ARCHITECTS ARE SPECIFYING SLATE FOR MORE AND MORE PROJECTS ACROSS THE COUNTRY
A familiar material across the UK's rural and industrial landscape, slate has been used for roofing and flooring since time immemorial. People welcome this most iconic of natural materials into both commercial developments and residential buildings across the country.
People like slate. They like it for what it is - a practical, natural material with a durable, traditional association, not manufactured, but quarried in an extraction process that goes back centuries.
Slate beds were formed millions of years ago from deposited silts and mud, subjected to heat and pressure modification and metamorphosed into new rock types. SSQ extracts Del Carmen slate in Spain from an ancient sequence of Ordovician strata laid between 505 million and 438 million years ago. SSQ's Argentine Riverstone slate was formed within a Pre-Cambrian rock sequence, with formation from over 640 million years ago.
SPECIFYING SLATE FOR A ROOF: KEY DESIGN POINTS When designing a slate roof, the calculation of the overlap needed is critical to long-term optimum performance. Take into account interrelated factors including site exposure, roof pitch and slate type selected, as well as the slate overlap. Consider the following:
? Roof pitch: in general, the lower the pitch of the roof, the greater the overlap for the slate tiles laid on it. A longer lap will compensate for higher capillary action. Smaller slates can be used on steeper roofs with free-flowing drainage. Exposed sites call for a wider slate with a greater overlap.
? Environmental conditions: local factors need to be particularly carefully noted with high buildings, or structures on the slopes or the tops of hills and along the coast. Any increase in the exposure grading for the site will influence wind-uplift considerations.
A longer lap will help compensate for wind uplift.
? Weathering: the degree of exposure to driving rain that the building will experience over its lifetime will influence the minimum lap of tiles to be specified.
? Fixing: all SSQ natural slates can be fixed by using either traditional holing and nailing, or a hook-fixing system.
? On site, ensure that the slates are sorted by thickness on the ground. See that slates of equal thickness are laid in any one course, with the thicker end at the tail. Thicker slates should be used in lower courses and thinner slates in upper courses.
? Construction: the rule of thumb for a natural slate roof is to have timber battens of 50 x 24mm, set out horizontally and nailed at 600mm centres. Underlay to BS 5534 Part 1 is needed; use Type 1F reinforced bitumen felt or other approved material if the underlay is not fully supported. Provide a 10mm continuous vent at both eaves for a cold-roof (insulation at ceiling level) construction, and 25mm for a warm roof (insulation in the plane of the roof).
A warm roof will also need additional ventilation at or near the ridge equivalent.
Sourced from established quarries, a properly specified SSQ slate roof will provide a service life in excess of 75 years, whatever local weather and environmental conditions prevail.
SLATE STANDARDS: CAUSE FOR CONCERN The new European roofing slate standard BS EN 12326-1:2004 has not been well received by suppliers. They are concerned that the dilution of compliance criteria will cause confusion. While the previous BS standard required slates to satisfy minimum performance criteria, the recently introduced standard has no pass or fail criteria, explains Alain Richard, technical director of SSQ.
'This new certificate gives different levels of acceptance for many tests and a simple statement of the values for others, ' he says. 'This means that any slate, regardless of quality, can now be CE certificated, even if the compliance levels to one or more of the tests are so low that a roof using such slate would fail.
'Under the new standard, a slate certified as complying with code T3 for durability will contain pyrites capable of burning a hole right through the slate, ' he adds. 'Busy architects are now expected to know this. Additionally, much of the required detail is not apparent on the certificate, so they will need to request it from suppliers to be absolutely sure of quality.' Barry Hunt of Independent Building Investigation Services, a firm of chartered geologists and chartered surveyors, says: 'This new industry standard fails to set minimum standards for compliance.' He recommends that specifiers use the following checklist to ensure slate quality:
? Flexural strength at standard thickness >70MPa.? Water absorption of less than 0.3 per cent - generally, the lower, the better.? Carbonate content of less than 3 per cent - generally, the lower, the better.? Proven satisfactory history of performance in UK environment.
? Avoid slates that have iron-sulphur and carbonate minerals in combination.
? Avoid slates with the potential for oxidation.
The French Standard P32/302 Class A has long been regarded around the world as an unofficial 'gold standard'. Ahmed El Helw, SSQ's chairman, makes a point of sourcing the best Class A slates. 'At SSQ we are totally committed to quality, ' he says. 'We provide single-sourced natural slates possessing excellent geological profiles, with full independent geological test documentation.' Richard says: 'SSQ avoids any frustrating tile-match issues through long-term strategic partnerships with established quarries, and a strict quality-control system.' He adds: 'Traditionally, slate was quarried using blasting.
