I have set-up a rooftop garden at home. I am too lazy to water it every day so I am planning to set up a drip irrigation system. But the question remains: is drip irrigation more water-efficient than conventional watering?
Drip irrigation is more efficient in general terms, but there are many variables to consider.
Drip irrigation is aimed at placing the amount of water required to supply the plants needs at a location where they can access it.
Conventional watering loses water both to the air during transit (small) and from evaporation from the surface and hear surface after deposition. Losses are potentially large as a proportion of all water supplied, depending very much on circumstances and specific layout.
eg loss of water will be increased by any of hot conditions, full sun (seldom a good time for conventional watering), shallow soil, lack of mulch, dark soil surface, hard soil surface not allowing rapid penetration, ... . Drip irrigation is somewhat affected by all these but to a lesser extent.
Wikipedia - drip irrigation useful.
Drip irrigation has probably existed in some form for millenia, but modern drip irrigation was developed in Israel as a response to its hot climate and low availability of arable land.
Brief but useful comment on Israeli irrigation practices is here - some of their large scale lessons are applicable to your home garden :-).
Lots of pictures - each one links to a relevant webpage.
Drip irrigation systems greatly vary in design, but I would recommend the pipe with built in self cleaning drippers as apposed to adding drippers into a reticulation pipe/hose.
The self cleaning internal drippers clean themselves when the water is turned on. Cleaning is necessary as ants sometimes block the drippers with sand.