Hello Rajkumar,
The following are various reasons for the development of cracks in RCC slab/ roof.    

1.Use of salty water
2.Use of sub standard coarse sand having silt contents more than 4%.
3.Use of fine sand instead of coarse sand.
4.Use of improper grade coarse aggregate i.e.  Bazri which has foreign material dust or earth mixing etc.
5.Use of cement made from mini plant
6.Mild steel not laid according to design.
7. Depth of slab is not according to the design
8. Rusted steel used in RCC slab/roof
9. Proper cover not given under mild steel bars
10. Removal of shuttering before 7 days
11.Vertical spots under shuttering settle down.
12. Curing not done properly
13. In some cases mason /Mistary continues work till the slab is wet and the laborer throws bricks on the slab which causes cracks.
14. Improper mixing without using vibrator while laying RCC slab.
15. Leakage in RCC slab which creates rusting in mild steel.
16.Top of RCC roof not covered by brick tiles or flooring etc.
17. Bottom of RCC roof not plastered.
18. Settlement of load bearing walls.

For details you may take help from the article ‘Repairs of Cracks’ at our website www.gharexpert.com at the link given below.

Repairs of Cracks

Thanks.