Correct fixing methods
With an increasing number of buildings owners upgrading to fibre cement profiled sheeting, Andrew Brown, Profiled Sheeting Technical Support Manager at Marley Eternit, explains the subtle difference in fixing methods compared to asbestos cement sheets.
"We are seeing an increasing number of building owners take the decision to replace their asbestos cement sheets, and there are now some interesting incentive schemes that definitely make this worthwhile. In many cases the client undertakes a refurbishment simply to gain the benefits offered by fibre cement – lightweight, low cost and robust.
Where customers do decide to upgrade, installers and contractors need to be aware that fixing methods are slightly different between fibre cement and asbestos cement sheets. Although relatively inexpensive, following the universally recommended guidelines for fibre cement profiled sheeting will mean that your client receives a robust and long lasting roof.
The difference in fixing method is inexpensive and can avoid costly remedial work, or even complete replacement of the roof. Essentially, there is only one difference, and that is the number of fasteners per sheet width. Non-asbestos sheets should always be fixed with 2 fasteners per sheet width per purlin (figure 2), whatever the wind loading on the roof. If necessary, resistance to high wind suctions can be enhanced by closing up the purlin centres, or by using a curved diamond washer under the head of the fastener to provide a larger bearing area between the sheet and the fastener.
Where wind suction loadings did not exceed 1.0kN/m² (generally in the more sheltered parts of England and Wales), asbestos cement sheets could be fixed with 2 fasteners at eaves and at end laps, but only one fastener at intermediate purlins where no end laps occur. We do still see fibre cement sheets being incorrectly fixed with a single fastener (figure 1) on the intermediate purlin and this can result in longitudinal cracking after 10-15 years.
Unfortunately this is one of the most common issues we see on site with all manufacturers’ fibre cement profiled sheeting. At wind loadings greater than 1.5kN/m², there was an option to fix asbestos cement sheets with additional fasteners per sheet width. This again is not acceptable for non-asbestos sheets and can also result in longitudinal cracking.
There are other ways the sheets can be incorrectly fixed, which may not immediately show up as a problem, but could result in a material complaint after many years, such as:
• Fasteners in the wrong corrugations.
• Drive screws punched through the sheet instead of pre-drilling the fixing hole.
• Fixing through tight holes, instead of having a 2mm clearance.
• Fasteners too tight
• Fasteners too loose
• Incorrect fastener for the purlin
Other manufacturers are now recommending fixing fibre cement sheets in the first full corrugation on each side of the side lap, which we have always adhered to.
Although there is little difference in the strength and appearance of the sheets, they do have different properties. Fibre cement sheets are much less brittle than asbestos cement sheets, which enable the current strip reinforced sheets to be classified as non-fragile Class C when tested in accordance with the ACR (M) 001:2005 test method.
There are many other advantages of refurbishing roofs using fibre cement profiled sheeting compared to other materials. Clients like the fact that they are vapour permeable, which significantly reduces condensation and can prevent damage to the building and contents. Fibre cement profiled sheeting also has a high level of corrosion resistance, which can increase life expectancy, along with preventing noise pollution externally and internally.
If your customer is looking to reduce their carbon footprint, fibre cement is a good choice too, as UK manufactured fibre cement profiled sheeting can achieve an A+ rating in the BRE (Building Research Establishment) Green Guide to Specification, based on generic rating for UK produced profiled air cured fibre cement sheeting (outer skin) - (Element Ref: 1012550002).
Following these fixing recommendations can help reduce many of the common issues associated with incorrect installation, helping your client take advantages of the benefits offered by fibre cement profiled sheeting. We are therefore encouraging all contractors to follow these recommendations – they are simple, inexpensive and might just save you and your company a future call back to site."