The great slate debate

Roofing material matters and slate specification issues can be confusing, says Ged Ferris, with guidance for Greenbuild News readers.

As many specifiers are aware, roofing slate is available in both natural and man-made varieties. In order to make the most suitable choice for a particular project, it is necessary to have a good understanding of the unique qualities of both, as well as the kinds of application for which each is best suited.

The natural choice

Natural slate is a traditional building material, with a heritage that goes back to Roman times. It offers a fantastic combination of aesthetic appeal and exceptional durability. Natural slate can become a positive factor in a building¡¯s whole life costs, which is key since environmental issues continue to increase in importance in all areas of construction.

Natural slate satisfies a common requirement of modern specifications ¨C sustainability. As well as lasting the lifetime of the building, natural slate can be salvaged and re-used. In fact, reclaiming natural slate for roofing and cladding has been common practice since the middle of the last century, with worked stone recognised as a valuable building material for many hundreds of years, as it is today. New natural slate, tested to BS EN 12326 Part I and meeting A1, S1 and T1 ratings with the required low carbonate content will be unaffected by normal extremes of temperature, and highly resistant to acids, alkalis and other chemicals. Man-made products often discolour quicker than top quality natural slate due to staining or the growth of lichens (once surface coatings have weathered).

Natural slate is often specified to respect the traditional or local character of the area in which the building is situated, as a natural product features variations in colours, sizes and textures. This is especially important in conservation areas, where slate selection must match existing roofing materials on surrounding buildings. Historically, slates were from a local source which means that a particular set of characteristics of roofing materials were common to a particular to a location.

UK availability

Worldwide there are many slate quarries from which slates are sourced for the UK market. The main source in terms of volume is Spain which supplies the bulk of Europe¡¯s demand. Spain is followed by Brazil and China, with smaller tonnages from areas such as North America and India. Within the UK, Penrhyn in North Wales remains the largest source of natural slate followed by Cumbria, with local availability from quarries in the West Country as well as continuing use of traditional quarried stone in some areas. The slates from each region have defining characteristics and it is useful for the specifier to have some knowledge of these when selecting slate for a particular project. The highest quality slates available in the UK are from Wales and Canada. The qualities of Welsh slate are well known and, in the case of Canadian, the regard in which it is held stems largely from its similarity to Welsh in terms of performance and consistency.

Fibre cement slate

The availability of fibre cement slate means that where budgets are under pressure or access to expert slaters is an issue, projects can still benefit from a  slate roof. Fibre cement slates come in a uniform size, thickness and shape, eliminating the time needed to sort on site, as well as offering added versatility when architects want to use complex roof designs on buildings.

Fibre cement slates are pre-drilled for quick and easy fixing saving time on site, and therefore also lowering costs. In addition, the slates can be cut and mitred using simple hand tools. This adds to the ease of installation and can also reduce waste on site through breakage. This consistency means it is easier for these products to be used on intricate roofs in that they are suited for use by the less experienced roofer.

Budget considerations may lead some to fibre cement slates, since they are cheaper than natural slate. Although natural slate may be favoured in conservation areas the visual quality of fibre cement slates means that nowadays they can blend seamlessly with historic surroundings when natural slate cannot be sourced or is too expensive. In less sensitive areas fibre cement slates can offer the architect versatility on complex designs as well as a huge variation in colours available. High quality products have an appearance close to that of natural slate.

Which slate for the job?

Natural slate is great to look at, and is extremely tough. That said, it does need to be sorted and worked with, which adds time on site. Fibre cement options provide a cost-effective and conveniently sized alternative, and the quality of modern fibre cement slates has improved their durability and strength, some coming with a 30-year guarantee. Modern slates are also resistant to fire, chemicals and fungal growth, and are unaffected by temperature changes as well as being vermin and rot proof. Meanwhile, the low fixed weight of fibre cement slates allows economical use of timber roof trusses. However, sometimes only natural slate will do, for example, when it comes to matching the roofing of a particular area or group of buildings. In a nutshell, fibre cement slates offer consistency and convenience, whereas natural slate provides authenticity and character.

Original:greenbuildnews.co.uk