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RCI January 2018

THE PITCH By John Mercer, technical manager at Wienerberger Roof Tiles 2017 marks 40 years in the roofing and building industry for me. So, what has changed in pitched roofing in the last 40 years? My first thought was; not much! Actually though, quite a lot has changed. One of the biggest drivers of change in domestic dwelling design was the energy shortages in the 1970’s. When energy was cheap and houses were heated primarily using coal fires, houses were naturally draughty to encourage the fire to ‘draw’ up the chimney. Cold air was free to move around under the floors, within the cavity walls and into the roof space. Towards the end of the 1970’s, the thickness of loft insulation started to increase and in the early 80’s, houses began to be constructed with closed cavities; cavity closures are now routinely installed at the top of cavity walls and with air bricks. This combination of greater insulation and limited cold air movement within the building structure began to cause problems with condensation. Roof spaces were particularly prone to condensation as warm, moist air passed through the ceiling into the colder loft space. The use of continuous eaves ventilation, soffit or over fascia types, combined with high-level ridge ventilation and tile vents has largely eliminated condensation. Roof underlays have undergone significant development since the 1970’s. When I started work in a builders merchant in 1977, two types of underlay were sold; BS 747 type 1F and non- British Standard. Perhaps unsurprisingly, the cheaper, non-British Standard type was the most popular. These underlays, being relatively heavy, tended to drape naturally between rafters and, being bituminous, formed a seal around the nail holes where the tile battens were nailed into the rafters. The disadvantage of these underlays was that they were vapour impermeable and so provided no contribution to the ventilation of the roof space. The introduction of lightweight vapourpermeable underlays, used extensively in Europe, marked a sea-change in the control of roof space condensation, though it has to be said that even today, there is still sometimes confusion about how these ‘breathable’ underlays should be installed in combination with roof space ventilation. In typical English roof construction, the underlay takes a significant proportion of the total wind load on the roof, therefore the wind load resistance of lightweight breathable underlays must now be determined using a test method described in BS 5534. The publication in 2014 of the latest version of the British Standard for slating and tiling; BS 5534, was another landmark in roofing. For the first time, as a default and regardless of the calculated wind loading on the roof, all single lap roof tiles, ridge tiles and hip tiles must be fixed, with perimeter tiles now being twice fixed. The Standard introduced higher theoretical wind John Mercer: “Roof underlays have undergone significant development since the 1970’s. When I started work in a builders merchant in 1977, two types of underlay were sold; BS 747 type 1F and non-British Standard. Perhaps unsurprisingly, the cheaper, non-British Standard type was the most popular.” loads, which means that more fixings are required. This has spurred roof tile manufacturers on to develop faster and easier to use tile clips to help the roofing contractor. The requirement for mechanically fixed ridge and hip tiles in particular, has driven an increase in the use of dry fix systems; ridge and hip tiles can still be mortar-bedded, but the mortar must be supplemented with mechanical fixings. The use of dry fix systems has increased significantly in recent years, as designers and their clients seek to achieve a maintenance-free roof. Dry fix systems, in eliminating the use of mortar at ridge, hip and verges have great advantages for the installer too, such as speed of installation and no concerns that rain or frost will damage freshly laid mortar before it has set. Until now, the manufacture and use of dry fix systems has largely been unregulated. With the publication of BS 8612 due early in 2018, the UK roofing industry will finally have a British Standard for its dry fix roofing products. The new Standard will deal with the materials that dry fix products are made from, by stating the appropriate Standard to test a material to, or, as in the case of plastics, specifying the material properties that should be declared. It will then cover the various functions of each system, such as wind and rain resistance, as well as the particular functions of each system. Increasingly, a tiled pitched roof can be regarded as a system combined with the internal ceiling construction, with particular importance attached to underlay performance, ventilation, fixings and dry fix systems. To improve the thermal performance of buildings, not only has the thickness of insulation increased but also there is now more emphasis on the air-tightness of buildings. This in turn influences how other parts of a building are constructed. For example, if a building has a well-sealed ceiling as defined in BS 9250, the roof space can be constructed with less ventilation as less water vapour can escape from the living space into the roof space. Roofs may not look a great deal different from the way they did 40 years ago, but there has actually been a lot of development in the last 40 years to improve the quality, appearance and durability of our pitched roofs. www.wienerberger.co.uk Visit www.rcimag.co.uk for more The Pitch articles from John Mercer 40 years in the roofing industry 070 JANUARY 2018 RCIMAG.COM


RCI January 2018
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