However, we use the safer and more environmentally friendly diamond-saw extraction method.' Skilled quarry workers cut large, measured blocks for SSQ, using a thin diamond-coated cable. Each slate is then hand split from the block to the required tile thickness.
Using these stringent production techniques, SSQ produces and exports almost 70,000 tonnes of natural slate from Spain and Argentina a year - enough to cover two million square metres of roof. A strong presence at each of the quarries ensures consistency of product - through a unique quality-assessment system, which sees each pallet of tiles inspected as it leaves the quarry.
Slates from different quarries differ in colour and quality, so it is important to make the right choice.
As architecture increasingly takes on the mantle of sustainability, with an emphasis on using natural materials to enhance the visual expression of a structure as well as performance, the use of slate is enjoying a renaissance both as a roofing material and for floors and other surfaces. Architects are specifying slate for more and more projects across the country. Two of the most frequently sought-after slates come from quarries operated in partnership with SSQ: the renowned Del Carmen quarry in Spain and the established Riverstone quarry in Argentina.
SSQ DEL CARMEN SLATE Lying in the Cabrera region of north-western Spain, an area that has geomorphic conditions favourable to the formation of high-grade seams of true geological slate, the quarry of Carbajal de la Romana is one of the largest in Spain. The quarry produces the distinctive SSQ Del Carmen slate, a traditional riven slate with a distinctive, faintly rippled texture, a characteristic longitudinal grain and a blue-black hue that does not fade under constant strong sunlight. With extremely non-porous properties and a low coefficient of expansion, SSQ Del Carmen is particularly suited to roofing projects located in areas where there are extremes of temperature. It is almost totally inert, showing practically no reaction when exposed to polluted environments or acid rain.
SSQ RIVERSTONE SLATE SSQ also imports another market-leading slate, SSQ Riverstone, quarried from an Argentine source where slate extraction has been taking place for hundreds of years. SSQ Riverstone roofing slate has a very low content of any of the usual problematic inclusions, such as pyrite and calcite, making it a stable roofing product that will not colour or suffer from pitting. It is sourced from La Represa quarry in the La Florida area of central Argentina.
With a grain slightly coarser than slate sourced in many other quarries, grey-green SSQ Riverstone slate roof tiles have a rougher surface texture that imparts a more rustic feel to a new or refurbished roof. Scottish heavy slates are available, alongside traditional English and Welsh slate thicknesses. Riverstone has a reputation among planning and conservation bodies across the UK, established through its aesthetic similarity to many local slates, its high quality and, equally important, its value for money. It has frequently been used on projects where the volume of slate required or the budget could not be met by local sources.
75-YEAR PERFORMANCE These durable slates are underwritten by SSQ with a guarantee of 75 years. They meet BS 680 Part 2, American Standard C406, S1, and are both rated Class A under the stringent French Standard P32/302. Snowdonia National Park has approved the use of SSQ Del Carmen slate as an alternative to locally sourced Welsh slate, and both English Heritage and Historic Scotland accept the use of SSQ Del Carmen and SSQ Riverstone slates in the refurbishment of listed buildings.
CHRIST COLLEGE, BRECON SSQ Riverstone grey-green slates are being used extensively in the refurbishment of a roof with an area of 2,500m 2 at the historic Christ College boarding and day school in Brecon, south Wales. The school was founded by Henry VIII in 1541, and includes Grade II-listed Victorian buildings as well as a Grade I-listed 13th-century chapel. SSQ Riverstone tiles were specified because of their close resemblance to the original Cornish slate used to roof the buildings. The specification had to meet with the approval of both the Brecon Beacons National Park authority and Cadw: Welsh Historic Monuments. To date, 16,000 400 x 200 size slates and 5,000 500 x 250 size slates have been used on the refurbishment of an octagonal common room and the library stairwell. By the time the entire refurbishment is complete, 50,000 slates will have been used. According to the school bursar, the slate was true to the original, but it came at a significantly lower cost.
WESTER DALMENY HOUSING A recently completed housing project at Wester Dalmeny Steading, Scotland, sees existing listed farm steadings restored and eight new houses added to the site, giving a total of 17 dwellings. Great care was taken by the architect Pollack Hammond Partnership and developer Bell Grant to preserve the inherent architectural traditions of the surrounding conservation area. Architectural sensitivity heavily influenced the choice of roofing, with the local authority approving the use of 88,000 SSQ Rocaber Ultra Heavy slates. The roofs were fixed by Braisby Roofing in the traditional method, using diminishing courses to deliver a superior aesthetic finish and enhanced environmental performance.
