FAQ's

Our experts answer common questions. From understanding which fasteners can be used for which composite panels to understanding electrolytic corrosion.

Question 1: Can fasteners and cladding envelope sealants be classed as “Replaceable”?
Question 2: Can fasteners and cladding envelope sealants be classed as “Maintainable”?
Question 3: If I specify austenitic stainless steel fasteners do I run the risk of electrolytic corrosion of the galvanised steel deck or purlin?
Question 4: I regularly specify composite panels for my roofing projects. How can I be sure that the long fasteners can accommodate the expansion, contraction and deflection of such panels and also maintain a weatherseal under concentrated foot loads around the fastener?
Question 5: On single ply roof systems over insulation on metal decks, I often specify the inclusion of a vapour control layer (VCL). Do fasteners penetrating this VCL and deck eliminate the effectiveness of the VCL and should the system be adhered as an alternative?
Question 6: I am working on a new project and wish to specify the fastener material. How do I determine what is the most appropriate fastener material - carbon or stainless?
Question 7: My metal roof and wall system is fixed to a steel structure. I am concerned that the fastener penetrations within the building will appear unsightly as well as being a health and safety risk in some areas. Can I instruct my contractor to cut off the visible fastener penetrations?
Question 8: I recognise that on my project, which is at pre-tender stage, carbon steel fasteners would not be suitable and that it will be necessary for me to specify a more durable fastener material, such as "stainless”. As “stainless” is a generic term covering many differing levels of performance, how should I specify “stainless” fasteners to ensure my client receives products with durability performance appropriate to the demands on exposed roofing and cladding applications?
Question 9: I am preparing a specification for a swimming pool project and I am aware that there could be problems with certain grades of stainless when used as components within pool halls. What advice can you give me regarding roofing fasteners?

Question 1: Can fasteners and cladding envelope sealants be classed as “Replaceable”?

A: Non-exposed fasteners and sealing products, both weather and air seals, can often only be accessed by removal of the external weathering sheet. In some cases removal of the weathering sheet in order to access concealed fasteners, within its required design life, may necessitate that it be totally renewed.

If fasteners and seals were to degrade within the design life of the system, then
installing new fasteners, both exposed and non-exposed, into their existing location would often result in greatly reduced pullout values and therefore present a structural risk (unless a purpose-designed “repair” fastener were used).
As a further example, insulated panel fasteners have enlarged threads under the head to “support” the outer skin and maintain a weatherseal (Downloads Designers Guide section 3.3a).
If the fastener prematurely corrodes, then the resulting leakage may damage the sheet around the hole such that simply installing a new fastener may not provide the required levels of sheet support.

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Question 2: Can fasteners and cladding envelope sealants be classed as “Maintainable”?

A: As noted above, non-exposed fasteners and seals can only be accessed by removal of the external weathering sheet. Exposed fasteners may, in theory, have their visible parts “recoated” but this should not be regarded as acceptable at the design stage of a project. Furthermore, applying a new corrosion protective coating to the visible head, providing an appropriate pre-treatment can be carried out, as well as being, in most cases, aesthetically unacceptable, would not reinstate corrosion protection to the concealed shank which may also be suffering from corrosion.

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Question 3: If I specify austenitic stainless steel fasteners do I run the risk of electrolytic corrosion of the galvanised steel deck or purlin?

A: No, this would not be considered a problem with austenitic stainless steel fasteners for roofing and cladding - as a general rule, the fastener should always be of a higher corrosion resistance (more noble) than the material it is fastening into. This makes the fastener the cathode and the purlin or deck the anode.
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Question 4: I regularly specify composite panels for my roofing projects. How can I be sure that the long fasteners can accommodate the expansion, contraction and deflection of such panels and also maintain a weatherseal under concentrated foot loads around the fastener?

A: As fasteners for composite panels tend to be long, they are subjected to different loading criteria than those for single skin or liner panel systems where the fastener clamps the sheet directly to the support.

