Slabs (and their reinforcement steel) should be designed by a structural engineer who will base the design on the soil conditions - this means that it is impossible to say from a photo if it will work or not. It also depends on the condition of the existing concrete. If the old slab cracks or moves there is a good chance the new slab will do the same. Most structural engineers (who are usually cautious and conservative) would suggest that you remove the old slab as they don't want to be responsible for any cracking in the new slab if the old slab fails with the extra weight that it was not designed for.

If you want a higher ceiling to your alfresco area then consider a new flyover roof like this one, which overlaps the existing roof at a higher level.

Best of luck,

Dr Retro

House Calls

I guess the concrete question was pretty much what I expected, but you never know what new techniques may exist if you don't ask. And yes, I like the fly over roof idea. When i get an architect in I will certainly discuss that option, thanks.