Tolerances in Sizes of Products
When working with paving products – new or modern units – although they may be produced
under strict quality controls, there will always be slight differences in size, known as tolerances.
Whilst most concrete styles, especially those that do not replicate natural materials, are
unlikely to vary sufficiently in thickness or dimension (e.g. textured Saxon slabs) to become
a problem, whilst those that are cast from moulds replicating Yorkstone may vary both in
thickness, length and width due to the semi-flexible nature of their casting moulds and volume
of wet mix concrete used during pouring operations. Only by a few millimetres perhaps, but
sufficient to require extra thought and care when laying. For this reason, joints should have an
additional tolerance of 3 – 5mm to compensate for the difference.
Larger sizes of concrete slabs may have a thicker profile than smaller sizes, essential during
manufacture to cope with the additional weight and resultant stresses place on the unit. This
extra thickness should be checked and noted if laying on a thin bed, as additional mortar will
be needed under the smaller/thinner slabs to compensate for the difference.
Natural stone products, even those sawn six sides, will be produced with dimension variations
of 3mm or more, as part of the sawing process. Even though the diamond blades used during
the cutting operations are changed/renewed on a regular basis, wear will take place during the
‘slicing’, affecting thickness, and dimensions during gauging. Once again, these variations
should be allowed for when laying the slabs, avoiding tight joints (less than 3mm).
Some sandstone paving slabs may have been produced by guillotining larger units into small
ones. For example, a (nominal) 600mm x 600mm slab may be ‘chopped’ into two halves, each
600mm x 300mm. These ‘chopped’ dimensions do not allow for a pointing joint, and may
require additional work to dress them to an appropriate size when laying. These are usually
found in lower priced packs.
Porcelain paving, although ostensibly pristine, with regulated dimensions, may be subject
to minor variations, and even a very small difference between individual slabs can create
issues over larger areas, where every slab laid may gain two or three millimetres, thereby
extrapolating the problem by adding width as work progresses. Trying to maintain equidistant
gaps between slabs can become a logistical problem. Rectified porcelain slabs have far less
tolerances than non-rectified at plus/minus 0.4mm and 0.7mm respectively.
It is always good advice to check and measure all products before purchasing or commencing
laying, making any decisions regarding jointing gaps to compensate – if necessary – by
adjusting accordingly
