What are the economical coolant chemicals available to apply on the r.c.c terrace slab to keep the house cool

Reciding on the top floor of the building it becomes very hot in summer afternoons as the terrace is directly exposed to sun. It also turns all the internal walls, even the furniture hot. And it remains hot till late night. It is also tedious and impracticle to sprinkle water all the time, or to spread jute bags or any such things in the terrace, or to erect any temporary shed on the terrace, being a common property of the housing society.