As the head of the fastener is “remote” from the purlin by the depth of the panel core, the fasteners are subjected to a bending load rather than a direct shear load.

This bending load is repetitive and results from panel deflections as well as expansion and contraction. This load is also more severe where the support is more rigid i.e. Hot rolled purlins.

These loads are well recognised and there is a European (ECCS) standard which defines a repetitive bending test for composite panel fasteners. This accelerated test repetitively bends the fastener to varying degrees to a total of 22100 cycles.

To pass the test, the fastener must not fracture and must retain at least 80% of its original tensile and pullout values.

All our composite fasteners were designed to comply with these tests prior to introduction into the market place.

To ensure the weatherseal under the foot loads that you referred to, SFS intec composite panel fasteners have enlarged threads, either 7mm or 8mm diameter, under the washer. These threads support the outer skin of the panel and can withstand a pushdown load equivalent to a mans dynamic weight. This is similar to the concentrated load test for profiled sheeting as defined in BS5427:1996.

As an additional means of ensuring this load is accommodated, even if the fastener is overdriven, the SFS intec range of composite panel fasteners has a thread free section immediately behind the washer. If the fasteners were fully threaded up to the head, any overdriving would partially / totally strip the top thread from the sheet giving reduced support and leading to potential leakage and failure.
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Question 5: On single ply roof systems over insulation on metal decks, I often specify the inclusion of a vapour control layer (VCL). Do fasteners penetrating this VCL and deck eliminate the effectiveness of the VCL and should the system be adhered as an alternative?

A: Adhering to the crowns of a metal deck may, in many instances, particularly on exposed roofs and around the perimeter zones, not provide the sufficient resistance to the wind loads and therefore additional performance as offered by mechanical fasteners must be considered. With regards the subject of air/vapour diffusion through the fastener penetration in the deck and VCL, extensive tests and studies were carried out by Heinz Wieland, an independent Swiss engineer, in conjunction with Icopal Monarflex Ltd. And it was concluded that, with the design of SFS intec isofast and when taking reasonably achievable site practice into account, fastener penetrations, even in high humidity buildings, results in insignificant reduction in resistance to air/vapour diffusion - too small to have a negative effect on the overall performance of the roof.
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Question 6: I am working on a new project and wish to specify the fastener material. How do I determine what is the most appropriate fastener material - carbon or stainless?

A: There are a number of factors you need to consider, namely:

1) The sheet material
2) The clients design life & warranty requirements
3) The external environment
4) The internal environment - building use.

In Section 2 of the SFS intec publication entitled “A designer’s guide to the specifying warranted austenitic stainless steel fasteners and sealants” each of these factors are individually explained. The Table on page 19 gives the concluding guidance on the functional life expectancy of carbon and austenitic stainless fasteners. In summary, carbon steel may provide a maximum functional life expectancy of up to 25 years in “non- aggressive” internal and external environments, when used with carbon steel and GRP/PVC/fibre cement type sheeting. Austenitic stainless steel would be considered as providing a functional life expectancy exceeding 30 years when used with all popular sheet materials noted in the table in virtually all environmental conditions. Warranties for carbon steel are normally a maximum of 10 years, whereas austenitic stainless may be warranted up to 25 years.

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Question 7: My metal roof and wall system is fixed to a steel structure. I am concerned that the fastener penetrations within the building will appear unsightly as well as being a health and safety risk in some areas. Can I instruct my contractor to cut off the visible fastener penetrations?

