It is a good practice to have safety nets for works at height to prevent injuries during fall of persons. Small time contractors usually avoid provision of safety nets and even if they were forced, they will provide nets only below the place of work and shift the net as the work proceeds. Because of the cost, the quality of these safety nets also is questionable. In case of doubt, it is better to test the capacity of the nets by dropping sand bags of sufficient weight from height and see whether the net is intact or strands are broken or the net sags. This will also give confidence to the workers and will be an eye opener for the contractor if the net fails. Normally, safety nets should withstand 2200 kgf. They should be provided such that the fall of person is restricted to 2 m and the width of the nets is atleast 6 m from the place of work. There should not be any gap between the wall and the net. Any construction material / debris that falls should be removed immediately to avoid injuries to persons falling on to the net. At many construction sites, I saw construction debris accumulated on the nets with nobody to remove them.
If the floor is good, then one can provide mobile nets which I saw at a construction site. A scaffold structure is made with the net tied all around. This will be located below the place of work at height and is moved as per requirement.
A few construction engineers question about the need for providing the safety net or its practicality to provide. At many places, I saw even for a small work, scaffold is erected (11 storyes height at one software company building, 5 storey height at one super speciality hospital). Therefore, what is required is commitment to ensure safety. Else, we get 1000s of excuses for not providing the scaffold / safety net.