One of the most common causes of complaints from customers is that of ‘moving’ paving slabs, not necessarily coming loose or ‘popping’ from their laying bed, but splitting apart even by a few millimetres, allowing moisture to enter the joints and loosening the pointing compound. Each slab appears to be shifting within its’ foundation, gradually migrating towards the edges.
On closer inspection, it is often found that the cause is a non-uniform foundation or laying bed. The base foundation of MOT Type 1 may be universally laid and well compacted, but if the actual sand and cement laying bed is not also laid as a uniform bed, but rather as individual ‘cushions’ of sand and cement (usually laid wet rather than semi-dry) to support the slabs, the result is that each slab is a separate entity and not part of a solid structure.
As you can see in the attached photograph, every one of the slabs on this project had been laid on such cushions, with the result that every single one had come loose (before any jointing compound had been applied, as the customer was unhappy with the obvious movement in the surface.)
It was possible to insert a sheath knife into the joints, to the full depth of the bed, down to the MOT foundation, and move the slab and laying bed as one unit.
For this reason, I suggest setting a single roofing slate on edge, in concrete or mortar, to secure the outer edges of a paved area (see attached drawing) to prevent side migration. The tile is not seen, but will provide you with a very strong barrier to hold the paving together.
