Can a green roof help with resource consent?

We were delighted to be involved in this exciting new build in Westmere, Auckland, for Evolution Builders.

The project incorporates 102m² of green roof across several areas, including:

• 22m² of pre-grown Sempergreen Click ’n Go trays
• 80m² of semi-extensive green roof, planted in situ with a combination of native and exotic species

The result is a lush, vibrant roofscape that looks fantastic from the upper levels of the house — while also providing valuable stormwater and environmental benefits.

We asked Chris Holmes of Caaht Studio, the project architect, why green roofs were incorporated into the design and what lessons he took from the project.

Chris says the idea came from the architecture firm and was considered during concept design.

The main objectives were:

• Aesthetics — the green roofs are visible from upstairs and provide a much more attractive outlook than conventional roofing.
• Thermal performance — improving the thermal performance of the roof.
• Stormwater management — reducing stormwater runoff.
• Resource consent — reducing the impervious area associated with the development.

One of the challenges during construction was getting everyone comfortable with the idea of installing a green roof.
The builder initially questioned whether the green roofs could be removed from the scope.
The solution was to install an ILD (International Leak Detection) system, giving the team confidence that any future leaks beneath the green roof could be accurately located and repaired.
Chris’s advice?
“I would specify an ILD system from the outset, for peace of mind.”
He also recommends that architects table the idea of a green roof early in the design process, so the structural design can allow for the additional weight from the beginning.

The finished roofs have exceeded expectations.

Chris says:

“The roofs have grown to be very lush and vibrant.”

But perhaps the biggest benefit is what they have done for the view from the house:

“They have made the harbour views from upstairs far more attractive compared with looking out over a standard roof.”

And when we asked whether the project had made him more likely to specify green roofs again, his answer was simple:

“Absolutely!”

We were particularly pleased with Chris’s comments about the installation:

“We have worked with Will on two green roof projects now and would not hesitate to recommend him to anyone contemplating a green roof project. He brings a wealth of knowledge which made for a smooth installation process and he came back to site to tidy up any loose ends.”

Chris Holmes — Caaht Studio

• Was the council supportive of the green roofs, or were there any       challenges during the consent process?
  Yes, we had no particular issues consenting the green roofs.
• Did the inclusion of green roofs influence your choice of waterproofing membrane or roofing contractor?
  Yes, it’s important to use a water proof membrane that is guaranteed under a green roof.  Builder chose the roofing contractor, who had an excellent track record with the builder.
• Were there any construction or coordination challenges that stood out?
  The builder was reluctant at first to install the green roofs, and queried whether they could be removed from scope. Following some debate the client agreed to pay for an ILD system (International Leak Detection) to enable any potential future leaks to be pin-pointed for repair. This gave the team confidence to proceed.

• Looking back, what do you think has been the biggest benefit of incorporating green roofs into this project?
  They have made the harbour views from upstairs far more attractive compared with looking out over a standard roof.
• Is there anything you would do differently on a future project?
  I would specify an ILD system from the outset, for peace of mind.
• Has this project made you more likely to specify green roofs again?
  Absolutely!

They can, although the benefits and requirements will vary between councils and individual projects.

A well-designed green roof can contribute to a project’s overall stormwater management strategy by:

• Reducing peak runoff during rainfall events
• Retaining rainfall and reducing the volume entering stormwater systems
• Delaying discharge into the stormwater network
• Improving water quality by filtering sediments and nutrients
• Reducing impervious surface area

This can be particularly useful where:

•  Impervious surface limits are tight
•  On-site stormwater attenuation is required
•  Downstream flooding is a concern
•  Low Impact Design (LID) or Water Sensitive Urban Design (WSUD) principles are encouraged
•  There is an opportunity to reduce the size of conventional stormwater detention systems

The key is to consider the green roof early — not as an afterthought.