Green Star, Homestar and metal roofing.

NZMRM and NZ Steel are members of the NZ Green Building Council 

We have previously discussed the long process of having metal roofing recognised by NZGBC as eligible for credit under the MAT-8 rating in the Green Star accreditation system which recognises the sustainability of products used in a building and their effect during the use and life (and after life) of the building.

The Green Star system was aimed at commercial buildings, initially offices, but subsequently other commercial buildings and schools.  Some of these of course have a relatively small amount of steel in the roof cladding if the structure is not also steel. The main issue against NZ made steel – which is used for the majority of metal roofing and wall cladding – was that it contained little or no post-consumer scrap, which NZGBC regarded as important from a sustainability viewpoint, even though NZ Steel’s production process has a number of very sustainable aspects, which may have gone unrecognised.

NZMRM and NZ Steel argued against this for a number of years and eventually MAT-8 was changed to remove this requirement, while still including some very demanding criteria before products can comply. Even so most of the criteria are aligned with the use of the building and its effect on the environment, rather more than the materials used in the building alone.

After the introduction of Green Star (along international principles, and aligned with some other international rating systems, it was realised that there is also a need to encourage sustainable homes, and there are schemes for such less complex buildings elsewhere e.g. (UK Code for Sustainable Homes and Energy Performance Certificate system for houses, mandatory on house transfer). Indeed it looks like NZ is well behind most of the OECD in this respect; just as well it is only quite cold here, not literally freezing mostly. NZGBC developed Homestar to fill this gap.  The first version of this was quite basic and awarded up to 10 stars for various aspects of house sustainability, but no real reference to the building materials used.

Then after some revisions to Homestar, in 2017 Version 4 was issued.  This contains a huge expansion in ways to obtain credits and specifically some features of the materials used.
We are going, in this first article, to discuss how to obtain Homestar credits under 4 different ratings. Note that there are still several mandatory criteria that must be met before any building can be eligible for a Homestar rating above 6 stars. These cover  thermal performance, ventilation, moisture control and efficiency of showers and toilets.. And of course while we are going to look at the credits available by use of metal roofing there are a lot of other ways to obtain points in the 192 page manual. 

Note the entire manual can be downloaded free from the NZGBC website, so I am including relevant extracts only.

So what are the features of metal roofing that provide your Homestar Assessor with information they can input to the Calculator to derive points for your building?

The four criteria we consider particularly relevant to metal roofing are  WST-1 Construction Waste Minimisation (up to 5 points); MAT-1 Sustainable Materials (up to 10 points); and for houses with rainwater collected from a metal roof – WAT -2 Sustainable Water Supply (up to 4 points) and STE -1 Stormwater Management (up to 4 points)

All this information (and much more) is available from the NZGBC website under Homestar. To avoid using too much space, we are going to go through these and summarise the points. All work in the same way -

  • What is the credit description;
  • What is its purpose
  • How to obtain points


  • How metal roofing will allow you to comply and get points towards the final rating. Most of this is summarised here from the Manual. 

WST-1 Construction Waste Minimisation
5 points

Aim: To encourage and recognise effective waste management practices by having a waste minimisation plan in place during construction and/or major refurbishment. To encourage and recognise a reduction in the amount of waste generated onsite during construction and/or major refurbishment.

Design Rating
Up to five points are awarded where the following can be demonstrated during construction and/or refurbishment of the dwelling:

Or -  Built Rating

Up to five points are available for this credit where it can be demonstrated that the waste generated from construction activities has been reduced or diverted from landfill and cleanfill. Points can be achieved via one approach only.

*Landfill is defined as any earth filling activity other than cleanfill.   Cleanfill is defined as per the MfE “A Guide to the Management of Cleanfills Jan 2002”.

Audit Documentation
Design Rating
All Projects
SWMP OR where a SWMP has not yet been written, an extract from either the building contract
or tender documentation (i.e. specification extract) that requires that a SWMP must be developed
in accordance with the REBRI guidelines, including any waste reduction or diversion targets AND
any requirements for onsite sorting.

Built Rating

All Projects

Monthly waste and RRR reports for the entire duration of construction works are to be signed and witnessed at each stage of reporting by senior company representatives of the waste and RRR contractor. These reports will clearly state the reported level of RRR that has actually taken place.

Reduced Construction Waste

Photo or copy of the completed waste records based on monthly reports for the whole of site which display the weight of waste sent to landfill/cleanfill measured in units of kg/m2.

Discussion points (by NZMRM) 

As we read this, you can obtain points in the Design phase by setting targets for waste reduction / diversion and having a Site Waste Minimisation Plan to show how you will achieve these targets. You can achieve another point by specifying on-site sorting stations. In the Built phase you will need to have implemented a Site Waste Minimisation Plan and have waste reports from a contractor to show that you have achieved a certain waste reduction or diversion benchmark. 

Now, what we know for metal roofing (long run or tiles) is that unlike most of the other materials used, the product is made specifically for the job – in the case of long run it is cut to length in the factory and for tiles the right number are supplied.  On-site trimming of corners and flashings (if appropriate) results in minimal scrap which is able to be recycled – and should be removed for this purpose by the installer. So, site waste from roofing will be minimal and in fact none should be sent to landfill/cleanfill, thus helping to achieve one of the higher waste reduction or diversion benchmarks. We anticipate that use of metal roofing, along with an implemented site waste management plan and smart decisions on other materials used, should help achieve at least 4 points in this credit.  

