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Theme: roofing

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A revised roofing code of practice, including guidelines on a wider range of products, new knowledge on wind and rain resistance, safety recommendations and international harmonisation, is set to make a large impact on the specification and design worlds

The recently revised British Standard BS 5534 code of practice for slating and tiling (including shingles) underpins most traditional forms of pitched roofing in the UK, and has implications for all those involved in specification, design or installation of slates, tiles and shingles.

The need for the latest changes may be summarised as follows:

Inclusion of a wider range of products to meet requirements of the Construction Products (Amendment) Regulations 1994 (thereby recognising the requirements of The Construction Products Directive - 89/106/EEC - for materials and products available throughout Europe).

Revised design requirements that reflect new knowledge and experience of wind and rain resistance of tiles and slates.

New calculation method for head- and side-laps in slating (revise the methodology of calculating head and side-laps).

New guidance on the use of hook fixing of slates (to define material specification and specific fixing advice for the use of hooks in slating).

New recommendations on species, sizes and grading of timber battens (reflecting the requirement to specify quality timber battens to improve the safety of roof operatives and provide a more robust groundwork for tiles and slates).

New annexes to give guidance on the use of insulation placed at the rafter line, reinforced underlays and the testing of nail hooks for slates (supportive guidance, calculations and test methods).

Closer harmonisation with international and European codes (align UK practice with similar international and European practice).

Give more accurate calculations for wind loads and fixings (to provide a more coordinated approach to calculation procedures).

The following gives a summary of some of the more important changes to the sections and clauses within the new standard.

Materials, fittings and accessories Great emphasis is placed on drawing the reader's attention to the use of proprietary products, with warnings concerning their use without reference to evidence of satisfactory performance and 'fitness for purpose' - a term that is well documented in the Building Regulations.

The foreword to the standard states: 'Where reference is made to proprietary products and to manufacturers' recommendations, the specifier needs to obtain evidence that they have been proven by relevant experience or relevant test method data based on methods of use in the UK climate to be fit for purpose.' Designers and specifiers are advised to consider using roofing products that are supported by recognised UKAS (United Kindgom Accreditation Service)-accredited third-party assessment.

Underlays With all the new underlay materials entering the market, it is important that clear performance criteria are referenced.

In addition to meeting the normal requirements of providing a barrier to minimise wind uplift load on the roof covering, providing a secondary barrier to the ingress of wind-driven snow and dust, and transporting any moisture in the batten space to the roof drainage system, the underlay must also be capable of providing a temporary roof covering for no more than three months.

Underlays should also be capable of withstanding the effects of 'tenting' - applicable to certain materials that allow the transfer of external surface moisture through the material if the internal surface is in contact with other components. The effects of shrinkage should not enlarge fixing penetrations.

Flexible underlays are referenced in a new Annex A, which has been specifically included, due to the impending replacement of BS 747 with EN 13859-1, which does not include specifications for Type 1F and 5U underlays.

Tiling battens Greater emphasis on health and safety in roof work has driven changes to the standard, with the emphasis on checking the type and quality of timber battens offered by the supplier, using a properly defined grading and marking regime. The standard makes it clear that battens are expected to resist the loadings on the completed slate or tile roof, but may not necessarily provide a safe place of work.

BS 5534:2003 prescribes the minimum sizes and tolerances, and the species to be used. It also provides a detailed list of permissible characteristics and defects, such as knots, wane, resin pockets, etc, which battens and counter battens must not exceed.

The standard acknowledges the need to specify the grading and quality of battens to bring them in line with other small-sized sections that are stress-graded. This has arisen because most tiling battens are smaller than the minimum size quoted in BS 4978 (for the stress grading of small-sized timber sections 35 x 60mm), so this does not apply to the grading of battens, unless they are more than 72mm in depth. A Design Method is also included for larger sizes of timber over 50 x 25mm section and for rafter spans over 600mm.

The choice of timber battens used on roofs must now meet the grading rules set down in BS 5534.

Recommended species These are as follows:

a) Imported (redwood, whitewood, sprucepine fir, southern pine); and b) British-grown (larch, British pine, British spruce).

It is acknowledged that best-quality imported species can be 17 per cent stronger than home-grown timber, although care still needs to be exercised in respect of the quality and grading rules.

Suppliers are required to identify and mark every length of batten claimed to comply with BS 5534 by stating the supplier's name, the origin of the timber, the grading, the basic size and the preservative treatment if used.

Fixings The new revision brings into line full references with new European standards applying to steel nails and other fixings.

The specification for the dimensions of hooks and rivets for slates has been updated, with the drive-in slate hook being recommended in preference to the wrap-around batten hook. The shank may be either straight or crimped, but the latter may reduce the capillary rise of water between the joints of the slates. Hooks should not be used at pitches of less than 25°, with crimped hooks used below 30°.

