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Green Roofs- What's it all about?

Green Roof - Marlborough Sounds, New Zealand.

Traditional roofs serve an important purpose, making sense for robust and simple materials to be used. Often out of sight and out of mind, simple materials have been used, particularly in New Zealand. Corrugated metal, chipped metal tile along with tiles and slates as the norm - usually for the very good reason of working pretty well. Membranes have their place too and its fair to say that they are reliable if installed correctly.

Why Green Roofs?

Although traditional roof materials have been proven over time, with more and more dense building occuring, the opportunity to break up the roof line, give back some 'green', help the local wildlife, build in the coutryside where a green roof roof can help the building blend into its setting or hilly settlements (i.e Wellington, Nelson, Christchurch, Dunedin etc) where roofs are seen more often by neighbouring property. With the development of more robust membrane roofing materials and more information about green roofs, why not? whats not to like about a green, living roof?

Green roofs, also referred to as 'living roofs' have become more appealing to residential and commercial building owners to help solve the problem of monotonous looking roofs by supplementing with protected and largely untouched vegetation. Making a useless space, a useful space for the environment.

Green roofs have lots of positive features and are claimed to last longer that traditonal roofs due to not being exposed to UV, claimed benefits include:

  • reduce energy costs due to their natural insulating properties;
  • create welcome green space for surrounding building users;
  • create safe and often isolated retreats for animals;
  • absorb rainwater/stormwater;
  • reduce the need for complex drainage systems due to slower water run-off;
  • Maintain a more consistent temperature;
  • improve air quality;
  • provide a quieter environment.

So, why are we not all putting them on? Oh yes, Cost and Risk... or are these myths?

Green roofs are more complicated than traditional roofs, with more components, so often more expensive from the outset and arguably more to go wrong.

There is additional work to ensure the initial waterproofing layer is waterproof, install a root barrier, drainage system and the earth, all of which add to the complexity and cost.

Green roofs are well proven around the world, but New Zealands often onerous compliance requirements may prove a stumbling block, frustration, cost and probably delay.

Compliance with the New Zealand Building Code (NZBC)

Building Consent Authorities (BCAs), usually councils will require more evidence to prove compliance with the Building Code, in this case primarily E2 - External Moisture, E1 - Surface Water, B1 - Structure and B2 - Durability. A green roof would be considered as an 'Alternative Solution' that requires proof of compliance.

B1 - Structure

A roof with a load of soil and vegetation is a lot heavier than without, This means that any standard design solutions (New Zealand Standard NZS3604:2011 etc) are not applicable and a Structrual Engineers design is lilkely required. This will likely result in a more robust (and expensive) roof structure.

B2 - Durability

There are a number of requirements of Durabilty, when NZ Code Compliance is concerned. Very simply, if something is easy to replace, there is a lower time period requirement for the eleemtn to last, but if its significant and hard to replace the durabiltiy requirment is greater - up to 50 years. A traditional roof would probably be assessed as a 15 year requirement - add soil and vegetation and a BCA would be wise to require 50 year Durabilty. This is above most, if not all membrane manufacturer guarantees,so why should the BCA accept it? Its much harder to satisfy council, so if you want to go down the green roof route, don't under estimate this stage in the process. A solution may be to make the 'green element' of the roof a removable item, essential stacked boxes.

It may be wise not to let the roots get so big!

This is what the council fear!

E1 - Surface Water

Requirements for spouting and downpipe sizes, pitch of roofs etc are very well established. Follow the requirements of E1/AS1 - Surface Water and you have proven compliance. Add a green roof into the equation and you may not. This will probably raise more questions by the BCAs, and although green roofs are also used to hold the water and control/minimise its discharge, BCAs will require calculations and explanation. 

E2 - External Moisture

Detailing for membrane roofs which is a common material used beneath is probably pretty standard, but adding the green element needs consideration. Membrane manufacturers generally don't like their products being buried! How can it be maintained? What if it leaks? likely big problems and expense. The manufacturers need to be fully behind the roofs use and the installation standard has to be the highest possible. It would be advisable to keep the roof design super simple. Minimise or avoid internal gutters, no changes in level, insist on a decent pitch and give it some eaves. Keep the essential risks to a minimum - think about whether skylights are required and why not take the plumbing terminal vents through the walls and around the eaves instead of penetrations through the roof?

BRANZ Schematic

What membranes?

As mentioned before, you need to do your research around the suitbaility and compliance of membranes, but these may be a starter for you:

Below are a few companies that offer membranes that are available in New Zealand for a green roof solutions;

Plants

When choosing your plants for the green roof you need to consider maintenace. This needs to be a set and forget exercise. The plants need to be tolerant of saturation and drought.

  • Grasses

Some grasses are suitable, but often grasses will need irrigation. The benefits are however a self seeding simple solution, for minimal soil depth.

  • Succulents

The mainstay when considering green roof plants. Tiny succulents thrive in low water areas, very tolerant to wind, need minimal soil depth and provide great cover. Plan differrent varieties to provide a wide colour range,

  • Herbs

Low growing whilst offering good ground spread, what a great idea to have an edible roof garden (although a bit of a hassle when your making dinner!), but ideal herbs could include oergano or thyme which both thrive in low amounts of soil and are drought tolerant.

  • Wild Flowers

Often thought of as the go too choice, but wild flowers often require deeper roots. There are however species that are very tolerant to water and thin soil whilst offering good flower colour and coverage.

Visit the forum discussion to learn from other peoples exeriences or share your knowledge with Ecobob community.

BRANZ, New Zealands Building Research Authority also have a few articles on Green roofs, pretty old now, but likely still relevant - visit 'The gen on green roofs' or 'Green roofs worth their weight in gold' 

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