How does BS5427 affect fixings and fasteners?

BS 5427:2016 + A1:2017 (Code of practice for the use of profiled sheet for roof and wall cladding on buildings) forms part of the British Standards. While not enforced by law, British Standards are recognised as best practice and may be referenced in any legal case resulting from an accident or building failure. As such, the recommendations they contain should be followed.

We have selected some of the basic points from the standard which relate to fasteners for the building envelope. The full standard contains much more detail.

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How does BS 5427 affect fixings and fasteners?

BS 5427 references fasteners in the following sections:

5.9 Fixings and fasteners

5.9.1 General

Primary and secondary fixings should be selected and positioned so that they can secure the cladding system against dead, imposed and wind loads in service, maintain water tightness, are durable for the service life of the cladding and, in certain types, accommodate some thermal movement of the cladding.

The design load of the fixings should not be less than:

a) the characteristic strength derived from C.5.6.3, Annex E (E.6), with safety factors in accordance with those in Annex C; and

b) level of deformation at which there would be a risk of leakage.

NOTE 1 The strength of fixings can be determined by the method of test and acceptance levels as specified in Annex E.

NOTE 2 The strength of a fixing is influenced by the combined behaviour under load of the fastener, washer and connected materials, and not merely by the strength of the fastener alone.

In the case of fasteners of the self-tapping or self-drilling type the thickness and strength of the material in which the screw thread is formed should be suitable for the particular fastener and be in accordance with the fastener manufacturer’s recommendations for use under practical site conditions.

NOTE 3 Fastener manufacturers define the range of types, thicknesses and material strengths that each of their self-drillers is suitable for and also, for self-tappers, the required predrill hole diameter.

For a given fastener size and type applied to flexible sheeting (e.g. metal or plastics), the strength of crown fixings is usually less than trough fixings, due to limited buckling resistance of the profiled sheeting under uplift load; large washers or saddle washers of adequate size and stiffness or sheet to support fasteners should be used to improve the strength of such fixings if required.

COMMENTARY ON 5.9.2

Proprietary fasteners have been developed, particularly self-drilling screws, rivets, washers and colour heads, to meet the fixing requirements of single layer and multi-layer cladding systems incorporating components which might include a combination of thermal and acoustic insulation, condensation and air control layers, spacing systems and internal linings.

When correctly installed, self-tapping screws and self-drilling screws should have hardness to form threads and, in the case of self-drilling screws, drill through the material and thicknesses of the profiled sheeting and structural element to be connected without stripping the threads in the fasteners or connected element.

Fastener and cladding manufacturers should be consulted on the choice of fasteners for the cladding design and the suitability of fasteners should be evaluated by examining the available performance data.

NOTE 1 In general, fasteners cannot be replaced during the life of the building as part of any routine maintenance.

NOTE 2 Examples of fasteners are illustrated in Figure 4. Annex E gives methods for determining the strength of fasteners and fixings.

The fastener material should be able to resist corrosion for a period comparable to the functional life of the sheeting, including any over-painting in the intended environment of use.

What are the implications of BS 5427 on the specification and use of fasteners on the building envelope?

Fasteners play an essential role in joining structural systems together so to meet the requirements of BS 5427 they must:

• Be capable of withstanding the forces placed upon them to ensure the structural security of the building.

• Be durable for the lifetime of the building – for example, the fastener material should be able to resist corrosion for a period comparable to the functional life of the sheeting or cladding being fixed.

• Ensure the weathertightness of the structure throughout the building’s lifetime.

Remember, incorrect specification or the substitution of quality fasteners with cheaper alternatives increases the risk of the envelope or its sub-systems failing. If the fasteners used do not comply with the guidance in BS 5427, you could be liable for repairs and compensation claims. The Skills Hub has lots of information about how to correctly specify fasteners, including:

How do I select the correct self-drilling fastener?

What are fastener performance values?

Alternatively, speak to your chosen fastener manufacturer for advice and support about the correct fasteners for the application and conditions.