SA Affordable Housing May - June 2020 // ISSUE: 82 | Page 19

FEATURE When constructing a roof framework, the rule of thumb is that any purlin should have a minimum nominal depth and width of 76mm or 50mm, and max centre-to-centre spacing between the purlins ought to be 1.2m. Joints between purlins next to one another should be staggered. But the tables that follow are a lot more specific. All roof trusses, rafters and beams that are supported by a brick or concrete block (or even a stone) wall must be securely fastened to the wall using galvanised steel strapping or galvanised steel wire that complies with the NBR. It is also important that fasteners are resistant to corrosion. If you order factory-manufactured trusses that are made with metal plate connectors, they may not comply directly with the requirements of the various tables in the SANS. But a “competent person” will be able to tell you whether they meet the requirements of the regulations. The NBR are not prescriptive - they were established as a guide to minimum standards, and therefore cannot be ignored. ARCHDAILY The roof can add massively to the charm of a house, but there are strict standards it has to comply with. POLE CONSTRUCTION IS A NEW ADDITION TO THE NBR SANS If this method of construction is used, softwood poles must comply with SANS 457-2 and hardwood poles must comply with SANS 457-3, and all poles must be treated in accordance with the requirements of SANS 10005. If poles are sawn or reshaped at the ends, any of the exposed ends must be treated with a Class W preservative. It is also necessary to cover at least 35% of the surface area of the end with a new nail plate to prevent or at least minimise cracking. Thatched roof construction – which utilises pole structures – is also mentioned, though there are additional standards that need to be referred to. For thatched roofs, laths must have a minimum diameter of 25mm and they must comply with the requirements of SANS 1288. Spacing must be done according to SANS 10407. If a thatched roof is constructed with gables, without hips, valleys or dormer windows, it must have a pitch of 45 degrees, and a clear span that is no more than 6m. Construction must also be in accordance with SANS 10407 and with additional specification in SANS 10400-L. In the drawings, specifications for rafters state that if the poles are 100mm to 125mm in diameter, then the truss clear spans may not be greater than 4 m. If the poles are 125mm to 150mm in diameter, then the spans may be more than 4m but not greater than 6m. ROOF INSULATION The thermal performance of thermal insulation is expressed in terms of an R-value, also known as thermal resistance. The R-value is a guide to performance as an insulator—the higher the R-value, the better the insulation (resistance to heat flow) it provides. Insulation is one of the most effective ways to improve the energy efficiency of a home or a building. Insulation of the building envelope helps keep heat in during the winter and out in summer to improve comfort and save energy. Insulation could add additional benefits such as being a vapour barrier and/or an acoustic barrier. The appropriate level of insulation intervention will depend on the energy zone, building construction type, and whether auxiliary heating and/or cooling is used. PERFORMANCE REQUIREMENTS The highest total thermal resistance for a roof assembly in order to comply with SANS 10400-XA is 3.7 m²K/W. The minimum added R-value for thermal insulation required in this roof assembly is 3.35 m²K/W. For high humidity energy zones the thermal resistance requirement is 2.7 m²K/W and the minimum added R-value for thermal insulation required in this zone is 2.35 m²K/W. Thermal Insulation Products have different thermal resistance properties and must be tested in accordance with SANS 8301 to determine the thermal resistance (R-Value). Ensure reputable contractors are used to install the insulation in accordance with SANS 10400-XA performance requirements and manufacturers installation specifications. For more information on other changes pertaining to thermal insulation please contact TIPSASA. Tel: [phone] or [phone] Website: www.tipsasa.co.za or E-mail: [email protected]. REFERENCES 1. SANS10400.co.za 2. Ecoroofing.co.za THE REVISION OF SANS 10400-XA: 2011 AND THERMAL INSULATION By TIPSASA (Thermal Insulation Products and Systems Association SA) Since 2011 specifiers, contractors, developers and new build clients are all exposed to the regulatory requirement for buildings to be efficient in their energy demand and usage. Sadly, they still do not recognise the benefits of thermal insulation, as some do not install insulation in roofs or, if they do, it is way below the deemed-to-satisfy requirement. The revision of SANS 10400-XA due to be published this year, will see a different map, based on energy zones. The revision will also see the introduction of seven energy zones, this is a modification from the six previous climatic zones. This has been established as the benchmark for the maximum energy demand, and maximum energy consumption in the design of a particular building under South African conditions. www.saaffordablehousing.co.za MAY - JUNE 2020 17