Use of terrace of a building

Roof is common area. Anyone can use it or all of them cannot use it. 

Whether you have submitted declaration as required under the provisions of State Apartment Owners Act (as applicable in your state) ?

You question is contradictory  if you sold a apartment to some one then the property being undivided share property .the owner of the property also have right to use the common place . So better to show all documents to a local lawyer and clarify your doubts 

Often Roof is common space mentioned on the Schedule of your deed of conveyance. Refer that with one Advocate. 

The purchaser of a flat becomes part co-owner of the building and has right of usage of the common open spaces of the building. However much will depend on the terms, conditions and covenants of the registered sale document.

While wholly agreeing with Mr. Kishore Mehta, it could be added that provisions of laws, if any, enacted by the State also matters. In the absence of legal provisions or mutually agreed terms and conditions the trrace cannot but be common ares accessible all the occupants of the apartment.

Sir.  In the agreement, terrace is not mentioned.  As except one flat, I'm the owner of rest of the building. In this case,  do I have right to completely close the terrace and make it a personal space. 

Yes, you hold the contract and rights here. 

Once again it will depend on the terms, conditions and covenants of the sale document, the wordings can only decide whether the purchaser was barred from the use of the terrace. Consult a practicing advocate of your area in your own interest. 

Admittedly, there are no covenants as to barring the purchaser from the terrace. As such, it is improper and unreasonable to take such extreme steps.