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Clay and Slate Logol (New)

BS5534:2014+A2:2018 revision of calculation method – Air Permeable underlays – Fixing Specifications

We would like to draw your attention to the fact that BS 5534 has once again been revised and a new version was published in February of this year.

One major change as far as fixing specifications are concerned is that there is now a requirement to take account of the Air Permeability of certain underlays.

Underlay provides additional resistance to uplift and the benefit is taken into account within the calculations we provide.  However, some more modern breather membranes are air permeable and with these products the contribution to the resistance is reduced.  Air Permeable underlays are defined as having not less than 20 m3/m2.h air permeability, measured at 50 Pa.  Examples of air permeable underlays are Proctor Roofshield and Klober Permo Air.

The revised calculations required to establish the correct fixing frequency and method make a significant difference to the uplift.

The result of this change often will mean that for example if nailing is deemed to be acceptable with non-permeable underlays this might not be the case with air permeable underlay, the consequence of which might be that a clip would be required instead of a nail.

We will be making suitable notes on our future fixing specifications, but we would urge you to advise us which underlay is to used on your projects on any fixing specification requests in order that this can be taken into account at the earliest time.

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