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How green is your roof?

7 March 2008 No Comment

Over the past year, everyone seems to have become an expert in sustainability.  But still when specifiers or clients ask “How green is my roof” there is confusion about how to measure the various elements of sustainability and what it all means. 

 What is meant by a “Sustainable” or “Green” solution for a building element?  It is now widely acknowledged that sustainable development refers to the balance of the triple bottom line, namely maximising economic, social and environmental benefit out of the decisions we make. It is easy to focus on only one of the elements of the triple bottom line.  In the past, this tends to have been the economic one, where maximising profits came before considering environmental or societal damage.  With the devastation predicted from climate change, it is easy now to focus purely on the environmental issues, and more specifically on carbon emissions.  While this is undoubtedly a key issue, it is important not to consider it alone, but to take a broader perspective. So, with so many often-competing factors vying for our attention, where should we start to answer the question of “How green is my roof?” With climate change being by far the most important issue to face, carbon emissions are a good place to start.  However even here, there is a mass of often-contradictory data on which to draw.   It is important firstly to consider both embodied and operational effects on the total carbon footprint of the roof.  Typically, CO2 emissions from the operating life of a building are at least five times those embodied in the building fabric, so the first consideration must be to minimise operational energy use through simple actions like applying suitable insulation.  It is now widely recognised that air leakage has a significant effect on heating bills, so constructing an air-tight building envelope is vital. Important as operational energy efficiency is, this is dealt with by building regulations and the required level of efficiency can be met with a variety of construction techniques and materials.  So, while it is important to minimise energy usage in a building, this does not answer the question of “How green is my roof?” The other part of the carbon footprint equation is the embodied carbon (embodied energy is often used) within the roofing materials.  Here, the important factors are the amount of CO2 emissions produced, either directly or indirectly, during the manufacturing and transport process, and the quantity of materials used.  Obviously, lightweight materials gain significant credit here, not only using less material themselves, but also often allowing lighter weight roof structures to be used too. Quantitative estimates for the embodied carbon emissions per unit area of roofing are notoriously difficult to find and even more difficult to compare.  Before attempting any such comparison, it is important to ensure that it is on a like-for-like basis and incorporates all necessary factors.  The science of carbon foot-printing has evolved a great deal in the last 10 years and so it is equally important that up-to-date data is used.   The boundary conditions used in any study of carbon emissions can have an enormous impact on the results.  For example, are figures given cradle-to-gate, cradle-to-grave or cradle-to-cradle?  How are recycled content and recyclability dealt with?  In many studies, recycled content is used as an important factor, but recyclability at end-of-life is not, but this gives a false impression. Pre-finished steel for roofing applications typically contains between 10% and 25% recycled content, but independent studies have found that 94% of demolition waste from pre-finished steel cladding systems is either re-used or recycled.  The steel recycling industry is the world’s most advanced recycling initiative, making steel the world’s most recycled material.  Meanwhile, closer to home, Corus have carried out research and development to ensure that the coatings in our pre-finished steel products provide no additional burden to the environment during the recycling process.  Pre-finished steel Colorcoat® products from Corus really are 100% recyclable. Even the term recycling means different things to different people though.  Steel, for example, is fully recycled. Scrap steel is an integral part of the steel making process, so steel is recycled without any down-grading of properties.  Steel is almost unique in this.  While many other products find some use after their initial life, there are few which can claim to be truly recycled.  For example, concrete products can be used as aggregate in a second life, but this doesn’t avoid production of virgin concrete, or lead to any future life.  While not necessarily so extreme, the case for plastics is similar.  While plastics can often be incorporated into the manufacturing process, in almost all cases only low-grade applications use recycled material. One of the fundamental problems with recycling most materials is separation from the waste stream, particularly from demolition sites.  The magnetic properties of steel have always made it very easy to separate from general waste, while segregation on-site is often carried out to ensure the value of steel scrap. Unfortunately, plastics, concrete, masonry, timber and other metals are more difficult to segregate, so generally become the victim of landfill, even when recycling is theoretically possible. So recycling, recyclability and recycled content are important factors to look for in any construction material, but even these do not give the whole story in terms of what makes a sustainable roof.  Unfortunately though, the whole story is a very complicated one.   Corus recently produced over 30 environmental product declarations (EPDs) for pre-finished steel cladding systems in support of our Confidex Sustain® carbon neutral building envelope scheme.  Within these, each system was measured on their impact in 6 key areas, including global warming, resource depletion and embodied energy, with a common basis for cradle-to-cradle comparison. While the results of this show differences, for example, in the amount or type of insulation used in a roofing system, the differences are small, highlighting the necessity to specify the correct product for a particular job, rather
than taking the simple approach of using the one with the lowest score in one of the categories.
 It is often the small things that make the difference and in measuring the impact of roofing systems this holds true.  Further studies by Corus have assessed the impact of the source of steel in pre-finished steel cladding systems and found that the embodied carbon content of imported steel is significantly higher than it is in steel which is made by companies such as Corus in the UK. Whilst there is no single accepted measure of sustainability of construction materials, direct comparison is very difficult.  The Green Guide to Specification is a good starting place and in the 3rd edition, it is evident that all pre-finished steel roof constructions gain an A rating while some flat roof specifications and artificial slates fare less well.  This represents possibly the best attempt yet at judging different roofing materials against each other, but it still suffers from an element of complexity beyond what most specifiers would want to deal with. Are there any simple rules for specifying a sustainable roof?  As we said earlier, functional performance is the first factor, since this defines how well the building will perform over it’s life.  Likewise, it is important to use long-life products, preferably with robust guarantees for as long as possible.  This minimises the need for maintenance and in many cases can eliminate costly repair or replacement.  Again, pre-finished steel scores well here with some products gaining BBA certificates for at least 40 years.  After that, recyclability, embodied energy and a whole host of other factors should be considered. So, to return to the original question, “How green is my roof?”  the answer is complex and comparing quantitative data from different sources is fraught with danger.  This doesn’t help the environmentally aware specifier though, so following the simple rules of good performance, long life and fully recyclable is a good start.  Added to this, schemes like Confidex Sustain® from Corus, which guarantees a carbon-neutral building envelope, can also help.

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