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Recommended Fixing Guide(pdf) » Contessa slate
Imported by Cembrit Ltd for over 15 years, Contessa has become one of the UK’s leading specified Spanish slates. Quality Contessa slates will last the life time of a building if installed correctly and have been widely used on major projects such as supermarkets, schools, hotels, city offices and prestige buildings. They can withstand the most severe weather conditions and are unaffected by sunlight, ultraviolet light and even acid rain.
British Standard BS 680: Part 2:1971 requirements for roofing slate. • Water absorption test (Clause 6.2.1)• Wetting and drying test (Clause 6.2.2)• Sulphuric Acid Test (Clause 6.2.3)General design considerations Contessa slate laid to BS 5534 will meet the strength requirements for the imposed and uniformly distributed wind and snow loads etc. The site exposure rating and the pitch of roof rafters will determine the size, pattern, lap and fixings for the slates. For UK and Northern Ireland locations, BS 5534: Part 1: 1997 will indicate the expected degree of exposure. Wind driven rain ratings less than 56.5 l/m2 per spell are described as ‘moderate’ (see table 1) and those above 56.5 l/m2 per spell are described as ‘severe’ (see table 2). Detailed guidance on wind load calculations is given in BS 5534: Part 1 and in BS 6399: Part 2: 1995 and Part 3: 1998. In locations where abnormal conditions may be anticipated such as elevated sites, coastal locations, areas of heavy snowfall etc., the recommendation for ‘severe’ should be followed. Additional information can also be found in BS 8104. Where the location or construction might make a lower rafter pitch acceptable, designers are asked to seek advice. Table 1 Moderate exposureless than 56.5 l/m2 per spell
Table 2 Severe exposuregreater than or equal to 56.5 l/m2 per spell
Supply Cembrit ’s natural Contessa slates are supplied directly to approved accounts, roofing contractors and builders’ merchants. Prices are available on request from Cembrit Ltd. Sitework Storage and handling Slates should be carefully stacked on their longer edges with timber bearers, battens or boards between layers. The lowest layer must be arranged on a firm level base. Working Slate can be cut and holed on site by hand or machine. Care must be taken to avoid undue spalling. Slates should be holed from the bed towards the face so that the nail-head is accommodated in the small cavity formed by the area of spalling. At the time of holing, it is recommended that the slates should be sorted into three or four groups of similar thickness. The thickest should be used in the courses nearest to the eaves. Individual slates should be holed so that the thicker end is the tail of the slate. The holing gauge = gauge + lap + nail clearance (8 to15 mm). The holing gauges for common laps are shown in tables 1 and 2. Fixing Contessa slates must be laid in accordance with the Code of Practice for Slating and Tiling, BS 5534: Part 1 :1997 Design, and the Code of Practice for Workmanship on Building Sites – Slating and Tiling, BS 8000: Part 6: 1990. Slates should be twice centre-nailed to horizontal battens etc., as described in BS 5534, using: • Copper nails to BS 1202: Part 2• Aluminium nails to BS 1202: Part 3 or, for hook-fixing• Stainless steel nail hooks to suit common lapsThe minimum shank diameter for nails used with natural slates is 3.35 mm and they should penetrate into the batten by a minimum of 15 mm. Slates, generally, should be not less than 150 mm wide. At all verges and abutments, alternate slate courses must start with half width or slate and half width to maintain bond. Slate and a half widths must be used if the half slate is less than 150 mm wide. At valleys and hips where slates need to be cut on the rake, wide slates must be used to maintain an adequate width of the head or tail. At mitred hips on low pitched roofs wide slates should be used. Battens In accordance with the recommendations in BS 5534: Part 1: 1997, the minimum batten size, for rafter spacing up to 600 mm for use with natural slates is 50 ´ 25 mm. To avoid splitting the batten, the maximum nail diameter should not exceed one tenth of the batten width. The roof should always be set out so that the long edges of the slates are parallel to the direction at which the water will run off the roof. In some instances this may result in raking all the eaves and the ridge. The ends of any batten should be fully supported and the length of any batten should not be less than 1.2 m (except where this is unavoidable) and nailed to a maximum of 600 mm centres. Where the roof is close-boarded, counter battens should be used down the slope in the roof on the line of the rafters. Counter battens should be at least 38 mm wide ´ 12 mm deep and, where impermeable underlay is specified, of sufficient thickness to provide ventilation between the boarding and underlay.Underlay Suitable underlay should have a minimum standard to BS 747: 1977 Type 1F and/or should have a BBA Certificate. The underlay should be draped over the rafters, or fully supported on boarding or sheathing, should allow any moisture to drain and should extend over the tilting fillet, fascia board and into the eaves gutter. Lead staining risk Lead develops a lead carbonate patina which, if washed over slates by rain or other moisture, causes unsightly staining. To prevent this reaction marring the work, it is advisable to treat all lead including soakers and flashings, with patination oil before any rain occurs, and not later than the day the lead is fixed. Patination oil, a low viscosity fluid intended for this purpose, should be applied evenly. It is readily available from builders’ merchants. Technical services For assistance with all aspects of the specification and application of any of our products, please contact Cembrit Head Office. Typical specification Roof to be covered with Contessa Slates, size ....... mm ´ ....... mm, laid to a minimum head lap of ....... mm. |
Contessa is a natural Spanish slate of the highest quality, selected from the finest deposits and produced by some of the most efficient quarries in Spain. It is available in two colours: blue/black and grey, in a variety of sizes, and with its textured surface can be relied on to create an attractive and long lasting roof.
Contessa slate is a tough natural material and samples are regularly tested to BS 680: Part 2: 1971.Cembrit Limited, 57 Kellner Road, London SE28 0AX Tel: 0208 301 8900 Fax: 0208 301 8901 sales@cembrit.co.uk |