Any solution to safety problems that relies on people just never making a mistake ever isn't realistically a solution at all. What you need is some process to e

Any solution to safety problems that relies on people just never making a mistake ever isn't realistically a solution at all. What you need is some process to ensure that when work like this is carried out, the area is appropriately marked as unsafe, and processes add cost. In this case, safety would probably have dictated not carrying out the work on top of the roof whilst the work underneath was in progress due to the risk of them misjudging where the weak area started and falling through, potentially blowing up the schedule or even meaning workers were paid to stand around doing nothing, which adds more cost.