Switch to energy-efficient air conditioning systems and the use of eco-friendly building materials would result in significant energy savings and reductions in CO2 emissions. Thermally activated building system (TABS) is a promising low-energy cooling technology, which provides better thermal comfort than the conventional air conditioning system. In TABS, cooled water circulated in the tubes embedded in one or any combination of roof, floor slab and walls, not only reduces the heat ingress but also cools the building. Integrating TABS with eco-friendly building materials like glass fibre reinforced gypsum (GFRG) could be an intertwining solution. A hybrid system that integrates TABS with GFRG roof is investigated in this study. The thermal behavior of this thermally activated glass fibre reinforced gypsum (TAGFRG) roof is analysed experimentally in terms of diurnal temperature gradients, thermal images of the interior and exterior roof surfaces, decrement factor and water temperature variations. The reinforcement zone of the TAGFRG roof handled a higher cooling load compared to that of air cavity zones. The diurnal temperature fluctuation of interior roof surface at the air cavity and reinforcement zones is reduced by 5.1 and 6.7 °C respectively by TAGFRG roof cooling. © 2020 Elsevier B.V.