A living green roof is an amazing thing. It benefits local wildlife, the occupants of a building and the wider environment. It can even be designed to support food production. However, to specify a green roof is more complex than standard architecture. Not least because it involves meeting the needs of living plants. For an architect who is neither a farmer nor a gardener, this can pose problems. Welcome to Harrowden Turf’s checklist on how to specify a green roof.
1. Consider the slope of the roof
2. Think about aspect
3. Load bearing capacity is THE most important factor
5. Choice of growing medium
6. Plant selection
There are two main reasons why slope is important. First of all, rainwater will run off a steep slope faster than plants can access it. Plants on a steep slope will need constant irrigation. A completely flat roof however, will drain too slowly. Some plant species do not appreciate having wet feet and you certainly don’t want poor drainage and water retention to add extra weight to the roof.
Secondly, the green roof build-up needs to be kept on the roof somehow. How will you stop everything sliding off during a heavy rainstorm?
The ideal slope for a living green roof is between 1 and 10 degrees. You may get away with 20 degrees for some green roof build-ups but you will need to install an irrigation system.
Some plants need sunshine, some are happy with a modicum of shade. Full shade is not ideal for a living roof – in fact it’s unlikely to succeed. If a green roof is high on the list of your requirements, be sure to position it so that it has as much sun as possible.
Consider too the prevailing winds. If the plants on your green roof are subjected to very windy conditions they may suffer from scorched leaves or dehydration. Think how you could protect them – perhaps with an upstand?
The load bearing capacity of the building will determine the character and the environmental efficiency of your living roof.
A deep layer of growing medium will offer plenty of insulation against cold, heat and noise. It will also help more with rainwater amelioration. You will also have a bigger plant palette to choose from but it will be heavy - in the region of 250kg per square metre.
A lightweight sedum roof build-up such as the Enviromat green roof system is more suitable for retrofitting. Allow for a load of 120kg per square metre.
Access for installing the green roof system is normally no problem. However, you must bear in mind that every green roof will need maintenance. Drainage outlets must be cleared at least once a year and the plants must be fed. Please consider this in your overall design. And don’t forget to include fall restraints for health and safety.
Topsoil is never an option for a green roof. Always specify a properly engineered green roof substrate. Yes, it does cost a little more than topsoil but it has been thoroughly researched, formulated and blended. Green roof substrate weighs less than topsoil, drains better, will not compact or oxidise and will last as long as the building does.
Not a plantsman? Don’t worry. You are not alone. If you are not confident about selecting plant species for a living roof talk to the team at Harrowden Turf. Harrowden Turf produces pre-grown mats of plants for easy installation onto a roof. Choose between Enviromat sedum matting for a neat, minimalist finish or one of the five varieties of Meadowmat for maximum biodiversity and visual interest.
Drainage matting and advice on the full green roof build-up are freely available from Harrowden Turf Ltd. Make the most of their experience and knowledge by booking a lunch and learn session.