Fibre cement vs Natural slate

John Dodd, technical manager for roofing at Marley Eternit, discusses the key considerations when choosing slates.

Slate has always been a fashionable choice of roof covering and over the years fibre cement slates have become increasingly popular as a cost effective and sustainable alternative to natural slate.  The introduction of the Code for Sustainable Homes in 2007 has been an important factor in driving demand for fibre cement slates because it makes the BRE’s ‘Green Guide to Specification’ product rating even more significant.

The Code for Sustainable Homes gives maximum credit for materials which provide the best environmental performance based on the BRE ‘Green Guide to Specification’ which grades the environmental impact of building products from A+ to E.  By using products with a higher rating, between one and 15 credits can be achieved which go towards the energy efficiency star rating of the building. 

Obviously choosing an A+ rated product gives a roof the maximum credits.  However, in order to get an A+ slate roof covering, the main issue isn’t whether you choose fibre cement or natural but actually where the product is manufactured.   UK produced fibre cement slates are listed as achieving an A+ rating (Element 812410008) in the BRE Green Guide and as Marley Eternit is the only UK manufacturer, we can claim to meet the highest rating possible for our range of slates. 
 
In fact the country of origin is becoming an increasingly important issue for new build housing and the carbon footprint of the products being used is the main reason for this.  As UK supplies of natural slate have decreased, specifiers have been forced to consider using imports from abroad with imported natural slates, particularly from China or South America, having a far greater carbon footprint than those produced within this country and in continental Europe.  Slates are normally transported by articulated trucks to a port and then shipped to Britain so the carbon emissions generated by road and sea transport will affect the slate’s overall environmental impact.  Another factor to consider is the embodied energy of a product, whether taken from the UK or abroad, the process of extracting natural slate from the ground is energy intensive.

UK manufactured fibre cement slates offer a viable alternative to natural slate because they provide roofers, architects, specifiers and builders the option of using a high quality product that is often cheaper, lighter and manufactured to a consistent quality and dimensional tolerances making it a cost effective alternative.  This is not only ideal for refurbishment projects, but also for new build developments, where the look of heritage traditionally associated with slate is sought.  However, interestingly it isn’t just the traditional look that is driving demand; we are starting to see fibre cement slates, like our own Thrutone product, being used to create a contemporary cladding on new housing developments.

Respecting traditional roofscapes and blending in with existing surroundings is still very important from a planning point of view and one reason why natural slate has been specified in the past.  However, recent developments in fibre cement slate production mean they can blend well with historical surroundings.  All four of Marley Eternit’s products - the Rivendale, Birkdale, Garsdale and Thrutone - offer various distinct characteristics such as riven surfaces and dressed edges, so that architects have the option of selecting a product that fits the planning and aesthetic criteria for the project they are working on.  

There is no doubt about the aesthetic and durability credentials of natural slate but they require a higher level of skill to install than fibre cement slates, which also result in roofers being on site for less time, thus shortening the  length of the roofing programme.  For example, fibre cement slates are very versatile. Not only are they lightweight, strong and durable (a minimum 60 years expected service life) , which allows them to be transported, stored and installed with ease, but the fact they are manmade means that they can be manufactured to uniform dimensions. This means that instead of struggling to sort slates to consistent thickness and size, as would be necessary with natural slates, fibre cement slates can be easily and quickly installed on roofs with a pitch of 20° or above. Not only does this benefit the project from an installation point of view, where wastage is minimal,  it also makes economic sense when considering the overall cost of the roofing contract.

Fibre cement slates go through a rigorous testing regime to ensure the quality of the product is of the highest standard, and Marley Eternit’s Widnes factory which produces the complete range of the company’s fibre cement  slates has recently been awarded the BSI Kitemark quality mark to BS EN 492, which includes tests for bending resistance, water impermeability, freeze/thaw and heat-rain cycling.  The Widnes -plant also holds BS EN ISO 9001, BS EN ISO 14001 and OHS 18001 for its quality, environmental and health and safety management systems, independently assessed by the BSI.

Natural slates have always offered buildings a distinguished look; however fibre cement slates, which offer an economical, readily available and environmentally friendly alternative, are certainly worthy of selection for any project where a slate roof is required.  As we move nearer to the target of ‘zero carbon’ in 2016, where and how roofing products are manufactured and their source of supply will become increasingly important as specifiers look to making the most of every opportunity to get the best  environmental rating that can be achieved.

For more information about the environmental rating of products, visit http://www.bre.co.uk/greenguide/

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