Guide to British Standards

The updated BS 5534 ‘Code of Practice for slating and tiling’ has generally been accepted as the industry’s most important of the last 20 years. Enquiries to Klober so far have certainly reflected a high level of interest, in complete contrast to what we would normally expect with a new British Standard.

We believe there are two reasons for this, the main one being that it affects fixing principles as a whole and secondly it has incorporated new wind load information contained in new Euro codes and the new regulations mean that a greater level of fixing is necessary.

Mortar Fixing

The first key element of the Standard was prompted by the fact that around two thirds of roofing claims to the NHBC under its Buildmark Warranty relate to mortar fixing.

Mortar offers little tensile strength and changes in construction technique have exposed its inflexibility. Failures had shown little sign of reducing in number, so much so that the move toward dry fixing had already been formally supported by the NHBC. Though mortar will of course continue to be used, all ridge and hip tiles must now be mechanically fixed.

The Standard also places particular emphasis on use of the correct material ratios when mixing mortar as site complaints had highlighted poor quality on a regular basis. There are many factors which can affect mortar quality, among them water purity, weather, use of admixtures and cleanliness of the tile surface. The NHBC’s input on failure rates also prompted withdrawal of the mortar test from the Standard and the requirement for mechanical fixing of all ridge and hip tiles.

Dry Fixing

Dry fixing, which has been in common use in Scotland for a number of years and is very much the quicker, more reliable option is considered to be best practice. It is important to follow manufacturer specifications for dry fixing of ridges and hips, particular in relation to such features as the ridge batten.

Klober dry fix kits contain all the necessary mechanical fixings needed to comply with the Standard while products such as Roll-Fix, designed for universal use, can also provide ventilation at the ridge.

As an alternative, the Klober Uni-Dry Ventilating Ridge provides neat, secure, maintenance-free, all-weather mechanical fixing for half-round and angled ridges as well as high- level ventilation. Fixing screws are not supplied by all manufacturers so care should be taken to ensure the correct type and length of screw is used, ensuring too that compatible clamping plates are supplied.

Verges

Dry fix systems vary between manufacturers so it is important to ensure that the specification for each is followed.

Verges must have two fixings while, when fixing natural slates, if using a hook fixing system verge slates should be hooked and nailed. With fibre-cement slates, verge slates should be nailed in addition to a hook or rivet (according to the manufacturer’s recommendations).

For all verges, where the nearest batten fixing is more than 300mm, an additional fixing must be provided.

Single Lap Tile Fixing

One element of the Standard which will impact heavily on contractors is the need to fix every single lap interlocking tile with either a clip or nail. Changes in wind loading calculations were prompted by improved knowledge of weather patterns throughout Europe.

Whether for new build or refurbishment, it is important to nail or clip tile and slate vents which are, after all, lighter than the roof coverings they are fixed with. All perimeter courses should also be mechanically fixed.

Where the new BS 5534 wind zone map and/or a manufacturer specification indicates a building will be subject to high winds, nailing alone may not be sufficient. The new calculation method should always relate to individual buildings to determine the level of fixing required: nailing, clipping or both.

Battens

BS 5534 requirements for battens have not changed though roofing battens should be clearly and indelibly marked with the name of supplier, timber type, origin, size and stamped to confirm compliance with BS 5534.

Accompanying documentation should also contain details of the type of preservative and method of treatment used on the timber.

The simplest way to ensure compliance is to use a supplier with UKAS quality assurance certification.

Membranes

The Standard carries considerable implications for manufacturers as well as contractors, particularly in relation to underlays. Following an increasing number of problems with lightweight membranes ‘ballooning’, a new wind speed map of the UK has been produced and manufacturers will be required to provide information on wrappers to show the zonal limit for each product.

A test for upward deflection has also been established, so labelling will at last provide a measure of comparison for the dozens of products available. Many have the support of BBA certification but this has, in many senses, caused confusion as it has masked variations in product quality.

Among air-open underlays, BRE tests have confirmed that Permo Air, for example, exceeds the minimum wind uplift resistance of 1600 Pa and can be used at a 345mm batten gauge in all zones (used with Permo TR tape for Zones 4-5 instead of the additional fly batten which is otherwise required). It can be used up to Zone 3, which extends as far north as Aberdeen, without it.

Membranes Continued

Underlay laps must be secured by a batten or by proprietary means such as tapes/sealants. If the latter, evidence will be needed from the manufacturer that this has been tested for wind uplift.

An unsupported underlay should provide a drape between rafters or supports sufficient enough to ensure that any water will run away from the batten fixing penetrations at supports or rafters, and allows it to drain to the eaves gutter.

Contact between the membrane and the roof covering should be avoided during and after installation. Klober membranes should be laid with a maximum drape of 15mm.

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