During the day, the sun heats the tiles, and the heat is transferred to your concrete deck. This deck accumulates heat for the full day. When the sun shines, the external temperature becomes cooler than the concrete deck and it starts radiating back the accumulated heat inside and oustide, which results in your roof heating your house, adding to the natural heat.
Studies showed that 80% of the heat in a house comes from the roof, which is permanently exposed to the sun radiations.