The objective of this study is to propose a light weight and suitable strength material as roof surface which can detain rainwater during rainy day and lower room temperature in sunny day by passive evaporative cooling. So blast furnace slag was used as the main raw material and clay as the subordinate ingredient to synthesize an alkali-activated cement matrix at room temperature. Furthermore the cement matrix were made into porosity by injecting prepared foam and attempted to improve strength by incorporation of short polypropylene fiber. Finally, the dry density of 453 kg/m3 and compressive strength of 4.5MPa porous roof material (slag-clay porous material) was gained. Variations of room temperature and heat flux transfer across porous material roof slab have been measured. The results showed that roof covered with slag-clay porous material can satisfactorily lower room temperature and reduce heat flux, as compared with OPC (ordinary Portland cement) roof layer simply. Covering slag-clay porous material layer as roof surface can create a more effective cooling system by utilizing its water absorption and evaporation capability during canicule, which provides a new approach to low carbon and energy saving technological development.