Construction of Roofs (SANS Part L) section of the construction regulations includes several interrelated SANS codes that must be adhered to during roof construction. These SANS codes serve as a reminder to designers and builders that other sections, such as Part A: General principles and requirements, Part B: Structural Design, Part C: Dimensions, Part K: Walls, Part R: Stormwater disposal, Part T: Fire protection, Part V: Space heating, and Part L: Roofs, are also important in roof design. It is standard practice in the industry to consider all of these SANS codes as a whole.
Roof Designs – Basic Requirements
The design of a roof is influenced by various factors, such as the type of roofing material and the span of the roof structure. Typically, a series of roof trusses are used to construct the roof, which is supported by wooden wall plates spanning the walls of the building. These trusses can either be assembled on-site by nailing or bolting them together, or they can be pre-made by a specialized truss manufacturer and delivered to the construction site upon request.
The trusses consist of rafters, tie beams, posts, and struts that are assembled based on a specific design. The regulations provide simple line drawings for the following truss types and maximum clear spans:
- Four-bay Howe truss with a 6 m clear span
- Six-bay Howe truss with an 8 m clear span
- Two-bay mono-pitched Howe truss with a 3 m clear span
- Three-bay mono-pitched Howe truss with a 4 m clear span
The regulations also specify that no member of any truss should have a length exceeding 60 times its smallest dimension.
For a secure roof structure, it is essential that roof trusses and other framing elements have accurately cut and fitted joints to ensure a tight fit. All trussed roofs require approved bracing to prevent any potential buckling of the rafters, tie-beams, and long web members, as well as to maintain the trusses in an upright position. It is crucial that those responsible for calculations ensure that no part of the truss exceeds 60 times its smallest dimension.
When constructing a roof framework, purlins should have a minimum nominal depth and width of 76 mm or 50 mm, respectively, and a maximum center-to-center spacing of 1.2 m. Joints between adjacent purlins should be staggered. However, the following tables provide more specific information.
For roofs supported by brick, concrete block, or stone walls, all roof trusses, rafters, and beams must be securely fastened to the wall using galvanized steel strapping or wire that complies with National Building Regulations. Additionally, fasteners must be corrosion resistant.
Protection from the Elements
Protection from the elements encompasses several factors, including fire resistance, combustibility, and waterproofing, which includes flashing and flat roofs. Regarding fire resistance and combustibility, light fittings and other components that penetrate the ceiling must meet non-combustibility standards, and no wooden or other combustible materials are allowed to pass through any separating element of a building.
Waterproofing primarily involves runoff water from the roof, which is directly affected by the slope of the roof and the type of roof covering used. The regulations specify minimum roof slopes and sheet end laps and provide useful drawings for waterproofing details such as parapet wall waterproofing on balconies, tanking against a cavity wall, waterproofing under timber and aluminium door frames, and waterproofing at a shower base.
Flashing is employed to prevent leaks from entering around chimneys and other projections. Flat roofs present a unique set of challenges, as they must have a fall of approximately 1:50 to allow for proper drainage.
The design and installation of roofs involve numerous components, and it is essential to prioritize safety measures during the process. Although there are no explicit safety regulations for anchor points and lifelines on roof wooden trusses, basic working at heights guidelines must be followed, such as ladder safety. When a worker is working beyond the third tread, a person below should hold the ladder to prevent it from slipping.
We simply and easily assist our clients with the relevant, hands-on application of the Occupational Health and Safety Act, Regulations and associated South African National Standards.
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CEO Cairnmead Industrial Consultants (Pty) Ltd
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