Fasteners as the key to optimised metal building envelope performance

[edit] Selecting fasteners early

Despite playing a crucial role in the performance of a metal building envelope, fasteners are an often overlooked component at the design stage. As the construction industry rises to the challenge of creating a safer and more sustainable built environment, one of the ways we can provide more certainty about the quality and performance of the envelope is to focus on the fastener specification early, and not leave decisions about these tiny components to chance.

Choosing incorrect fasteners can be detrimental to the performance of the envelope. In many cases, however, the issue may not be due to the wrong fastener being used – it could be because a poor quality product has been selected, or that the correct fastener has being incorrectly installed. All these issues can be mitigated with careful specification.

Early consideration of the fasteners, however, is not only about minimising the risks of issues developing. It could actually determine the form that the envelope takes as a whole. In fact, the fastener choice not only influences the aesthetics of the building, but it can even offer a completely different perspective on the proposed envelope design or system choices.

[edit] Types of metals used for fasteners

The majority of metal roofing and cladding sheets, composite systems and sandwich panels used in UK projects are manufactured using steel or aluminium. The recommended fastener material for use with these metals will usually be one of two types of steel:

• Coated carbon steel – commonly used for building envelopes in low-risk environments where the potential for corrosion is low. A service life of approximately 25 years is provided for C1 and C2 environment classifications, but this can be improved upon when using high-grade nylon over-moulded heads.
• Stainless steel – offers enhanced resistance to corrosion, making it suitable for a much wider range of applications. Given their superior corrosion resistance, stainless steel fasteners can be used in roofing and cladding systems with a service life in excess of 40 years depending upon the stainless steel grade and environmental classification. This can also be enhanced with an over-moulded head.

As stainless steel is a relatively soft metal, many of these fasteners incorporate a carbon steel drill point to create a bi-metallic screw. This ensures they can penetrate (self-drill) and form a thread in substrates for ease of use on-site.

In projects where the environmental conditions are particularly aggressive, such as with swimming pools or when there are certain industrial processes, HCR stainless steel, such as 1.4529, may be required. This has higher molybdenum and nitrogen content, making it even more resistant to corrosion.

Best practice for most projects is to specify stainless steel fasteners as these offer greater certainty in terms of resistance to corrosion. Choosing coated carbon steel may be attractive from a cost-saving perspective, but the risk of corrosion developing will always be greater. Hence why stainless steel offers the lowest whole-life cost.

[edit] Maximising air and water-tightness

Given that every fixing point in the metal building envelope penetrates through the outer roof or cladding sheets, when a fastener is installed it must create a reliable seal. Without this, air leakage and water ingress will occur and undermine the performance of the envelope and building as a whole.

A high quality self-drilling fastener will have an integrated washer, typically formed of EPDM bonded to aluminium. This helps to distribute an even clamp load when the fastener is driven into place, ensuring a tight seal to a point where it will compensate for moments when installation cannot be 100% perpendicular.

This type of washer reduces the potential for installation quality to let down the performance of the finished envelope, and it supports the professional installer who is constantly aiming for ‘right-first- time’ installations. This is a big expectation when a metal building envelope project could involve tens of thousands of fixing points.

Best practice is also for the quality of the self-drilling fastener seal to be tested at regular intervals during installation using a seal testing kit – something that leading contractors are increasingly writing into their own quality procedures. This is a relatively simple process that uses a cup to cover the fastener head in-situ, which can then be connected to a pump to withdraw the air, creating a vacuum and allowing the pressure to be measured.

[edit] Unlocking aesthetic potential through colour matching

When self-drilling fasteners feature a head coloured. to match the panels, there is enormous scope to add interesting creative features and achieve aesthetically original metal building envelopes, without compromising performance goals.

Three approaches are generally available to achieve colour-matched fastener heads. A powder coated screw could be used, or a standard self-drilling fastener with appropriately coloured cover caps. Let me advocate a third approach, which is to use self-drilling fasteners with an over-moulded head.

Over-moulded fasteners have the metal head completely encapsulated using engineering-grade nylon, which is composed to include a colour pigment to match the sheets it is being used with. It provides a durable, lasting colour with material characteristics that mean it is unlikely to be damaged during installation in the way that powder coated screws can be.

Providing that a high quality over-moulded fastener is specified, it will hold its colour as robustly as a powder coated fastener, despite early myths that they are prone to UV fading, whilst also enhancing long term performance. This is because the moulding process results in total protection for the screw’s hex head, thus virtually eliminating the risk of corrosion, and the head design also contributes to an enhanced seal.

[edit] Suitability for their structural role

In addition to satisfying the durability, weather tightness and aesthetic functional objectives, fasteners must also be capable of withstanding a range of different loads. This means they are also critical from a structural perspective.

The loads that the envelope fasteners are subjected to include tensile, shear and installation loads, such as the fastener’s ability to resist over driving. In cladding systems, they may also be required to offer resistance to bending and push down and provide clamping. Here, a well-designed fastener has advantages, particularly in terms of the installer being able to achieve the right results on site, as it will give them a clear indication of when a correct clamp is achieved – thus reducing the risk of over-driving.

[edit] The importance of third party approvals

Another consideration when determining the fastener specification is whether the product has third party approvals. Amongst the most important approvals and certification schemes available for fasteners are European Technical Assessments (ETAs) and FM Approvals. Whilst FM Approvals are a relative newcomer to the UK market, this global standard is becoming increasingly sought, given the depth of the assessment process. This examines not just the fastener, but how it works within a complete roof or façade system.

ETAs are another valuable indicator of a fastener’s capability. An ETA provides fastener manufacturers with a voluntary route to CE marking, given that the products are not covered by a harmonised standard (hEN) under the Construction Products Regulation (EU) 305/2011.

This article appears in the AT Journal issue 154 summer 2025 as "Why fasteners hold the key to optimising metal building envelope performance" and was written by Brian Mack, Technical Business Development Manager, EJOT UK.

--CIAT

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