MikeC was asking what is an allowable handrail under the IPMC, not whether it is required for existing buildings as regulated for new construction.
fatboy provided some guidance to MikeC's reference but it obviously doesn't go far enough except for pre-manufactured handrails.
201.3 Terms defined in other codes.
Where terms are not defined in this code and are defined in the International Building Code, International Existing Building Code, International Fire Code, International Fuel Gas Code, International Mechanical Code, International Plumbing Code, International Residential Code, International Zoning Code or NFPA 70, such terms shall have the meanings ascribed to them as stated in those codes.
IRC definition: HANDRAIL. A horizontal or sloping rail intended for grasping by the hand for guidance or support.
201.4 Terms not defined.
Dictionary definition: RAIL. A long horizontal or sloping piece of wood, metal, or other material that is used as a barrier, support, or place to hang things.
It would be the AHJ as to whether to require retrofit to be current standards which I would caution since there may be hundreds if not thousands of existing structures that have 2x4, 2x6 and other materials; pipes, chains and ropes as handrails in existing buildings that was acceptable at the time of construction.
Francis