A: It is our general recommendation that fasteners, once correctly installed, should not be tampered with in any way that may detrimentally affect their performance. Cropping off fasteners, for either safety or aesthetic reasons, may result in reduced pullout performance depending on the method used. The thinner the support member, the worse this effect may be. Where it is desirable to reduce the penetration length of the fastener, then this could be considered providing the steel support is at least 6mm. Care must be taken not to apply a “lever” force to the fastener and therefore bolt croppers or grinding would be the most practical method. Where appropriate, corrosion protection should be reinstated. An alternative to cropping would be to apply a plastic “screw tip cap”, usually white, to the visible part of the protruding fastener.
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Question 8: I recognise that on my project, which is at pre-tender stage, carbon steel fasteners would not be suitable and that it will be necessary for me to specify a more durable fastener material, such as "stainless”. As “stainless” is a generic term covering many differing levels of performance, how should I specify “stainless” fasteners to ensure my client receives products with durability performance appropriate to the demands on exposed roofing and cladding applications?

A: The British Standard for profile sheeting is BS5427:Part1:1996 entitled “Code of practice for the use of profiled sheet for roof and wall cladding on buildings - Part 1. Design”. Under the section dealing with fastener material, clause 2.8.3. states “Stainless steel for fasteners should be of grade 304, 305 or 316 to BS1449” ie 300 series. These grades are all within the Austenitic type of stainless. The other type of stainless from which threaded fasteners could be made is referred to as Martensitic. These include the 400 series stainless grades. Martensitics are not considered suitable for roofing and cladding fasteners, and are therefore not included within BS5427, due to their known poor corrosion resistance.

To enable austenitic stainless fasteners to drill through steel purlins they have a carbon steel drilling point, which would be heat-treated and hardened. This carbon steel element must not extend along the shank to such an extent that it is within the purlin when installed. A concise and safe specification for “stainless” fasteners may therefore read;
Fasteners must be manufactured from austenitic stainless steel in compliance with BS5427:Part1:1996. When installed, all threads, both within and above the purlin, must be austenitic stainless.
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Question 9: I am preparing a specification for a swimming pool project and I am aware that there could be problems with certain grades of stainless when used as components within pool halls. What advice can you give me regarding roofing fasteners?

A: It is widely documented within the Nickel Development Institute publication entitled “Stainless steel in swimming pool buildings” and other publications that some grades of stainless steel, including austenitic, are susceptible to chlorine induced stress corrosion. This can occur if the stainless steel is subject to prolonged condensation in the presence of chlorine and acids (low pH values). This type of corrosion is not visible unlike normal “red rusting” associated with carbon steel.

With regards roofing and cladding fasteners, the total system is usually designed to prevent condensation forming within the system or on the fasteners.

Important factors that should therefore be considered at the design stage include the pool and air temperatures and the relative humidity. To avoid condensation these are normally designed in the range 25oC-30oC and 50%-65% respectively. It is also vital that these environmental conditions are controlled and maintained by the management of air handling systems and pool control.

The NiDI document advises that stainless steels which are susceptible to stress corrosion must not be used for components or assemblies which are classed as “safety critical” i.e. those which would, in the event of failure, result in a major risk of injury to people within the building. Handrails, suspended ceiling hangers and staircases typically fall within this “safety critical” category.

We would not regard our 304 grade austenitic stainless steel fasteners which we would recommend to secure the roofing system in an over-purlin application as safety critical. Austenitic stainless steel fasteners have been used for nearly 40 years in swimming pool roofing applications and we are not aware of any failures due to stress corrosion. If you so require, we can provide references for our products from other architects and engineers who specialise in the leisure industry. BS5427:1996 also states that, where stainless steel fasteners are to be used, the stainless should be of grades 304, 305, or 316 i.e. austenitic.

The only other readily available fastener material is coated carbon steel. We would not consider this appropriate because, in such an aggressive environment, the fasteners would corrode very rapidly. The rate of corrosion would also be accelerated where the fasteners coating is inevitably damaged whilst passing through the metal sheet and steel purlin (where applicable).

The area of risk is often within the system, therefore not visible, and therefore post “painting” after installation would not address the corrosion problem which carbon steel would suffer.

300 series stainless fasteners provide the best overall solution.

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