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Winds Of Change..!

Some 200,000 buildings are damaged by high winds in the UK every year, costing the British economy up to £800+ million.

High winds and storm damage are becoming commonplace with gusts in Scotland and Northern Ireland reaching over 100mph causing severe damage to roofs.
With a predicted increase in the frequency and severity of storms in the UK and Ireland, and the increase in wind speeds it is now more important than ever to recognise the part that roofing underlays play in preventing tiles or slate being dislodged from roofs.

Wind uplift is one of the main causes of roof covering failure. Wind blowing over the roof causes pressure differentials and where negative pressure or suction occurs, the roof underlays play a crucial role in acting as a primary line of defence.

 

 
 
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The Role of Roofing Underlays..

In unsupported applications, some two thirds of the total wind load has to be absorbed by the underlay, the remaining one third by the correctly fixed roof covering. (BS 5534: 2003).

BS 747 Type 1F and 5U re-enforced bitumen have been traditional roofing underlays for this country for decades and have shown by site experience and by testing, to resist the worst wind uplift conditions expected in the UK and Ireland without extending unduly under load.

However the disadvantages of the BS 747 underlay types are well documented including weight and usability. If an underlay has too much inherent elongation or extensibility under load it will deflect upwards in such a degree that it comes into contact with the underside of the roof covering and can dislodge the slates and tiles.

Underlays that are air permeable are by definition unable to resist all of the wind load pressure. Whatever wind load pressure that escapes through the underlay will be transferred onto the roof covering requiring higher frequency of fixing of the slates or tiles.

With the trend towards vapour permeable (breather or type LR) underlays and the modern lighter weight impermeable, (non-breathable or type HR) alternatives to 1F and 5U, the issue of wind uplift has been largely overlooked
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