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The importance of good workmanship |
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BS 8000: Part 6: 1990
BS 8000: 1990 Part 6 Code of practice for slating and tiling of roofs and claddings is still in force. Section 4.3 Natural slates fittings and accessories, applies to all natural slates regardless of origin or grade. The key recommendations appear in section 4.3.1.
“a) Sort slates into three or four groups of equal thickness.”
“c) Lay slates of equal thickness in any one course, with the thicker slates in the lower courses and the thinner slates in the upper course.”
“Choosers” “users” and other stakeholders
BS EN 12326 is a product standard. Aspects of natural slate roofing not covered by BS EN 12326 relate primarily to brand selection and grading. Before a quality finished roof is achieved designers, specifiers, roofers, even clients and distributors have roles to fulfill. These groups have their own expectations and priorities. The phraseology they use in discussions on natural slate may be misunderstood or misused. Furthermore when slate is described generically the definition will change depending on the speaker.
Commonly used terminology
Bests, premium, firsts, seconds, thirds, mixed, thicks, clumps, heavies. These are examples of descriptions of grades of natural slate. The layman could be forgiven for thinking there is a standard rating for the quality of slates and that for example a “best” will be better than a “third”. One could also assume that durability is defined by these terms. Unfortunately this is not the case. These everyday terms do not appear in the BS EN 12326.
Quarries and importers
Each quarry and their importer will use these phrases to describe the output and amount of quality control a slate grade has undergone. That does not mean that a first from one quarry will necessarily be better than a second from another. The opposite could in fact be the case. Producers and importers use this terminology merely as an internal comparison. Furthermore not all quarries will provide a separate ACD for each grade of slate they produce. The less quality control and grading undertaken by the quarry on a particular shipment, the lower the quarries costs are likely to be. This allows the quarry and its agents to offer lower prices. It also means that tolerances on thickness and dimensional stability will be greater, putting an additional burden on the installer to sort and grade the slate. It also increases the amount of wastage.
Roofers
The installer is ultimately interested in laying the slate. He is concerned with efficiency of laying, how fast the job will go, how much breakage and waste he will encounter and how much sorting is necessary on site. To the roofer consistency is vitally important. “Best” or similar terminology for a roofer will not necessarily be strongest or longest lasting, it will be the most regular in thickness and dimension. Historically sorting was undertaken by the roofer when he was counting out the slates for a particular slope (usually by weight, heavier slate will be thicker) and holing the slate (slate will be thicker at one end, the nail hole would be made at the thinner end) to receive the nail. Much natural slate nowadays is pre-holed either by the quarry/producer or the importer/distributor. The nail hole is not always correctly pre-holed.
PRE-HOLING DOES NOT NEGATE THE NEED FOR SORTING!
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Natural slate - Quality & Standards
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