Project1_Layout 1 07/05/2013 PITCHED ROOFING Safety in slate: Important elements to ensure a secure installation Natural slate has become one of the most widely used roofing materials, and is favoured by architects, roofing contractors and homeowners alike. Here, Allan Liddell, sales area manager for Scotland at CUPA PIZARRAS, explores the key factors to consider when it comes to both specification and installation As contractors will be aware, environmental factors must be taken into consideration, as a roof can be severely affected by the level of its exposure to wind and rain. Buildings on slopes or hills, and also tall buildings, would be classified as highexposure and those positioned within 30 miles of a west-facing coastline as severe. The type of slate is also crucial to the success of the project. For a steep roof, small slates are a suitable solution, and wider slates with an increased lap should be used for those with a lower pitch, as well as for highexposure sites. Calculating the correct slate lap is the third and final aspect that contributes towards a robust specification. This is determined by measuring the roof pitch and considering the amount of weather exposure. If the thickness of the natural slate reduces its pitch by 3° or more, the next lower rafter pitch should be selected to ascertain the correct head-lap. The importance of quality control Once a specification has been established and the slates are received on-site, best practice dictates that the necessary safety and quality tests be completed. Slates are sent in pallets and when arriving on-site, should be assessed and divided into three or four thicknesses. Roofing contractors should ensure that the slates are laid on the long side with battens between the layers. During this grading process, it is vital to carry out essential checks for imperfections and to tap each slate to determine soundness. Once slates have all been safely and securely examined, installation can begin. Roofing contractors may well have a preferred method, but it is worthwhile having a clear understanding on the two main methods: nailing and hook fixing. In both instances, slates can also be supplied pre-holed for ease. 090 MARCH 2018 RCIMAG.COM Whichever fixing method is chosen, it is vital to calculate the gauge. The following table provides a useful guide: Nail fixing • Firstly, slate nails should be either copper, silicone bronze, aluminium alloy, or stainless steel in coastal areas for corrosion protection • Each slate is double-side nailed with two nails onto a soft wood sarking board with 19mm minimum thickness • Holing by machine is the advisable approach to help ensure quality control. Only one slate should be holed at a time. It is important to pay attention to the risk of damage if the machine uses a punching technique. The holing machine will also need to be checked, cleaned and adjusted frequently • Holing by hand is sometimes necessary, for example, if a small number of slates need re-holing for repairs or when a hole needs to be repositioned in-situ. To hole a slate with a spike hammer, place the slate securely over a narrow iron. Holing should always be from the bed of the slate and each slate should be holed twice Fixing with hooks • Roofing contractors must be aware that this cannot be done if the roof pitch is below 25°. In addition, if the pitch is below 30°, the hooks will have crimped shanks to reduce the capillary rise of water that could occur within the joints between slates • Fixing with hooks as opposed to nails can have many advantages, and roofing contractors, architects and specifiers sometimes choose this method to prevent wind uplift • When fixing slates with hooks, it is important to note that all perimeter slates should be hook-fixed and nailed. Slate hooks should also be stainless steel, and should be formed from stainless steel wire. Natural slate has many qualities that have made it so prevalent in the roofing industry, including attractive aesthetics, eco-friendliness and durability. As the popularity of natural slate continues, and with it now used for a multitude of different buildings, it is vital to make both an informed specification and to follow best practice when it comes to installation. www.cupapizarras.com/uk Gauge for nailing Gauge = (length of slate - lap) 2 Gauge = (length of slate + 10mm) - hook length 2 Ensure nails are not long enough to puncture membrane/felt.
RCI March 2018
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