After 6 months down the road nightmare back again as the Flat roof area leaking again as could saw water mark at the plaster ceiling

Yesterday have re-apply water proofying (DIY) using PenTens...cross finger and hope it solve out.

For protection against rain, I'd say metaldeck roofing is the best because it has less  joints.
As far as I know, there is no permanent cure for cracked concrete roof.
Consider doing a layer of metaldecking over it.

Cement roof tiles  depend on the overlap between tiles and the roof pitch, whch must be at least 20 degrees. Other factors can also cause leakage - flashings, structure sagging, cracked tiles, etc.

P.S.
There is one method of repairing leakage whereby holes are drilled at close intervals on  the concrete slab. A liquid is pumped under high pressure into the holes. The liquid finds its way into the pores of the concrete, expands into a foam material and clogs up the pores thus preventing leakage through the slab.

I don't know what material that's called. I believe it is still in the market.
It worked to stop leakage through my bathroom floor slab so that I did not have to hack out all the tiles and redo the waterproofing.

OK, polyurethane the foam packing material.

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Added on February 18, 2012, 12:50 pm

What 400mm thick wall?
We are talking about a car porch roof which is under direct sun and rain.

Car porch roof slabs are typically 100-125mm thick. Wouldn't a thinner slab be more difficult to waterproof than an thicker slab?

The question is also, how long will it long under outdoor conditions?
The same job can be done using sealant or flexible coatings but they don't last.

That's true, it's not as if you have other choices though it may not be 100%.

How much does it cost?