I was on top of one of the iconic London buildings earlier this week and was struck by the number of green roof buildings scattered around the city.
A few years ago I worked with an architectural firm who was thinking of investing in a green roof. The company eventually decided against a green roof for technical reasons.
Nonetheless, the analysis demonstrated that a green roof can be a great investment for a company, with the added benefit of being good for the environment.
In this article I cover all you need to know about green roof buildings.
What is a green roof?
A green roof, or what is sometimes called a living roof, is a building that has a roof that is partially or completely covered by vegetation. Typically the growing medium is separated from the roof by a waterproof substrate that allows water to drain or filter off the roof into drainage pipes.
In general, a green roof can be classified into two main categories – intensive and extensive. A green roof which has a reasonable depth of soil to allow larger plants to root and grow is typically considered an intensive green roof – the intensity refers to the level of labour, maintenance, irrigation etc. that is required to care for the roof. Intensive roofs typically support 40-70kg of vegetation per square foot.
On the other hand, extensive green roofs require very little, if any intervention, to grow and prosper. They are typically established on a thin layer of rockwool which can support mosses and sedum species. Extensive roofs support a lighter weight of vegetation (5-12kg)
Green roofs can be built on both pitched and flat roof structures. Pitched green roof structures typically require less attention to water-proofing as water naturally runs off the roof. Flat roofs are more prone to the risk of water penetrating through the roof structure and therefore require a complex array of water proofing materials and substrates.
Benefits of a green roof
The benefits of installing a green roof are multifaceted and include environmental, social and financial elements. From an environmental and social perspective, green roof structures reduce the carbon footprint of a building by reducing energy requirements and acting as a carbon sink for CO2. Moreover, green roofs reduce stormwater runoff which is good for urban areas that suffer from bloked drainage and localised flooding. Green roofs provide natural habitats for urban wilderness and also help filter urban pollutants from the air and rainwater.
From a financial perspective it has been shown that green roofs increase the life span of a roof by protecting the building substrate from erosive forces. Moreover, real estate prices on properties with green roof structures are usually higher than those of traditional buildings with normal roof structures. The obvious direct financial benefit from having a green roof is the reduction in energy requirements for a building. Energy savings are typically achieved through the additional insulating layer provided by the green roof as well as by the increased evaporative cooling that reduces the need for excessive air conditioning.
Green roof costs
The cost of a green roof varies depending on the type of roof, the required waterproofing and the level of vegetation planted. In the US costs range from 15 to 25 dollars per square foot, whereas in Europe the costs are between €100-€200 per square metre. Costs per square metre / square foot do typically come down the larger the area that is covered.
A key consideration is the maintenance cost of the roof. As noted above, intensive green roofs require significant manual intervention which comes at a cost. I would factor in the same cost as that of a normal size garden (i.e. maintenance, irrigation etc.). Extensive green roofs are less costly as they require very little manual intervention.
Green roofs can be a great initiative for a business to undertake. I have seen a few companies that have used a green roof initiative to engage staff in the building and development process as well as use the space for staff recreational activities (i.e. a place for staff to eat lunch, relax and have after work functions).
Moreover, the direct environmental and financial benefits can really drive value for an organisation. Typically, paybacks on a green roof are quite long (>5 years); however, the intangible benefits such as the increased visibility of green initiatives in a company can be a key differentiator that helps a business stand out to win new business and attract talent.