Green Roof How To: Installing Your Own

Maddy Harland
Friday, 1st August 2003

Maddy Harland describes how to convert a pitch and tar flat roof into a green roof: a beautiful and enduring paradise for birds and bees

The state of our front garden had become legendary in our neighbourhood. Our house eco-renovation had at last been completed, solar hot water was flowing in abundance during summer months, and the back garden was planted up with wildflower meadow, top and soft fruit and some veggies. We were fairly sorted. But years of work had turned our front garden into a junk yard. Even cleared, the Portuguese laurel had become overlarge, blocking out the light, the lawn had disappeared under black plastic mulch, the pond liner had perished and drained, and the garage roof was a leaky abomination that not even a Russian vine could disguise. And as all our bedroom windows looked out on to the garage, it was spoiling for a change.

Choosing a green roof system

Reluctant to replace the roof with another tar and pitch roof that would only perish in ten years, we decided to plant an alternative. I remembered the turf roofs at Findhorn and then recalled John Talbot, the community's eco-builder, telling me how they needed cutting in the autumn and watering in hot summers to remain green. Turf is also heavy and the garage roof's joists were not particularly robust.

On a visit to Ben Law's woodland workshop one spring, I was impressed by the yellow flowering sedum cascading down the roof and the nest of baby thrushes in the eaves. Though I couldn't emulate the thrushes, I could plant a sedum roof instead of grass. Sedum is drought resistant, needing no extra watering after it is established and doesn't need trimming either, so it is low maintenance.

Experimenting with green roof plants

You can buy sedum impregnated mats for roofs. BedZED, the eco-housing development in South London covered their expanses of roofs with this product but, for a small project like this, it was unnecessary and expensive. You can grow your own. The first step is to collect appropriate plants from all quarters and propagate them. Tim and I decided that this was an opportunity to test out a range of drought tolerant plants and began propagating sedums, saxifrages, house leeks and other alpines the year before. A trip to our local garden centre run by people who actually grow their own plants (rather than buying them all in and flogging them straight on) was instructive. So too were visits to friends who gave us plants to grow on. We ended up with a wide variety of plants with which to experiment.

Then to the build. Firstly, we stripped the roof of Russian vine and deteriorated roof felt. Luckily the chipboard covering the joists was damp but not perished, so we left it in place. Next we refelted the surface to reseal the roof. This was a fail-safe approach in case the top membrane was perforated at any time. Purists may well wish to ignore this step. We removed the rotting fascia boards and raised the sides with arris rails to accommodate the organic matter that would later be our planting medium. We laid a thick layer of carpet on top of the felt to protect the pond membrane which came next. In an ideal world we should have used a rubber pond liner which would have been more environmentally benign. I confess that instead we used an extra heavy duty PVC liner with a 20 year life expectancy. This was because it was a fraction of the cost of a butyl liner. (Lack of cash sometimes enforces compromise, but in retrospect I'd rather have found another way around this problem.)

Benefits of a green roof: wildlife and water

A couple of our local pals, Shane and his dad, Terry, gave us a hand. Once the pond liner was in place new fascia boards were placed around the front and sides for finishing and to hold the liner in place. We then mixed compost with Pro Gro, a municipal soil conditioner from green waste, and laid it on the roof. It is obviously important not to incorporate any stones into the mix and risk damaging the membrane. It is equally important not to be clumsy with the hobnail boots! The growing medium was then topped by woochip to suppress any weeds.

Finally, an arris rail was fixed to the back of the roof on the downside of the slope to stop the organic matter flowing into the gutter. The rail had grooves cut into the underside to allow rainwater to drain off into the gutter and we made it detachable in case it rots.

Then for the fun bit: planting all our alpines, sedums, thymes and a few whimsies like tiny pinks and delicate blue flowering alpines along the way. Tim and I placed a scaffold board across the width of the roof and got to work planting in blocks of colour. No clashing reds next to pinks! What fun to plant in terms of just colour schemes for a change rather than multifunction!

Job done? Even that day we quickly discovered that the roof had become a playground for hungry blackbirds foraging for worms. They had no consideration for our baby plants and tossed them out of the mulch enthusiastically. Not a problem. We erected a temporary blackbird guard which comprised old upended bricks and garden netting that we happened to have lying around. That deterred them without entangling them or the plants in the netting. It also stopped the local cats treating the roof as an outsized litter tray.

It has taken three years for the plants to establish properly and grow together into a colourful, flowering bed. In the first year, we did water the roof regularly when it dried out in the summer. We were less attentive in the second summer (and it rained alot). This year, we haven't really bothered despite being a dry season. The main work has been weeding the roof, a job that has diminished as time has passed. Grass, hairy bittercress (yes, I know its edible but I'm not climbing up for a salad), dock, willowherb and rogue yellow flag iris from the pond, and the occasional hawthorn and bramble have all volunteered. Neighbours walking by have been amused to see us on roof. "What are you doing?", one asked in the first year. (I confess, we do have rather a reputation for being eccentric locally. It's probably the legend of the compost toilet among other things...) "Weeding our garage roof. Come and see," we replied.

Today, our roof is a delight and a beautiful part of what is now a lovely cottage garden and our local reputation is on the road to restoration. The sedum has begun its glorious descent over the fascias and, from spring onwards, we have a succession of flowering plants and a wonderful richness of colour. The blackbirds and sparrows can now forage on the roof as it is suitably robust, and the flowers attract bees, hoverflies and butterflies in abundance.

Cost of a green roof

And the cost? All this pleasure for less than £300.00 in materials and plants bought in February 2000.

Further Resources

Small Green Roofs - low-tech options for greener living is a useful guide to building resilient green roofs. 

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