SOMETIMES this column just writes itself. There I was, standing in front of the house, watching the swallows dart around. Whenever my gaze was drawn to the leaking guttering, unstable-looking chimney and patches of harling threatening to fall off the cottage walls, I directed my attention back to the wonders of nature.
Suddenly a voice behind me asked: "So, Louisa, what's the point of having a green roof?" Oh no, I thought, have weeds now started springing up between my roof slates? The dandelions by the front door are already bringing my good name into disrepute.
As it turned out, it wasn't my roof that was up for discussion, but green roofs in general. I don't make these things up for dramatic effect – my neighbour genuinely did pose this question. Adopting my best eco-guru expression, I replied: "Em, er, it's to create a habitat for wildlife."
"Doesn't it act as insulation, too?" asked the neighbour's wife.
"Yes, it does," said I, nodding sagely. I then made a sharp exit, lest the Q&A session get any more in-depth.
More research was needed. Especially as I'd been harbouring a desire for a green-roofed shed ever since I saw Joe Swift knock one together on Gardeners' World.
Extensive green roofs (as opposed to intensive ones, which are roof gardens you can walk around on) usually consist of layers of waterproofing and drainage materials with soil and plants on top. Pre-prepared blankets of plants can be rolled out across the roof or they can be planted manually. But whether you're planning on greening up the roof of your garage or your whole house, you'll need the advice of a structural engineer to make sure it can take the extra weight.
A study by English Nature estimates that 200 million square metres of roofs in Britain could become green without requiring much structural change, which sounds promising.
Getting back to the original question: why would anyone want a green roof? They do provide habitats for wildlife, but the insulating properties require more scrutiny. It's air pockets in the soil that trap the heat, but in a rainy country soil can become waterlogged, squishing out the air. One study showed that growing 30cm of grass on a similar amount of soil can insulate your roof as effectively as installing 15cm of mineral wool, but this only holds true when it's dry. Research published in the journal Energy and Buildings 2005 found green roofs can actually emit 40 per cent more heat in winter than a normal insulated roof.
According to the website www.living roofs.org (which bills itself as the independent UK resource for green roof information), more than 30 million square metres of green roofs have been erected in Germany since 2000, so they clearly have something to offer. From Scara Brae to the Scottish parliament, there is evidence of green roofs closer to home too.
Beechgrove gardener Carole Baxter was recently involved in planting the green roof of Aberdeen's Union Square development. During Ken Livingstone's tenure as mayor of London, a technical report was commissioned into living roofs, and found they do have a role to play in combating climate change. Green roofs reduce rainwater run-off (good news with regard to flood risk), reduce the urban heat island effect (the plants evaporate moisture and cool their surroundings), improve air quality and increase biodiversity.
Since I live in the countryside, my wildflower shed roof will be more a style statement than a weapon against climate change, but I am looking forward to seeing Scotland's city skylines go green.
