would survive and colonise the roof.
Sound proofing cannot have been a great issue here but the appearance is a great advantage being situated on the edges of a SSSI. The roof covers a large classroom area and must also be a great benefit in terms of thermal insulation.

Green roof design considerations

I shall deal with planning and finance issues later, but before setting out on your green roof construction project there are some important design factors to take into account. The first of these is the additional weight of a green roof compared to a conventional roof.

Weight of a green roof
Water or soil weighs roughly 1 tonne per cubic metre, so an additional thickness of say 75mm adds 75Kg to every square metre of roof - equivalent to a small person standing every metre apart on your roof. This means that the supporting roof structure has to be considerably stronger than that used for a conventional roof.

Flat or pitch roof?
In times past when people were building with heavy roof materials such as stone tiles or thatch they reduced the demands on the strength of the structure by increasing the pitch, or angle of the roof. Lighter materials such as corrugated sheeting, need only a very shallow pitch. By increasing the pitch, more of the load is transferred to the walls - which were usually substantial structures of stone.

This could be a solution to a green roof but clearly the steeper the pitch, the harder it is to maintain a good vegetative covering. Specialist roofing contractors will be able to advise on how steep a pitch their particular roof covering can tolerate and a structural engineer will be able to advise on the size and strength of the supporting structure that will be needed.

I understand that much work has been done in Germany on using green roofs on flat roof structures - originally as a means of extending the life of flat roof coverings by protecting them from the environment. This opens up far more possibilities for the type of plants to be grown but, as I said before, there is an environmental balance to be struck between the benefit of the roof and the environmental cost of having a stronger supporting structure.

Thursday, 4 November 2010

Benefits of Green roof Construction

There are numerous benefits to green roof construction and perhaps one of the most important – and hardest to quantify – is thermal insulation.

A layer of turf, comprising a soil base and grass covering, is a non-conductive material. This means it doesn’t readily allow heat to pass through it from one place to another. On a roof, this keeps the warmth in during the winter and keeps the heat out during the summer. How much insulation it provides (its ‘u’ value) is difficult to say as it depends on the specific turf, the roof pitch and various other conditions, but studies suggest it can improve a roof’s insulation by as much as 75%

The soil part of the green roof layer absorbs low frequency noise and the vegetation part absorbs high frequency noise producing sound deadening effects of up to 45 to 50dB. This is clearly a great benefit in areas with, for example, noise from traffic, trains or aircraft.

Amenity value
It is to my mind a very subjective view as to whether green roofs add much to the local amenity. I think, for example, a single green roof in a conventional street could look, at best, slightly bizarre but it would be intriguing to see how a whole street of green roofs could look. On the other hand turf roofs on cottages in exposed locations – such as the Highlands or along the coast – look in keeping with the landscape. I shall seek the views of planning experts to find out how they would regard an application for a new green roof building in a rural, suburban and urban location.

Water runoff
There are other environmental benefits claimed for green roofs but, having been involved in land drainage in the past, I think one of the greatest immediate contributions could be in slowing the water run off from developed areas following rainfall. Years ago in Sussex we experienced the problems that occur when major green field housing developments caused rainfall to run off impermeable surfaces and erode the surrounding ‘natural’ surfaces – rather than being absorbed by the ground and contributing to groundwater stocks. There is no doubt that a green roof housing development or industrial development could hold back a lot of this water, giving time for more of it to be absorbed and reducing the damage from flash flooding.

Tuesday, 2 November 2010

What is a Green Roof?

A green roof is generally accepted to be a roof covered with some form of vegetation. Mostly this refers to some kind of grass or turf roof, but more elaborate green roofs may also incorporate larger plants and shrubs – or even small trees.

Turf roofs were used in various parts of the world in the past as they were seen as a good way to keep heat in and cold weather out. Gradually however the practice died out as developments in the manufacture and availability of building materials meant it became unnecessary – even unfashionable – to simply use locally available building materials.

Turves, reeds and straw thatch gave way to modern, smarter roofing materials like slate, tiles or even corrugated iron. It is perhaps easy in this new environmental era to dismiss this change, but it should be remembered that many of the older roofs required more maintenance than slate or tile and were often harder to construct.

Today improvements in techniques and materials are seeing a resurgence in the popularity of green roofs. This rise is popularity is due to a number of benefits green roofs can offer, such as improved thermal insulation, improved noise insulation and various environmental benefits.