Although it is widely admired for its aesthetic qualities, slate is a highly variable product that can pose plenty of problems for the specifier. We explain how to minimise them
1 - Sourcing and quality checking
Slate is a natural material and, although this offers unique aesthetic quality and long durability, it also poses problems for the specifier. Not all quarries (mostly overseas) can be relied upon to provide serviceable products. Slates with iron pyrites or bedding plane weaknesses are common and these imperfections are not identified in most quality checks.

Careful sourcing and checking of the batches to be used prior to installation is strongly recommended. Some slates seem competitively priced and it is a great temptation to use them. However, ask for full batch testing prior to ordering.

3 - Samples
Additionally there can be significant aesthetic variation. Before choosing a slate, obtain a minimum of six samples to be clear of the variations that can occur. BS EN 12326-2 and BS 680-2 contain import specification information and it is wise to use this when selecting a product, but specifiers should understand the limitations of the standards.

4 - Quality control
Seek the advice of a specialist during the specification process to minimise potential problems in quality. Be aware that slate is a natural material and will vary dramatically between sources and in some cases even from different areas of the same quarry.

5 - Pitch
The proposed pitch should always be confirmed with the manufacturer to ensure that it is suitable for the product in question. This will be governed by the size of the slate, the lap and exposure of the site. Generally the minimum pitch for a slate roof is 20° from the horizontal. Bear in mind that low pitches on exposed sites may require additional fixings.

6 - Details
It is important to provide detailed drawings for features and junctions to ensure that the builder has sufficient information to complete the task to the architect's satisfaction. Guidance can be sought from the roofing subcontractor who may offer a design service. Details should always have a clear reference to specification clauses. Eaves, ridge, hip and valley details should all be provided.

7 - Battens and underlay
BS 5534-1 is the code of practice for slating and tiling. It recommends batten sizes of 50 x 25 mm for support centres of between 450 and 600 mm. These items must be added to the specification and, where possible, duplicated on the detailed drawings to avoid confusion. Any variation on this should be agreed with the manufacturer. Advice on the underlay to be used as well as information regarding standard slate sizes and minimum head laps should be obtained.

8 - Condensation control and ventilation
With the advent of vapour-permeable flexible underlays, a range of possible moisture control methods is now available. At the time of writing there are a range of views as to the appropriate use of these new materials (see "The rules", page 17).

The specifier should carefully consider the application including the building type and use, and the roof build-up and void shape. All of these will have an influence on the amount of moisture that will be generated and the way in which it can be controlled beneath the slates.

The specifier should always ensure that condensation on the underside of the slates does not occur. In most circumstances ventilation through the batten space should help to minimise this.

9 - Health and safety
Slates are mainly used for roofing, which implies working at a high level. It is the designer's responsibility to minimise the potential of risk to the installer. A method statement should be provided for the health and safety file to ensure the contractor follows the safety principles. Issues covering safe access and safe working on roofs must be considered from the first stages of design. Consideration must be given to the maintenance of a completed roof in the future and the provision of a man-safe system may be required.

Natural slate itself does not present a significant health risk. However, protective face-wear and gloves should be used when cutting slates to avoid the dangers of sharp edges and the inhalation of slate dust. Care should be taken in the fixing of tiles to prevent falling from or through the structure.

10 - Guarantee
Advice can be sought from the National Federation of Roofing Contractors with regards to a guarantee. Generally trade members will offer a 10-year triple insurance scheme covering materials, workmanship and solvency of the roofing contractor.)

Reference documents

The following documents provide further guidance:
  • BRE Defect Action Sheets

  • BS EN 12326-2: 2000 Slates for roofs method of test

  • BS 680-2: 1971 Specification for roofing slates

  • BS 5534: 2003 Code of practice for slating and tiling (including shingles)

  • BS 5534-1: 1997 Code of practice for slating and tiling (including shingles): design

  • BS 8000-6: 1990 Workmanship on building sites: code of practice for slating and tiling of roofs and claddings

  • Lead Sheet Association Rolled lead sheet – the complete manual 2003

  • National Federation of Roofing Contractors (NFRC), various documents

  • Building Regulations (or national equivalents

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