The following BS clause is frequently ignored to save time and money often with disastrous and unsightly consequence.
BS 8000 Part 6 1990 Code of Practice for Slating and Tiling
“Slates should be sorted into three or four groups of equal thickness.\r\nLay slates of equal thickness in any one course with the thicker end at the tail. Slate the roof with the thicker slates in lower course and thinner slates in upper courses”
Sorting slates by thickness is as an important part of the slating process as is fixing them. Slates must be graded on the ground, before being lifted to the roof. This is usually done during the holing process, but as slates are increasingly supplied pre-holed, the sorting and grading tends to be by-passed.
Sorting and grading gives the roof an improved aesthetic finish and a stronger resistance to wind lift. When slates are not sorted before fixing, they do not lay true on each other, they kick-up and create unsightly and dangerous gaps. In such instances the roof generally looks awful.
Most of the time the blame is directed to the supplier of slates, slates are a natural product and unlike man-made product, they cannot be exactly the same thickness. A tolerance of 20% in the variation of thickness is accepted, and this can mean a significant difference when applied to the product.
It should be noted that anything other than the first quality slates will tend to need more diligent sorting before fixing. It is to be expected that the roofing contractor will need to cut down some of the less acceptable slates for use on eaves and under-weaves courses to minimize the waste factor. This should be taken into account when deciding which grade of slates to purchase. There will be a reason for the difference in price between grades and quality of product.
If the slate roof looks un-even and unsightly, look no further for the reason. Pose the question – were the slates sorted and graded before fixing as required by the British Standard?