Lojikglos - don't forget that with body harnesses you need to provide an emergency recovery plan and manpower to deal with that should the individual fall.
Stephen

The hierarchy of control is:-
AVOID
PREVENT
PROTECT

But you can broaden it by adding:-

AVOID (ENGINEER OUT THE NEED FOR WAH LIKE WINDOW CLEANERS WITH TELESCOPIC POLES)

PREVENT (COLLECTIVE PREVENTION - GUARDRAILS)

PREVENT (INDIVIDUAL PREVENTION - FULL BODY HARNESS USING FALL PREVENTION SYSTEMS SUCH AS RESTRAINT)

PROTECT (COLLECTIVE PROTECTION - MINIMISE THE CONSEQUENCE OF A FALL BY USE OF CRASH MATS AND OR SAFETY NETS)

PROTECT (COLLECTIVE PROTECTION - MINIMISE THE CONSEQUENCE OF A FALL BY USE OF FULL BODY HARNESS WITH FALL ARREST)

Like added earlier, ensure you cover rescue plans.

I would add that the duration of the work and the type of roof needs to be considered.

Safety nets (which are also a collective measure) may be preferable to a harness for longer duration works where there are fragile roof lights, for example. Fall restraint harnesses are preferable over fall arrest harnesses but people don't like wearing them and forget to clip on etc.

The advice I give is that companies need to assess each roof and have a clear policy for long and short duration roof work for that specific roof. That way, the individual carrying out a specific risk assessment for a specific task has clear guidelines and expectations.