Laying a patio

It is never a good idea to lay a new patio on top a old one, what about the height difference.The fall is it in a sound condition what is it sat on has it sunk or likely to twenty other questions. However if you satisfy yourself that none of it is going to move then depending on The type of slabs I would lay it on a full bed of mortar. Good luck Pete.

Provided they don't rock, aren't hollow beneath and cannot shift in any way, then they will be as good as any hard core, however the obvious question one would ask is, do you have the height tolerance? The finished slab level needs to be 150mm below the damp course. if you can lay your new patio within this measurement parameter then go ahead and leave them down. Make sure the existing slabs are free of crud and moss and grease. you can give them a stiff brushing using some sharp sand and water as an abrasive. we always solid bed slabs on a continuous bed of sharp sand mortar. The only downside will be that there is no suction in the substrate due to the impervious nature of the slabs below. This may cause some water to sit on top of the slab joints for a while, and may delay the pointing.