It's a frequently overlooked fact that extensive green roofs need maintenance in order for them to deliver all of the benefits they were installed to supply.
A simple maintenance regime requires two visits to the roof per year; one in the Spring to check plant health and apply fertiliser if necessary; one in Autumn / Winter to remove debris and unwanted plants and, most importantly, to make sure that all of the drainage outlets are clear of rubbish and working properly.
Extensive green roofs frequently consist of mixed sedum species growing in a shallow layer of growing medium. Sedums are chosen because they are particularly well adapted to living in poor soils in exposed conditions and they do very well on roofs. However, there are two conditions that sedum plants cannot survive under: one is shade; the other is soggy ground.
It is therefore vital for the plants' survival that once Autumn leaves have finished falling, it is important to get out onto the roof to check that all is well.
The first job is to collect up all the dead leaves, carrier bags, rubbish and detritus that have found their way on to the roof in the last 6 months or so and remove them.
Be especially vigilant in the corners, because that's where everything tends to congregate. Debris blocks sunlight from the plants, effectively keeping them in the shade and killing them.
Next, pull out any unwanted plants such as tree seedlings. They can form strong roots that are capable of damaging waterproofing.
Finally make sure that drainage outlets are clear and that rainwater can escape from the roof quickly and easily - water sitting on the roof for long periods increases the weight loading and causes sedum roots to rot.