Building guidelines can be long and confusing but they're worthwhile, says Peter Mayer
The weather is getting worse, so we are told. Evidence from weather records is compelling: there is a gradual increase in the annual number of severe storms, in recent years individual storms have been more severe and about 33 tornadoes are reported each year.

Roof coverings are particularly vulnerable to wind damage. After 12 years of exposure to design and construction practice for insurance purposes, our company, Building Performance Group, has built up a great deal of inside knowledge about the causes of building failures. Feedback from Building LifePlans insurance shows that many failures result from guidance in published codes and standards not being implemented.

In short, most failures – apart from being costly – are avoidable.

The problem
The faster the wind blows the more it produces areas of suction when it is deflected and accelerated over the roof of a building. The degree of suction increases at edges – verges, hips and eaves – and at low roof pitches; a roof at a 17.5º pitch is more vulnerable than one at 30º. Flat roofs are notoriously prone to wind damage.

The local action of wind generates suction forces strong enough to dislodge slates, tiles and even sheet metal coverings.

The storm of October 1987 caused an estimated £1.9bn in damage, most of which could have been avoided if protection had been provided in accord with modern good practice.

There are well-defined solutions to minimise the risk of wind damage to roofs, but unfortunately they are all too often buried in the detail of good practice guidance, British and European standards and manufacturers' information.

What to look out for on drawings and in specifications
Designers should ensure that explicit fixing specifications are clearly defined for the type, size and "lap" of roof covering, the pitch of roof and the exposure of the site.

The "lap" is the amount each tile or slate covers the one below. This should be indicated on drawings or specification, and be suited to the exposure to avoid uncertainty on site.

Resist the temptation to simply quote a British Standard – the Code of Practice for Tiling and Slating is 84 pages long and only parts of it will be applicable to your roof. For instance:

  • large slates (500 mm × 250 mm and bigger) should be centre-nailed, not head-nailed, especially in exposed locations. Nailing at the top of the slate – the "head" – increases leverage, which means the slates are more readily lifted by the wind

  • the size and type of nails to be used should be stated

  • the nailing frequency should be stated. Typically, the end tile or slate in every course at verges or abutments, at each side of valleys or hips should always be nailed, bedded in mortar or clipped. Similarly, every eave and ridge course (two courses for plain tiles), and all types of ridge and hip tile should be nailed.

    What to look out for on-site
    Roofers need to realise the design details as above, but they also need to consider that:

  • thick mortar bedding – for ridges over deeply profiled tiles, for example – should be packed with small pieces of tile ("slips" or "gallets") to reduce the volume of mortar, and so minimise the risk of shrinkage, cracking and displacement

  • hip irons should always be provided to provide restraint to the end of hip tiles

  • care must be taken in the selection and setting out of slated and tiled roof coverings to avoid small, poorly-fixed (and sometimes unfixed) cut pieces at junctions

  • one-and-a-half width slates and tiles must be used at verges and abutments.