(by livingroofs.org)
A green roof system protects the waterproofing membrane from climatic extremes, UV light & mechanical damage and in so doing almost doubles its life expectancy. Therefore a good quality root resisting waterproofing system with a normal life expectancy when exposed to the elements of 30 years, can be expected to last up to 60 years, thus saving the client the cost of re-waterproofing during the average buildings expected life time.
Green roofs have a positive effect in terms of thermal insulation through their ability to cool buildings and insulate them during the winter [dependent on daily conductance of the green roof].
The German Green roofing manufacturer Zinco International has estimated that green roofs can contribute to savings in fuel heating costs of 2 litres of fuel oil/m2/year in Germany and an industrial plant in Frankfurt [Possman Cider Cooling and Storage Facility], which had a green roof installed recovered the cost of the green roof in 2-3 years through the savings in heating and cooling costs, and the reduction in industrial equipment associated with these activities.
A conversation with a building services manager in London happened to uncover that the application of a retrofitted green roof on a building had reduced the need for cooling/heating of industrial plant in the floor beneath. Since the green roof has been installed cooling and heating fans have not been used. It is estimated that about 25.9 MW per year is being saved. Using current electricity rates this is approximately a £4,300 saving per year. If the green roof had been installed as part of the original design of the building and the ability of the green roof to reduce cooling and heating requirements had been known, there would have been a potential saving of £10,000 due to reduction in the need to replace heating and cooling equipment in the floor in question.
A recent article in Heating and Air Conditioning Magazine www.emap.com is availble as a pdf here [500Kb]
There are potential savings for developers and owners in that a green roof installation could reduce the number of drainage outlets. A number of developers and green roof manufacturers have pointed out that quantity surveyors are unlikely to factor in these cost benefits, as they tend to separate the costs of roofs and drainage. However, depending on the size of the roof and the height of the building, the cost benefit of drainage outlet savings could significantly offset the cost of a green roof.
Industrial buildings, if designed at the planning stage, can take the weight of a green roof. The additional cost of a strengthening deck is outweighed by the reduction in drainage capacity requirement both on the roof and at ground level.
The reuse of local or secondary aggregates can provide a cost saving during the construction of the roofs within a development. The cost of growing mediums is in the region of £10-£15/m2. Although there will be a cost incurred if materials such as brick and concrete are used from site in terms of crushing to the required grade, screening and transportation within site this will be minimal. A 1000m2 green roof could potentially save the developer £10,000 off the cost of the materials needed on the roof. Inverted roofs require hard landscape features such as paving slabs to prevent the possible flotation of the insulation. The cost of paving is in the region of £15/m2. The required weight to prevent flotation can be achieved by the use of secondary aggregates in a green roof system thus leading to a cost saving by eliminating the use of paving slabs.