1. Well,no permanent structure is permissible unless it is made as per sanctioned building plan.
2.Since the flat owners relinquished their rights over the roof the builder can make constructions on it but beyond this the owner can not restrict the usage by other flat owners.
3.So immediately file a case before the consumer forum and seek interim relief to use the same or injunction not to restrain you from using this.
4.the roof is a common space and hence its use can not be restricted either by the owner of penthouse or the builder.
5.So if you file case beofre the consumer forum you are expected to get result.