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Beware the Green Roof

By Lauriston Muirhead, Wodonga Albury Towards Climate Health (WATCH)

We don’t think about roofs much, or should that be rooves?  See!  We’re not even sure how to spell them!  When did you last think about, or even look at, your roof?  Chances are that it was because something went wrong such as damage or a leak.

You want your roof to be strong – so it doesn’t blow away or get damaged by hail, lightweight – to keep the cost of your house down and, above all, waterproof.

There has been a fashion over the past few years for “green” roofs.  I am the first person to promote almost any opportunity to grow plants but I have strong reservations about putting them on the roof without very careful thought.  If, after careful thought, you still want this sort of green roof – think again!

Most plants need significant amounts of soil (or similar material) and water.  Soil and water are heavy.  A 10mm rain-shower absorbed into growth material on your roof of, let’s say, 300 square metres, adds three tonnes to your roof!  A good solid couple of days rain putting 100mm on the roof, adds 30 tonnes of water!  Not to mention the weight of the soil and the plants themselves.

All this extra weight requires extra engineering and additional materials with all the extra embedded energy that involves.

Now let’s assume the building is strong enough to take all that extra weight of water, soil and plants, the next challenge is keeping it all on the outside!  Water is incredibly good at finding its way through the most apparently impenetrable materials.  Add the strength of growing roots and you have powerful forces working to penetrate your roof and let water in.  Even concrete is not a guaranteed barrier over time.

Another consideration is all the maintenance of what is, effectively, a roof garden.  A quick look at the statistics for admissions to Accident and Emergency  from falls from ladders and rooves should make anyone think again.

There are nearly always better, safer, cheaper and more enjoyable places to put plants than on your roof.

In my long experience, the “greenest” thing you can do with your roof is to put a solar hot water unit and as many photo voltaic panels as you can on it.