I agree and think a bunker type shelter would be best. Concrete block walls with rebar and filled with more concrete would seem to offer the most protection. The roof could be poured, reinforced concrete. Should be tornado proof too.
Major problem with flat roof--they can collapse from water weight when the drains become clogged with debris. Saw this first-hand in Stuart FL last year. Eleven condo buildings condemned in one complex when the flat roofs collapsed.