The paper analyses the efficiency of increasing the thermal mass above the floor to reduce summer overheating in one-storey residential terraced buildings in temperate climate. The effect of floor slab thickness on total yearly energy demand is also investigated. According to EN ISO 13786:2017 standard, the useful thermal mass thickness of building envelope is limited to approximately 8-10 cm distance from its surface. Increasing the floor slab thickness from 8 to 18 cm lowered the average and maximum indoor air temperature during 9 day heat wave by 1.25 K and 1.52 K, respectively. Since these effects were obtained for a room with a relatively high total thermal capacity, it can be expected that it would be higher for rooms with a low thermal capacity enclosure. Cooling, heating and total energy demand decreased by 27.1%, 7.5% and 17.0%, respectively. The experiment results show that increase of floor slab thickness over the maximum values given in EN ISO 13786:2017 standard can lead to significant decrease of indoor air temperature during summer heat wave as well as heating and cooling energy demand. The cooling effect remained stable both during the 24-h cycle and throughout the entire heat wave period. (C) 2019 Elsevier Ltd. All rights reserved.