Understanding your fasteners

The specification market is characterised by the stricter demands of architects and social housing landlords who often seek windows and doors that offer enhanced security, optimum safety and robust warranties for long-term assurance.

But in the pursuit of achieving a strong, secure window, remember that it will only be as strong as its weakest component – and that could be the fastener.

Despite being only a tiny part of a complete window or door, the fastener can be the difference between success and failure. That’s why it is vital that you and your specifier customers fully understand the differences between fastener properties and ensure that an inferior product has not been specified or, conversely, that you are not running the risk of over-specifying the fasteners, which could impact on the client’s aim to achieve best value and make your tender uncompetitive.

As with friction stays and other hardware, the key issue is the type of metal used to manufacture the fasteners. The baseline material is coated carbon steel – electroplated zinc, plus a clear or yellow passivation layer. Fasteners of this type will need to show evidence of being salt spray tested, showing no corrosion to 240 hours in accordance with BS EN 9227: 2012. This provides a service condition of Grade 4 to BS EN 1670: 2007.

Coated carbon steel fasteners are ideal for certain applications, being easily capable of providing the integrity necessary to achieve a window or door service life of at least 10 years. But, there are two main reasons why specifiers may seek a higher performing product.

Firstly, the location of the properties. If they are situated in a relatively harsh environment, where the local atmosphere contains the elements that have a more corrosive effect on metal hardware, for example coastal regions or industrial areas, coated carbon steel screws are unlikely to be able to equal the service life of the particular system in which they are integrated.

Secondly, the specifier may want to extend the service life of the window or door to 25 years. PVCU and aluminium frames are easily capable of retaining their integrity for this period, so there is a powerful case for specifying all stainless steel metal components to deliver this.

In both cases, the fasteners will need to be stainless steel. Best practice for friction stays is to specify austenitic stainless steel, in grades 304 or 316 depending on the location and intended warranty. These will be salt spray tested to 1,000 hours and have corrosion protection grade 5 under BS EN 1670: 2007.

This enhanced resistance to corrosion makes austenitic stainless steel a pretty clear choice to achieve the higher level of window and door performance – and it should be the only fastener metal specified with aluminium profiles due to the particularly aggressive galvanic reaction that takes place between carbon steel and aluminium.

But austenitic stainless steel is not the only option for countering corrosion. Fasteners manufactured from martensitic stainless steel could also deliver PVC windows or doors with the performance that matches the client’s specification. Martensitic is a hardened type of stainless steel – so fasteners of this type have the advantage of being suitable to drill into steel reinforcement, unlike austenitic which is too soft.

In certain circumstances, martensitic fasteners are ideal but, as with all fastener options, it is important to seek technical advice before choosing a particular route.

One additional material consideration is a bi-metallic fastener which combines the benefits of an austenitic stainless steel shank with a hardened carbon steel drill point. This overcomes the issue of drilling into steel reinforcement, but bi-metallics are significantly more expensive. They may be the best option, but it is definitely worth assessing all the different materials before choosing bi-metallics.

By Andy Holland, Technical and Marketing Manager at Rapierstar