Calibrations in Materials
Almost every paving product has variations in size, either/both thickness, length and width. These variations need to be taken into consideration when planning or specifying a project, especially when designing large scale works, as the smallest difference – when extrapolated over a distance – can cause problems with detailing.
Concrete paving slabs for example, may be described as 450mm x 450mm x 37mm in a brochure, yet when they arrive on site, they may be 455mm x 455mm x 37mm, and if they have been designed into a grid pattern, using a brick edging detail, with an allowance of (say) 900mm x 900mm within a square, the slabs will not fit without being sawn to size.
Porcelain paving is nominally manufactured to a strict size, yet the majority of products will have plus or minus 2mm tolerances. When laying a patio allowing less than 5 – 6mm joints, this differential can become problematic, unless recognised by the installer.
Riven Sandstone is perhaps the worst material for variations in size, both surface dimensions, but also in thickness. Nominally based on a 150mm scale i.e. 300mm x 300mm, 450mm x 450mm, 600mm x 600mm etc, they arrive on site with variations as much as plus/ minus 20mm.
Some sandstones have been loosely calibrated by quartering, i.e. a 600mm x 600mm slab cropped into 4 no 300mm x 300mm squares, which allows for no mortar joint between the sizes, requiring the units to be reduced to size with a cutting blade.
Many sandstones have been passed through a ‘thicknesser’, which reduces the thickness of the stone to enable more slabs to be fitted into a ‘one ton crate’ to save money on transportation costs. These slabs are barely 20mm thick, sometimes even 17 – 18mm, which has the effect of creating bowed and almost flexible units, especially on some of the larger sizes (750mm x 900mm for example) which are liable to flex and bend even after placing on a mortar bed.
These curvatures can cause trip hazards or ‘lips’ if laid corner to corner, or cross-jointed. If possible, try and achieve a minimum of 30% steps in the paving pattern, i.e. third bonding or preferably half bonding to minimise the problems with uneven surfaces.
Other ‘calibrations’ or dimensions/tolerances to be aware of include setts, especially those that are produced by cropping, e.g. granite setts, nominally 100mm cubes. In theory, and on a plan, you need 100 granite setts to cover a square metre. 100mm x 100mm x 100 number. In practice, you will have difficulty in laying more than seventy granite setts in a square metre, as they vary in size between 115mm x 115mm down to 90mm x 90mm in a wide range of different sizes that require a skilled hand to lay in any form of pattern.
