For 1850 sq.ft flat roof ,what mm tmt bars used my hall size 21*21 and 11*17 and

         It is better if you have structural drawings design. In my opinion you have to lay consolidated/ hidden beam in the hall size 21' x 21'. For RCC slab / roof  12 mm tar i.e  tmt bar is sufficient. For 1850 sft area slab 2500 kg 12 mm tmt bar is required excluding beam steel.