Building Regulations as they apply to roofs

General rules for the construction of roofs

As with most of the National Building Regulations, those that apply to roofs relate to SANS other that the one specific to that particular element. For instance, where any roof is to be supported on the wall of a building as described in the relevant section of Part K: Walls, the roof MIST be constructed in accordance with the rules laid out by the relevant SANS (in this case 10400). In addition, the new SANS remind designers and builders that other sections are also vitally important when it comes to roof design, including Part A: General principles and requirements; Part B: Structural design; Part C: Dimensions; Part R: Stormwater disposal; Part T: Fire protection and Part V: Space heating. 

Since anybody building a house MUST either BE a “competent person” in terms of the regulations, or must EMPLOY a “competent person” to put in plans and oversee the building operation, either you or the person you employ should purchase the updated section of SANS 10400 Part L Roofs from the SABS to double-check details and specifications. Also be acutely aware that circumstances vary from site to site. 

Basic requirements
Roof design depends on a number of factors including the type of covering you are going to use and the span over which the roof structure is to be supported. More often than not, the roof structure is assembled from a series of roof trusses. These rest on wooden wall plates, and are designed to span the walls of the house. They will be delivered to site on order by a specialist truss manufacturer. 

The trusses themselves are made up of rafters, tie beams, posts and struts, all of which are assembled according to a specific design. The new regulations have simple line drawings for most types of truss, as well as tables that specify the maximum clear spans for rafter and/or purlin beams.

Allowable timber grades for softwood and laminated rafter beams are addressed in considerable detail in the Regulations, including, for example, the following: maximum clear spans for laminated SA pine supporting tile or slate roof pitch; for SA pine supporting metal or fibre-cement sheeting or metal tiles; for SA pine purlin rafters or purlin beams supporting profiled metal or fibre-cement sheeting; for laminated SA pine rafter supporting profiled metal or fibre-cement sheeting or metal tiles; and for gum pole rafters. 
In addition to maximum spans, there are also minimum requirements in terms of slope (or pitch) and minimum end laps. 

Some important factors regarding connections
It is vital that roof trusses and other roof framing elements have joints that are accurately cut, securely made and fitted so that the component parts are drawn tightly together. All trussed roofs MUST be provided with approved bracing that prevents any possible buckling of the rafters, tie-beams and long web members. The bracing also needs to keep the trusses in an upright position. Whoever is doing the maths needs to be certain that no section of the truss has a length that is greater than 60 times its least (or smallest) dimension. 

If rafter construction is used instead of roof trusses, and the roof covering is regular sheeting or tiles (as already mentioned), it is important to accurately assess the parameters for rafter spans and the size and grade of rafters. 

All roof trusses, rafters and beams that are supported by a brick or concrete block (or even stone) wall must be securely fastened to the wall using galvanised steel strapping or galvanised steel wire that complies with the National Building Regulations. It is also important that fasteners are resistant to corrosion. 

 

Extracts were taken from an article in the South African Builders Magazine, September 2013 issue, written by Penny Swift