The Hierarachy of Control is designed to control risks. The controls are ranked according to their effectiveness in controlling hazards at their source:
Level 1 eliminates the hazard altogether. To achieve this, building owners could relocate plant and equipment away from fall hazards. In the case of roof work though, it is the roof we need to access - so it is not possible to eliminate risk altogether.
If Level 1 is not practicable, Level 2 calls for a passive fall prevention device, such as a catwalk or walkway, to eliminate the hazard presented by the non-load bearing material and guardrails on the side to prevent anyone stepping off the walkway. Alternatively, an elevated platform (Or Cherry Picker, etc.) can be used to do roof work or to clean the gutters. This may not be able to be possible if it introduces other hazards, such as proximity to overhead power cables, or the cost makes it not practicable.
Work positioning systems, such as roof anchor points or static lines, require a high level of user skill. They also demand ongoing involvement by the PCBU (controller/owner of the workplace) in the form of training, regulating access, risk assessments and consultation. Level 3 controls won't prevent a fall through fragile surfaces (Such as Asbestos roofs, laserlite, etc.), making them effectively Level 4 controls.
Level 4 controls include fall arrest systems such as tested and annually certified roof anchor systems and static lines. The use of Level 4 controls acknowledges that injury is almost unavoidable in the event of a fall but aims to save lives. The workload associated with Level 4 controls is high. Apart from the ongoing responsibilities of training, etc. the controller of the workplace is responsible for emergency rescue procedures and medical evacuation.
If Level 4 isn’t practicable, then, as a last resort, Level 5 controls such as signage or training are used.