Sunday, 31 January 2016 08:54

Considerations when specifying concealed fix roof systems

Safintra explains the technical details and installation options when constructing a concealed fix roofing system.

The metal roofing market has seen a spike of growth and product innovation in recent years, driven in large part by design developments within the construction and architecture industry, often based on considerations of sustainability. Concealed fix systems in particular, used for large-span metal roof applications, have improved their engineered performance to provide a high-performing and durable solution.

Given the large scale of investment buildings often covered by a concealed fix roof system, specifiers need peace of mind regarding the engineering performance of the entire roof system if they are to serve the long-term interests of their clients and reduce liability for all concerned.

When a concealed fix roofing system is installed, the sheets are fixed to the purlin with a clip system, and are not pierced with any fasteners. The clips that are needed to fix the sheets to the purlin serve two purposes – they circumvent the need for holes in the sheet (which prevents the possibility of leakage through the perforation) and they allow the roof to expand and contract in a sliding movement over the clips.

By being able to move over the clips, the sheets can rolled in long unbroken lengths which can thermally expand without restriction. Without concealed clips, the sheets would be positively fastened through the sheet into the purlin and the fasteners would be subjected to the entire force of the expansion; this force is considerable, and either the fasteners will tear larger holes in the sheet, or shear and break. The continuous lengths of a concealed fix system also obviate the use of end laps which further reduces the risk of leaks.

How coated steel expands in roofing systems
Coated steel expands at a nominal 1mm per 1m, and it is apparent that a sheet of a mere 15m will move by 15mm in heat. Best-practice installation is to not install a pierced fix sheet in lengths of more than 15m. If the sheet is over 15m, a pierced fix sheet should be end-lapped to allow for thermal cycling.

Concealed fix roof profiles are aesthetically pleasing and functional

The profiles of concealed fix roofs are both aesthetic and functional. When designing a low-sloped roof, a defined rib and a wide, deep pan are needed in order to deliver excellent water run-off at slopes as low as 2 or 3 degrees. The height of the rib is important as it also provides structural strength, acting rather like a mini I-beam.

The design of the clips is also something that needs to be considered as this is the “anchor” that will secure the sheet to the sub-structure and hold it down in high winds. The strength of the clip will be determined by the material that is used as well as the thickness of the clip, which in turn helps to determine the minimum wind-uplift resistance of the system.

Consider the complete roofing system before specification
The roofing system design will define the manner in which the clip locks into the sheet profile. Every rib of the profile should be engaged by the clip to prevent wind cushioning under the unsecured rib, and distorting the sheet off the clip. Generally, the more ribs that are secured with a positive clasp holding it to the purlin, the stronger the wind uplift resistance. If ribs aren’t positively secured, they should be shaped to provide a strong and stable interlock with the clip at every point. A complete system will also offer a high wind load detailing for particular terrain categories or design parameters such as wide overhangs.
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One of the most widely specified systems available in the market is the Saflok™ concealed fix roofing system. The precisely engineered clip and the dimensional consistency of the Saflok profiled sheet provide a robust and reliable roof. Over 10-million square metres of Saflok™ concealed fix roofing products are installed in South Africa alone.

Specifiers and design professionals are encouraged to request full technical details of the system they intend to specify – and request technical support throughout the installation itself. Safintra offers this service free of charge to all customers from the company’s seven branches in South Africa (Johannesburg, Durban, Cape Town, Polokwane, Nelspruit, Port Elizabeth, Bloemfontein). Safintra recently appointed an agent for Namibia, operating from Windhoek. Further operations include Mozambique, Zambia, Malawi, Tanzania, Kenya, Angola, Uganda and others.

Safintra Roofing & Steel
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Website: www.safintra.co.za