Laying new slabs over old. - Getting the right levels

[alanccurrie]

I want to resurface a patio area of approx 80sq mtr which is currently laid in old 3x2' concrete slabs. The base is thick sand (local subsoil is also all sand). Due to slight uneven settlement over the last 20 years the existing slabs are a bit cracked and uneven in places but overall are relatively sound. More of a problem is a failing retaining wall along the bottom level of the patio which has bulged leading to a drop in the level of the retained patio by a few inches in one area (about 3-4 sq mtr).

My first job is to fix the retaining wall but thereafter I want to overlay the old concrete slabs with indian stone slabs. I have picked up from responses to other questions on this site that i should be able to to bed them using something like a 1:6 mortar mix, however, i am not sure how to tackle the area which has subsided.

Should i try to bed as is just using a thicker layer of mortar (which will need to be about 3-4" thick in places), or should i try to lift the level of the subsided area by some other means first.

Alternatively should i lift all the old slabs and start from scratch.

Hope you can help.
Alan

[ABILITY]

Lift them up and start again, no question

[Tony McC]

This is yet another FAQ - can I lay new flags on top of old? The short answer is that you can if you wish, but it's a bad idea for all sorts of reasons, not least of which is that you're literally burying your problems. What would happen if we all just laid new carpets over the old one in the living room? Or new wall tiles over the old 'uns in the bathroom?

Unless there are very good reasons not to do so, it's always better to get shut of the old stuff before investing time and money on the new.

[Maite]

Can I lay new terracota slabs on top of tarmac?

[alan ditchfield]

As said above the job that you do is only as good as the base you put it on.