Multifunctional small green roofs: drought-proof shelters with wildlife habitats

Tim Green
Friday, 2nd December 2011

Whilst building low cost small green roofs Tim Green had a brain wave. His roofs could not only keep his buildings weather proof, he could also make them into wildlife habitats and dust them with symbiotic mycorrhizal fungi to make them more drought tolerant. Genius!

Obviously the primary function of any roof is to keep the weather out of a building but, in my mind, no roof should be constrained to this single purpose.

For the last couple of years, anything I have constructed around the farm has been roofed with that old farmers' favourite, the galvanised roof sheet. It's quick and easy to work with and for us has served the very important function of providing a means to capture plenty of rainfall.

However, we have now reached the point where our roof capture area has exceeded our water storage capacity so the latest two roofs on my construction escapade have been designed with different functions in mind. One is to provide a cool summer escape inside a dog kennel for our excessively hairy sheepdog and the other is to create much-needed habitat for creepy crawlies and fungi.

Although both are living roofs, one was carefully thought out and built with care; whereas the other was thrown together with reckless abandon.

In the doghouse

Firstly the dog kennel roof. Now it has been said that maybe I've gone a little overboard with the doghouse. It's constructed out of cob and is nearly as big as our kitchen. I've justified the effort by considering it a 'cob practice project' as a cob house is somewhere in our muddled future plans. But, if I'm honest, it may also have something to do with the fact that we dote on our dog far too much.

Cob dog house

A living roof was the obvious choice for this building because of the cooling effect provided by the vegetation on hot summer days. I've never tried being a border collie but I can certainly sympathize with anyone who has to wear a fur coat in 30 degrees.

The roof itself is about 12 feet by 9 feet (apologies for imperial units but they somehow seem fitting for archaic building techniques) and the support is provided by three small larch tree trunks from our woodland and half a dozen 4 inch round jump poles. I shan't go into detail about the timber framing here, suffice to say it was beefy to the point of over-engineering.

Tim Green at work

Fascia boards to retain the growing medium were from the bed of an old bale trailer we were renovating and the beams were covered in a layer of 1 inch thick tanalized planks. The all-important waterproof liner is a PVC pond liner (as with most people making green roofs, we would have preferred a butyl liner but couldn't afford it!). For drainage I followed the advice from the CAT Turf roof tipsheet and used a gravel channel and plug hole.

Newly completed in November

The big job was adding the growing medium. It's amazing how much a 6" deep 12x9ft container can hold. There's a lot of information about proper combinations of materials to make a green roof substrate but, owing to cost restrictions, I largely went with whatever I had lying around. Maybe I'll find out in the future if this was a mistake.

Here's my recipe:

4 wheelbarrow loads (whlbrw from now on) purchased soil conditioner

3 whlbrw topsoil (stones sieved out)

3 whlbrw clayey subsoil (left over from cobbing)

6 whlbrw homemade composted manure

2 whlbrw sharp sand

2 whlbrw woodchips (an attempt to "slow down" the fertility)

2 buckets horticultural vermiculite which I found somewhere

1 wheelbarrow homemade biochar (pre-soaked in compost tea)

2 buckets wood ash

1 sack of crushed seashells (ripped bag at garden centre – on offer!)

Mix it all together on a tarpaulin and chuck it on the roof.

Creating a micro-habitat too

Having applied the soil and raked it to a fine, even tilth that would have made an RHS gardener proud I realized I was becoming excessively controlling. Aside from the vegetative cooling function of the roof, the wildlife habitat function was also important and no decent habitat looks like a finely cultivated flower bed. As well as adding a bit of soil 'topography' I came across a wonderful document about living roofs that suggested the blindingly obvious (but unthought of by me!) idea of creating micro-habitats for invertebrates and the like. So on went the rotting logs and piles of stones.

During one of my occasional 'planning ahead' episodes earlier in the year I had pot grown a load of pink purslane and semi-domesticated strawberries for the purposes of populating a roof. There weren't enough to cover the whole area but as they were both edibles I decided to concentrate them round the edges for easy reach.

Strawberry edge

Each plant that went in had its roots dusted with a mycorrhizal innoculant. I will write about this in more detail some other time but my experiments with symbiotic mycorrhizal fungi have shown them to convey a great deal of drought tolerance (amongst other benfits) to their plant partners. Drying out is always going to be the biggest risk for a living roof so my hope is that a combination of the moisture holding potential of the biochar and the nurturing effects of the mycorrhizae will reduce the need for watering in dry spells.

A few random garlics, bugles, saxifrages and thymes also went on and then the remaining roof area was sown up with a very broad selection of wild flower seed. When spring arrives the Darwinian battle will begin and maybe in a few years everything will have settled into a wonderfully harmonious entangled bank.

Roof two: the log store

As I've rather rambled on about the first roof I'll keep the bit about the next roof brief and simple – as was its construction.

The existing roof on the log store (made from a demolished wardrobe) was failing to fulfil even the primary roof function so, buoyed up on green roofs and microhabitats, I went about replacing it. The underlying structure was some old chestnut fence posts and a load of strips of wood from a neighbour's kindling pile. Our neighbour mills fancy wood for yacht interiors so his off-cut pile is rather fine. The off-cuts were anywhere from 3mm to 10mm thick (arbitrary move to metric) but all bendy enough to follow the contours of my wobbly log store.

Recycled yacht interior offcuts

When all fixed down they looked rather pretty so it was almost a shame to cover them up, although they can still be admired from underneath! On top of these went flattened cardboard boxes and another pond liner. Instead of fascia boards to hold everything back I position a load of vaguely interlocking logs and sticks. Then the whole roof was covered in rotting logs, harts tongue ferns, pennywort, compost and leaf litter. Job done.

The construction took about 3 hours and cost just under £15 all in. This compared favourably to the still modest £200 cost of the kennel roof.

I will keep you posted with photos and updates as the roofs mature and grow. If either of them collapse I will keep very quiet about it. 

Mycelium running

Tim Green is a filmmaker and gardener. He made the BBC2 permaculture classic, A Farm For The Future, with his partner, Rebecca Hosking, and now lives on the Devon farm in question where they are testing out many of the ideas featured in the film (when the 'Old Boys' let them!). They write a brilliant blogs for Permaculture online and features for Permaculture Magazine. To find out more click HERE

Permaculture recommends

Mycelium Running by Paul Stamets priced £34

Small Green Roofs by Nigel Dunnett, Dusty Gedge, John Little and Edmund C. Snodgrass priced £16.99.

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