One important change to the fixings of natural slates is that the head diameter of the nails must not be less than 10mm.

Mortar The standard has been revised to reflect the NFRC (National Fenestration Rating Council) technical bulletin 27, dealing with mortar, and now recommends five alternative mixes:

cement/sand - 1:3 (type S);

cement/sand - 1:4 (type G);

adapted site mortar/cement - 8:1;

cement/hydraulic lime/sand - 1:1:6 (type G or S);

cement/hydrated lime/sand - 1:1:6 (type G or S).

If contractors have any doubt as to the correct specification, they can test the mix using the mortar tensile-adhesion test, as recommended in Annex J.

Weathertightness The recommendations have been revised to reflect new knowledge and experience gained on rain- and wind-load resistance.

The categories of exposure to driving rain remain largely the same, although a small area around Carlisle has been redesignated from severe to moderate, as it is in the lee of the Lake District mountains.

Slate head- and side-laps This topic has created some controversy within the industry, with a new calculation procedure for establishing the minimum head- and side-laps for double-lap slates, replacing the old angle of creep method.

The new calculations are based on the combined use of a 'C' factor for driving-rain exposure to calculate the head-lap of the slate for a given roof pitch, with an 'E' factor applied to calculate the required side-lap.

The length of the slate should never be less than three times the head-lap.

The nature of single-sized double-lap slating ensures that a notional half-width of slate overlaps the slate below. The suitability of any width of slate for a given pitch must be determined so that it provides a side-lap not less than the safe minimum and in relation to the position of the nail holes, where there is greatest risk of water ingress.

The old angle-of-creep method was based on a diagonal line from the top of the perpend to the nail hole on the covered slate.

This controls the width of the area of sidelap between the parallel lines, but in effect does not work effectively for all size/pitch combinations.

The latest version of BS 5534 presents the old angle-of-creep calculations in a modified way, which avoids the trigonometry, and presents them as a series of multipliers known as the 'E' factor.

These 'E' factors are used to multiply the margin (batten gauge) to give the minimum side-lap. One anomaly of these factors is that they can give an incorrect progression for small slates at low pitches, with a direct result of the side-lap being related to the margin.

Note that for centre-nailed slates the nail holes should be positioned 18-30mm from the side edge of the slate.

Minimum laps for double-lap slates The new calculations mean that the minimum recommended head-laps for the slate sizes commonly used at low pitches in exposed areas have been increased by as much as 23mm, thereby increasing the number of slates required on the roof.

Alternatively, longer and/or wider slates should be used to achieve both the head-lap and side-lap requirements.

Wind uplift The following summarises the main changes to the calculations covering winduplift resistance on tiles, slates and roof groundwork:

Improved layout of wind uplift calculations and tables.

'R' values of uplift pressure reduction revised to reflect revision to BS 6399.

'The new calculations mean that the minimum head-laps have been increased by as much as 23mm, thereby increasing the number of slates required on the roof or longer and/or wider slates should be used' l Revised table of nail penetrations for fixing into timber against wind uplift.

New guidance on fixing of underlays.

Guidance on aircraft vortices revised.

New guidance on nail- and hook-fixed slates.

Double-lap slate calculations A new force diagram for double-lap slates has been added, together with wind-uplift calculations for hook-fixed slates.

Annex B (insulation placed at rafter line) Annex B gives much-needed guidance on the design and installation of pitched roofs where insulation is placed at rafter line.

It also provides minimum ventilation requirements when using high vapourresistance and low vapour-resistance underlays (vapour permeable).

A calculation method is also provided to check the bending stress and deflection of fixings that pass down through the counterbatten and insulation into the rafter.

Annex D (preservative treatment) This entirely new annex gives guidance on the use of timber preservatives to reflect recent changes governing their use and health and safety issues:

Revised to reflect new guidance on use of preservatives.

Battens are Hazard Class 1 (no risk of wetting in service).

Hazard Class 2 to BS 8417 or BS EN 599-1.

Warning about CCA (chromate copper arsenate) in contact with metals.

No CCA preservatives in domestic or residential construction after 30 June 2004.

Implications In summary, the latest revision to BS 5534 means the industry will now benefit from:

improved design guidance to resist winddriven rain;

improvement in the security of tiles and slates;

better design information on sub-roof systems;

reduced risk of falls and injury on site;

third-party accreditation of proprietary products;

an easier method to determine tile-fixing specifications;

robust roof design details (linked to Approved Documents C and L);

urgent revision of BS 8000-6: 'Workmanship'; and l better-trained peratives.

John Dodd is general technical manager at Marley Roofing

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