So can you or should you look to incorporate both a green roof and a rainwater harvesting system?
Rainwater harvesting - the requirementsFirstly, it is important to stress the basic requirement for a rainwater harvesting system to function and deliver a good quality of water. Virtually all experts in this field (our competitors!), agree that a clean, traditional and pitched roof produces water which can be simply filtered to remove general leaves and grit type debris. Flat roofs can sometimes have areas which drain very poorly and have "semi-stagnant" pools which only flow under more intense rainfall. After all a rainwater harvesting system can only deliver the expected quality of water, if it is supplied with it in the first place. The rainwater system isn't going to increase the quality and of course, a poorly designed/sized system could adversely affect the water.
Why are green roofs specified in the first place?Green roof technologies have emerged in the last decade or so and add value to the function of a building in a number of ways. Firstly, planning in certain areas of the country see them as a way of blending into an area, for example if the development is within a green space. Secondly, the roof off rates are much reduced, by the plants holding on to the water, before transpiring it back into the atmosphere. This also means that the often the lightest of rainfall would never reach the ground or the surface water discharge system. This is important, as attenuation design can be reduced. Finally, adding there must also be some carbon reduction from the plants themselves.
So can you combine rainwater harvesting and green roofs?Green roof technology is improving all the time, in terms of stability of the sub-structure, the membranes and the overall understanding of the maintenance requirements. This all helps to keep the resulting water clean and in some cases probably cleaner than some traditional roofs. So, technically you can, but we at Freerain, advises our customers that the choice is really an "either or" situation. The main reason behind that, is not as above (water quality), it is actually a much more practical reason. The amount of water available to a rainwater harvesting system is about 90% for pitched roofs and around 75% for flat roofs. For green (or even brown roofs) the amount is 40% at best.
So for example, a 100M2 London roof (pitched) might send 52M3 of water to the tank per year. But if it was a green roof, this would fall to only 23.3M3.
We also recommend additional filtration to remove the higher organic load. Even with the use of fine filtration, our expectation is that the resulting water might still be slightly dis-coloured. Obviously, adding more kit, adds to the overall cost of a system.