The growing popularity of increased thermal insulation, in association with code and standard mandates, assists in mitigating exterior ambient temperatures and heat flow migration influences on building interior environments. Some have tried to mitigate these exterior influences on the interior by roof surface color alone; an incorrect precept. Roof color alone, a single roof system component, cannot mitigate exterior influence in and by itself. Insulation, roof system design, roof deck, etc. all have a role to play. HVAC designers have not been informed as to how roof system design can detrimentally affect HVAC performance: Increased air temperatures above the roof surface, high temperature heating of roof top piping, and the heating of roof top units by reflection of roof surface have all resulted in HVAC performance well below that which it was designed, expected, or anticipated for. Appropriate roof system design is the result of the architect, engineer and building owner working together, taking into consideration building use and the effects of the climatic and environmental conditions expected to be experienced. This paper will draw on the author's 30 year history of roof system design and forensic observations to review and explain the effects of roof system design on HVAC design in hot climates, from the perspective of a roof system design. Concerns such as heat flow, reflected ultraviolet light effects, roof top temperatures and their potential detrimental effects on HVAC performance will be reviewed. Additionally, design recommendations and detailing suggestions for achieving long-term roof service life performance in hot climates will be reviewed. Proactive design recommendations for HVAC designers on how to deal with roof borne effects will also be provided.