No renovation contractor will give you this type of warranty.
Reno contractors are small timers. 
They can give you warranty on paper but what's the point?
Withholding of payment is the best warranty!

With regards to payment with-holding, I don't think the contractor would agree to hold his final payment(10%) for 6 months, let alone 1 year as the moment the works is completed, he will ask for full payment.

The reason of my question is as my intended porch renovation is  of concrete roof slab, the defects like leaking would not be possible to detect within 6 months as normally such occurances happens after 1-2 years, as what my friend experience, right in the middle of her porch. Contractor - deny responsibility and cabut. Cost of repair plus waterproofing - 8k. 

Is asking a 2 year warranty against leaks sounds reasonable?

Is there  any particular to use a concrete flat roof? Is the slab to be used as a balcony?
If not, go for metaldeck roofing - solves all your worries about leakage.

Agreed, become Project Manager or Site Supervisor.