The information below is based on OH&S legislation, keep in mind that if your company or the company that hires you has stricter rules in regards to control zones you must follow the higher standard.

A control zone is required on a roof where workers will be working within 4 meters of the unguarded roofs edge, the warning line can be as simple as a taut rope with a tensile strength of 5 kN with coloured flags stretched between stanchions that are no more then 2 m tall and 0.9 m short.
Workers do not need to be tied off as they access the roof provided that they proceed directly into the control zone and remain in there.
If workers need to work outside of the control zone than a fall protection system or travel restraint system needs to be put in place to protect the worker in the event of a fall.
If workers are working on a large flat roof and will at no point be going within the 4 m area of the roofs unprotected edge the is no requirement to have a control zone.
Any roof that has a slope of more than 4 degrees cannot use a control zone and all workers must be tied off.

Photo Source: http://bluewater-mfg.com/product/perma-line/