Of prime importance to all parties involved was finding a material that would match the traditional slates that have been used in the region for centuries. For colour, texture and, importantly, thickness, SSQ Rocaber Ultra Heavy was a natural choice for selection.
'We were particularly pleased with the successful integration of the old and new buildings achieved on the site, ' says architect Tom Pollack. 'We had to overcome problems such as the irregularity of the existing walls by fitting timber-frame structures within the shell of the old buildings. These supported the slate roofs, which look all the better for the extra effort put in by Braisby and SSQ to deliver the diminished coursing.'
HAFOD HOUSING RENEWAL SCHEME An ongoing renewal scheme in Hafod, Swansea, involving the refurbishment of the outer envelope of approximately 1,100 housing properties, will use in excess of 55,000m 2 of roofi ng over a 10-year period up to 2011. The whole project, funded by the Welsh Assembly, is being designed, specified and managed by the City and County of Swansea. A third of the scheme is located in the Vivianstown Conservation Area, once home to the Hafod Copperworks, established in 1810, which was at one time reputed to be the world's largest and most up-to-date copper-smelting enterprise. All the houses in the conservation area were built in the 1830s for the families of copper workers and are enclosed by Grade I-listed boundary walls made, uniquely, from copper slag, a waste by-product of copper smelting.
Over the years, many of the houses have undergone private refurbishments to repair the damage resulting from weathering and pollution. Much of this refurbishment has destroyed their original nature - chimneys and timber entrance canopies were removed, slate roofs replaced with concrete tiles and timber sash windows replaced with uPVC.
The refurbishment is aimed specifically at restoring the traditional uniformity of these humble dwellings by reinstating the chimney stacks and pots, recovering the roofs in traditional slate, providing new guttering and fascias, rendering in a uniform colour and reinstating the porticos to the original design. The choice of vernacular materials is critical to successfully meeting the required effect and quality standard. The selection of blue-black SSQ Del Carmen roofing slates was approved by Swansea council as matching the high standard of the traditional Welsh material.
A spokesperson for the Hafod Area Renewal Office says: 'The choice of SSQ Del Carmen was made easier after confirmation that it was the only non-indigenous slate to be used in the Welsh National Parks, so we know its pedigree. We were also pleased with the level of technical, design and on-site support we received from SSQ.'
NATIONAL MARITIME MUSEUM, FALMOUTH The National Maritime Museum in Falmouth, Cornwall, a RIBA Award-winning design by Long & Kentish Architects, brings together the collections of the Cornwall Maritime Museum with the national small-boat collection from the National Maritime Museum in Greenwich, south London, under an SSQ Riverstone slate roof.
Commissioned by the South West of England Regional Development Agency, this landmark regeneration project was inspired, in part, by the surrounding industrial sheds traditionally used to house boat builders.
Construction of the museum itself posed a number of challenges, including building part of the museum over the dock wall into the harbour, constructing a gallery below sea level and developing a design that could use the natural environment to assist in controlling the internal conditions of the gallery to reduce energy usage.
The use of vernacular materials, such as the English green oak cladding and a roof tiled with SSQ Riverstone slates, fixed in diminishing courses creates a striking, contemporary, yet authentic look to Falmouth harbour's waterfront.
DURABILITY TAKES THE FLOOR With the increasing use of SSQ Riverstone slate as a flooring material, SSQ decided to launch into the flooring market. An indepth IBIS report (1009-001) into the properties and performance of the slate as a flooring material was commissioned.
The report classified SSQ Riverstone as a 'phyllite' - a metamorphic rock closely related to slate but of coarser crystal grain size. It concluded that the slate is suitable for flooring and other applications. In most instances SSQ Riverstone outperformed its closest UK equivalents: the Cumbria-sourced slates that include Burlington and Westmoreland. Its benefits were found to include:
? Its natural riven surface enhances an inherent low slip potential under most wet surface conditions.
? The slate offers good resistance to abrasion.
? It offers good all-round resistance to weathering extremes, if used as an external flooring material, as it is unaffected by temperature and does not fade in the sunlight, while its low porosity does not allow concentrations of ice, salts or other materials to lead to the breakdown of the stone.
In most applications, SSQ Riverstone's hard constituent materials, high density and low porosity give it a good floor and stair-tread surface that resists dirt infiltration. One of the main benefits of a slate floor is its inherent resistance to scratching.
With a natural surface that is riven, or smoothed to a patina, larger scratches will not show up, and will quickly be integrated into the overall appearance of the floor.