Laying Paving Over Existing Paving
Sometimes, the temptation is too great. The customer wants to keep costs to a minimum, and suggests that new paving slabs may be laid on top of existing slabs. Why not? There is plenty of room to lay them without breaching the house damp proof course. Why spend lots of money digging out the area and laying new foundations when the existing paving is sound?
Trying to make someone understand the practical aspects of landscaping can be difficult.
Customers watch too much television! Watching alleged experts tinkering about for a ‘factual’ t.v. garden makeover programme, where everything costs fifty pence to build, people do get strong and wrong ideas about construction. If a famous faced gardener says it’s OK, why can I not have the same?
When you prepare a sub-base or foundation to build a driveway or patio, the area should be excavated and graded to levels and falls. The depth of the foundation will be dictated by the purpose for which the area is to be used. A driveway to hold cars and perhaps oil delivery lorries will require suitable foundations. A patio to be used to games and parties, or simply lounging on will not need to withstand heavy traffic and weights.
What they both have in common is the sub-base. This is comprised of either MOT Type 1, crushed concrete or broken bricks and tiles, with a face size no larger than 5cm x 5cm, small enough to be able to be rammed or compacted into position. The reason why the materials have to be so small is to prevent rocking in the ground caused by traffic. Anything larger than a quarter of a brick will be liable to rock in its’ space causing movement in the foundation.
Although the existing patio paving slabs may have been secure for years, they are extremely prone and vulnerable to rocking once a new surface is constructed on top. By ‘rocking’, I do not mean visibly or measurably so, but sufficient to prevent any bonding between the two skins or layers. Hence the upper slabs will work loose and break free from their laying bed, no matter how strong that grip is due to the movement in the lower base foundation.
In the case of the patio shown in the two photographs, the new paving has been laid on top of an existing paved area, and extended beyond into the lawn – without building new foundations at least matching the existing in strength, rather simply a ‘dot and dab’ of spot bedded mortar!
Far from being a suitable base or foundation for the new paving, it very quickly proved to be no better than quicksand, with the slabs becoming de-bonded and loose a few days after the project was signed off and paid for. As you can imagine, customer was less than happy. The whole area had to be taken up, excavated to new levels and an MOT Type 1 base installed, and the slabs re-laid over a suitable full bed of laying mortar at great expense to the original contractor.
