Pouring A Concrete Patio

[Lemmons]

I want to Pour a concrete patio over an area 4000mm x 3000mm and then tile it. My question(s) are:

Can I pour over exisiting stone flags. They've been down for 20 years and are solid, or am I being lazy?

Also I'd like it to be level (within 50mm) to the french doors at the back of the house. So i'd be looking at pouring to an overall depth about 150mm. If I put in an adequate drainage channel separating the patio from the house will that satisfy the damp course building regs?

[The Builder]

If I were doing this job for someone, Lemmons, I would have to lift the slabs to see what was under them. I wouldn't want to risk a problem later because of an assumption that the bed was solid.

Doing it for yourself, I think it would be well worth doing it over the existing slabs. 20 years is plenty of time for it to settle. I would say, though, that I'd have to be very sure there was no sinkage.

Run a straight edge or a string line over the surface. Assuming they were laid straight in the first place, any undue settlement would show up under the line.

Having said that, even if the surface is "dished", it could still be stable.
It's one of those decisions that only you can make, I'm afraid.

Why not pull one slab up and let us know just what is underneath (and its thickness)

[malagabob]

If pouring over existing slabs you will have to take into account the level between damp proof course and finished level will decrease.
I seem to remember the recommended distance is a minimum of 150mm 6"