Slate Selection Best Practice

Natural slate has many qualities, including strength, durability, low carbon footprint and fire-resistance. For many architects it is the organic nature of the material that is central to its appeal, particularly with regards to colour and textural variation. However, unlike factory-made and other processed roofing and cladding products, roofing slate can vary greatly in terms of material composition and quality. While two products may appear visually similar, inherent weaknesses in one could lead to future performance issues, such as discolouration, delamination and even failure. Mark Boardman, Technical Manager at SIGA Natural Slate, examines the key areas architects should consider when it comes to specifying roofing slate that is fit for purpose.

Testing criteria
The principal standard for evaluating slate is BS EN 12326, Slate and stone for discontinuous roofing and external cladding. Architects should be aware that this is not a simple pass or fail test, rather, it is an indicator of quality based on a series of assessments. The most important of these cover sulphur dioxide resistance (S), water absorption (W) and thermal cycle (T).

The sulphur dioxide test examines the slate’s ability to resist atmospheric pollutants. Samples are subjected to extremely acidic and humid conditions over a number of cycles to see if they soften. Most products, including SIGA slates, will achieve the highest standard: S1 (no change). S2 and S3 slates may be supplied by others, but specifiers should be cautious.

Architects should only specify a slate that absorbs 0.6 per cent or less of its mass in water. Excessive water absorption may make the slate susceptible to frost damage, resulting in delamination. All SIGA slates are classified W1 (≤ 0,6%), with most products attaining 0.3% water absorption.

The thermal cycle test examines damaging inclusions, in particular pyrite, which can cause rust or oxidisation within the slate. This can result in surface staining, pitting, and in the most serious cases failure. Both T1 (no changes in appearance) and T2 (some discolouration, including runs, but without structural changes) classifications are permitted on UK building projects. It is worth mentioning that the NHBC only approves T1 slates, and in light of this decision, SIGA Natural Slate will only supply this category of T1 slate, irrespective of project type.

Tracing the source
Provenance is crucial when it comes to specifying slate. Quarries with a good pedigree will test their stone comprehensively at least once a year to ensure quality standards. Architects should ask suppliers which quarry they source from, how long have they supplied it into the UK market, what volume of Slate they produce and what quality control procedures they work to. They should also ask to see the supplier’s slates on completed building projects. For example, the Gallegas quarry in Spain supplies us with the SIGA 39, 35, 37 and 65 via SIG Pizarras who operate strict quality control audits on our behalf at the quarries.

Selection process
Selection is one of the most important aspects of slate production. Slate is generally mined or quarried in up to lorry-sized blocks. These are diamond sawn into smaller cubes and then manually split to form slates. The slates are then sorted and graded by hand. SIGA natural slates are divided into four categories: Excellence, Specification, Commercial and Classic.

Excellence & Specification are the finest selections available. They are slates classified as flat, clean and regular, which at installation means minimal sorting and greatest visual consistency. The Commercial category closely resembles Specification, with slightly more variation in terms of appearance. It is often used on volume housebuilding projects. Classic is an affordable slate that is best suited to repairs and small domestic projects, including house extensions.

All natural slates must be sorted prior to installation in accordance with BS5534 Code of practice for slating and tiling, and BS 8000-6 Workmanship on Building Sites. It is worth remembering that a skilled slater can make an affordable slate look great, whereas a high-cost slate can look distinctly average if poorly laid.

Reproduced by kind permission of Architecture Today.

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