Check for splinters, and loose, warped or rotted boards. Sand down splinters or rough areas on boards. Refasten loose boards with wood screws and an electric screwdriver. Replace warped wood or rotted pieces with new lumber.
Concrete or stone balconies should be checked for cracks and crumbling. If you have thin cracks, you can usually use a cement-based patching compound or masonry crack filler to make the repair. Apply the crack filler as you would apply caulk, moving along the length of the crack. You may use a putty knife or trowel to apply vinyl concrete patching compound, as directed by the manufacturer’s instructions. Press it firmly in to the crack to force the initial layer into all nooks and crannies and allow drying time between layers. For cosmetic repairs, you may use a polymer premixed concrete patching compound. All of the repair materials will likely differ in color from the balcony. You may prepare some of the patching material and let it cure and dry in order to compare color. For best appearance, you may paint with an appropriate textured concrete coating.
For large cracks in the balcony floor, you may need the help of a professional, as structural issues can be very dangerous when concerning a balcony.
Inspect support posts, balusters and railings for any looseness, cracks, rotting, excessive rust or missing fasteners or other issues that can affect balcony structural integrity.
Check the balcony railing wall and floor brackets. If they are loose, screw loose brackets back into the wall and floor with an electric screwdriver. Refasten or replace screws at the base of loose balusters (vertical bars of the railing).
Replace any structural elements you cannot repair. When in doubt, call in a professional for help.