Paving Over Existing Paving
Whilst it may seem an obvious cost saving in the eyes of a home-owner to survey an existing patio, realise that there is sufficient room between the damp-proof course and paving to allow for an additional layer of paving to be installed on top of the old one, extreme caution should be employed when making any decisions.
Unless the original paving is in very good condition, firmly bonded to the bedding mortar over a sufficient base, there is a great risk of the paving slabs becoming loosened, and any slight movement under new work may cause debonding issues with the second layer of (probably) thinner units.
There is a very good reason for using MOT Type 1 as a sub-base for paving, in that it is a material that can be compacted, with a blend of larger stones mixed with fines, thereby having the ability to bind together. Even crushed concrete should be broken into small pieces, no larger than 75mm, which once again, requires a blinding material of fines mixed in to enable compaction to be achieved to prevent underground movement. Even on rough farm tracks, rubble is broken into a maximum size of ‘half a brick’, as anything larger will ‘rock’ when vehicles pass over the surface, causing the drive material to break up.
From a contractor’s viewpoint, any decision to lay new paving on top of old (unknown) foundations is fraught with problems. Under the Consumers Rights Acts, responsibility for any work involves a six year ‘warranty’ period, during which any faults that manifest during that time will have to be addressed. It is always advisable to have full control over every element of a project, and laying over an unknown foundation is fraught with potential problems.
A classic example showing why paving on top of existing paving is not a good idea. Using an existing ‘foundation’ of any kind is not sensible, as you have no idea of the construction methods used by the previous contractor. In this case, the sandstone slabs had been laid over soil with spot bedded mortar, and the fresh work lasted only a couple of months before failing.
