A flat roof can be strengthened either by reducing the span of beams that supports it, or by adding beams to reinforce the structure. Reinforcing existing beams is usually the most practical solution in residential construction. Reinforce a flat roof to carry the additional weight of new mechanical equipment, or to use the roof as a deck or a roof garden. Hire a structural engineer to determine new load factors required by local codes, the reinforcing beam sizes and the locations necessary to support new uses, and to prepare construction documents fully describing the work.
Remove existing ceiling materials to expose the beams that frame the flat roof. Use a reciprocating saw to release ceiling-finish materials from the walls and a pry bar to remove finishes from roof beams. Remove insulation materials from cavities between the beams.
Inspect the roof structure, noting damage to any existing beams and replacing them, if required.
Insert additional beams between existing beams, following plans prepared by the structural engineer.
Repair the roofing membrane to prevent future water damage to existing beams.
Reinstall insulation materials and finished ceilings to complete the work.
Remove all personal belongings and furnishings from the areas below the roof beams that you will be reinforcing. This is a dirty, difficult job that will be much easier in unoccupied space.
Never change the use or add to the existing load of a flat roof without knowing building code requirements. Code requirements are enforced for public safety. Ask about your city's regulations and follow the rules to prevent unintended damage to existing structures.