Some of you may have noticed the sedum roof on our house here at Muddy Carrot HQ. Looks good, I think you'll agree, but is there any more to it than that? A lot of effort to go to perhaps, so there must be more logic to it than mere aesthetics?It's made up of an absorbent substrate which we planted with 4,000 assorted sedum and alpine plug plants that we grew in the polytunnels here. Now, four years on, the plugs are getting knitted together to create a living carpet which changes colour with the seasons and will ultimately help the building blend in with its surroundings.
But the recent heavy rainfall has brought home another benefit of green roofs - recognised in Germany where something like one in ten new builds in built-up areas includes a green roof. That benefit is rainwater runoff attenuation. Underneath the planting substrate is an egg-box like structure that provides thousands of little rainwater reservoirs to sustain the plants through dry periods. Above that is a membrane followed by the planting material which itself is absorbent (it's made of little pellets of aerated brick material) plus of course the plants' roots also absorb a certain amount of moisture. So from a dry state there are three ways in which rainwater is held before it finds its way to the gutters and downpipes.
Although this house is surrounded by fields so flooding isn't a problem, where green roofs really come into their own is in heavily built up areas. The abundance of non-absorbent surfaces such as concrete, tiles and tarmac exacerbate the effects of heavy rain, but this can be slowed down by absorbing the water of an initial downpour so it can trickle away slowly over time. Where better to do this than above our heads on rooftops, making good use of otherwise inaccessible areas?
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