MAT-1 Sustainable Materials 10 points

Aim: To encourage and recognise the specification and use of responsibly sourced materials that have lower environmental impacts over their lifetime.

Credit Criteria
Up to ten points are available where there is a selection of reused, eco-preferred (see definition right) or responsibly sourced (see definition right) materials as follows:


This credit is only applicable to new dwellings that have been built, and existing dwellings that have undergone major refurbishment within the previous three years (from the date of assessment). Points are awarded for each of the following material categories only where at least 50% of the total material content is reused, eco-preferred or responsibly sourced. Up to two points can be awarded to each material category depending on the means of compliance. Table (right) outlines each material type where NZ Steel products would be included and the measurement unit to establish that 50% of the content is compliant. 

Material categories consider materials that are typically used in large volumes in construction. While Home Star ratings apply to individual homes and apartments, in large developments of multiple houses the total material volumes can be considered rather than a per house basis.

Audit Documentation

Design Rating

Drawing(s) or specification(s) clearly showing selected products;

  • Accompanied by product data sheet or certificate demonstrating compliance; AND
  • Completed Materials Calculator tab.

If exact products have not yet been selected, specification extract(s) stating requirements to be met.

Built Rating
Invoices or supplier/installer confirmation letters clearly showing selected products.

  • Product data sheet or certificate demonstrating compliance of each product claimed; AND
  • Completed Materials Calculator tab when targeting ‘materials which don’t have 100% compliant products.

Discussion (NZMRM)
As covered in our MAT-8 Green Star discussion, metal roofing made from steel manufactured by NZ Steel complies with items 2 and 3 and so are eligible for 3 points.  Because metal cladding is made to size for each installation (which minimises waste) the use of reused metal cladding is not practicable (or indeed desirable from a quality viewpoint).   

So these two criteria (WST and MAT) relate to and are inherent in the use of metal roof and wall cladding in any house. The remaining two criteria relate to houses where rainwater is collected from the roof for all household purposes, as occurs over the whole of New Zealand outside urban areas (where doing so is not allowed for commercial reasons). Water collected from unpainted metal roofing made with NZ made steel or metal painted in New Zealand has been demonstrated to be safe to drink and contain no unacceptable chemicals.  Such water is suitable for all household purposes. This may not be true for product painted outside NZ or for other roofing products.

WAT-2 Sustainable Water Supply 
4 points

Aim: To encourage and recognise reducing a dwelling’s demand on water supplies through the collection and use of rainwater on and around the dwelling and by promoting responsible water use behaviour through separate metering of apartment water consumption.

Credit Criteria
Up to four points are available for dwellings which reduce the consumption of potable water in and around the dwelling through the collection and use of rainwater. A water calculator is used to estimate the percentage of household water demand able to be met with rainwater:
Distribution of Rainwater Use Points

The credit then discusses the various ways in which water can be used in houses and allocates points up to 3.5 for 75% of water being rainwater collected.   

Discussion (NZMRM)
While some dwellings may collect rainwater and only use it for e.g. watering the garden, or for restricted household, use the vast majority of rural rainwater collection is used exclusively for all household purposes and so qualifies for maximum points.  The manual shows how to calculate points for lower % use.

Water Calculator
The Water Calculator includes calculations for WAT-1 (Water use in the Home), WAT 2 (Sustainable Water Supply),  and EHC-2 (Hot Water Heating). Central to this calculator is the household water use summary, which estimates daily per person water use in litres for the dwelling.

The Homestar Assessor will use this Calculator as part of the overall assessment.

STE-1 Stormwater Management
4 points

Aim: To encourage and recognise houses/sites that reduce stormwater run-off from buildings and hard surfaces, in order to mitigate flooding, pollution and stream erosion.

Credit Criteria

Up to four points are available for managing site and roof stormwater runoff.

Site stormwater runoff

Up to 2 points are awarded where it can be demonstrated that a percentage of the site (not including area under roof) is permeable or designed to capture stormwater runoff through permanent on-site stormwater management systems e.g. vegetated swale, on-site rain garden, pond, sandfilter or stormwater detention tanks.

But this also covers the detention of stormwater from the roof.

Roof stormwater runoff

Up to 2 points are awarded where the stormwater associated with the roof is effectively managed on site with either a living roof or designed to capture stormwater runoff through permanent onsite stormwater management systems e.g. vegetated swale, on-site rain garden, pond, sandfilter or stormwater detention tanks.

Discussion (NZMRM)

We would argue that roof rainwater collection and retention systems by definition comply with this last as long as retention / detention tanks are sized appropriately (see manual for detailed advice).

Summary and Conclusion

The Homestar accreditation system was designed originally to encourage people building new houses to consider how to make the whole project as sustainable as possible, using some quite basic criteria. This followed more complex models used elsewhere (and note that in the UK, such a rating is mandatory when selling or renting a house).  Homestar originally did not consider things like actual materials, more the sustainability of the house as built.

This most recent iteration now aligns more with Green Star in making allowance for the sustainability (both inherent and in use) of the building materials.  We have considered only those which are affected by the use of metal roofing (and although not covered as such) wall cladding.

We acknowledge with thanks the comments from the staff of NZGBC involved in Homestar, who have read and checked this article. 

Next time we will revisit Green Star.