Rainwater harvesting is increasingly recognized as a sustainable solution for water management, especially in areas affected by water scarcity. However, the potential pollution risks linked to asbestos cement roofing materials, widely used in residential areas, have not been thoroughly explored. This paper addresses this gap by assessing the effects of asbestos cement roof degradation on the quality of harvested rainwater, focusing on its suitability for irrigation. The research combines controlled laboratory experiments and field-based sampling of rainwater in contact with asbestos cement surfaces. Significant changes in water quality were observed, notably, the analysis highlights a substantial increase in heavy metal contamination, including a 75.4% rise in mercury concentration and a dramatic 127.3% increase in lead levels. The findings underscore the need for extensive investigations into the impact of asbestos cement roofs on water quality in Hungary, considering the varying characteristics and contamination levels of asbestos cement products across countries as well as the limited existing scholarship on the subject in Hungary. This research is novel in its pioneering examination of the effects of asbestos cement roofing on harvested rainwater quality in Hungary, with a particular focus on heavy metal contamination (e.g., lead, mercury, zinc), which poses significant environmental and health risks.