It depends.......why is it there, what is it expected to do, what species are present, where is the site [ location /climate] and how it was built.
A recent article in Scientific American online has had a huge response in comments, many debunking the assertion the green roofs don't work, as claimed by the author.
Truly, there is no real evidence presented - either way, in the many words delivered on the topic.
But how can a successful green roof be assessed?
While not an exclusive list consider the following claims that are made.
- Reduces roof temperatures and thus air conditioning and /or heating loads for the building
- Produces more oxygen in a city environment
- Absorbs CO2 and other gases by the plants and even some particulates or metals into the soils
- Uses rainwater and so reduces stormwater loads in the neighbourhood as well as absorbing materials in the rainwater [gases and particulates]
- Improves roof aesthetics
- Costs more to install and maintain
- May require an additional water source to meet plant needs in periods of water drought
- May allow food production on the site as a small food garden
- Must be maintained at some additional cost
Careful design can do a lot to improve the situation and allow a green roof to function effectively at minimal costs. While some push for native species on green roofs, that is not an essential feature by my views. It has to function at modest cost and inputs, once established.
In the warmer regions, grass can be a useful species. Yes, it may have to be mown sometimes, but many are tough species, and you can select those with minimal nutrition and mowing needs.
In Darwin there is a great green roof, which many do not even think about - the Speakers Green at Parliament House. That is built over the roof of the basement car park, and uses zoysia grass - it generally looks fantastic! This area is highly maintained and used intensively for functions, while being decorative.
In Singapore, the new water facility at Marina Barrage has a very large green roof - again it is grass!
Some types of green roofs can be identified as intensive or extensive. [ Both examples above are extensive types, although the former could be argued over and is used frequently]. The intensive type often has garden beds, small trees, formal paths and grassed or untended areas with mass plantings as a mixed development and may be used intensively, while extensive green roofs are ones that are sometimes just grassed, or with mostly less tended scrambling plants, less if any in the way of paths and garden beds, and often not greatly used.
The article is worth reading but many comments are very critical of the author, often for failing to really understand the subject. So yes, there are a range of views.
In my view the green roof can be a great asset, but need a degree of planning involving several different professional inputs is needed for successful development.