You might ask your contractor about installing safety tiedown brackets on your roof. They are special stainless steel brackets for a fall arresting harness that are made to be installed on roofs and left there.
Obviously the type of roofing, along with design on roof, pitch. obstacles (vent pipes, skylights, etc) will be a factor in using them as well.
Most any roofing contractor will try to change your mind about installing things on roofs as it does "tend" to promote problems in the long run with not only the roofing material but with the underlying materials as well. The arresting system tie downs are about the only thing that is really approved for roof installation on a permanent basis.
Also, you should be aware that going up on a roof causes damage every time you go there even though you may not see it!
Shingles get the granules rubbed off them and worse if it is hot, they can get stretched and/or ripped which can make leaks.
Metal roofing gets a lot of strain when people go up on it. The sheets tend to want to slide from the weight of people or additional items placed on it which can (and does) cause stress on the screws that hold the metal on and can over time cause the holes for the screws to become elongated and possibly leak.
Tying things off to vent pipes or other "roof projections" can stress them and cause the roofing sealant around them to loosen or crack and again a possible leak.
Stepping too close to the flashing around chimneys, skylights, vents, etc, can break the seal on those and promote leaking as well.
I have seen cases of people throwing ropes over the peak of a roof to hold themselves or items and cause damage to the roof cap which resulted in water penetration and severe damage to the roof and home years later (damage to the tune of 10,000 of dollars easily).
Roofs are also one of the most dangerous areas of falls and deaths!
I have repaired and replaced a lot of roofs and roofing material over the last 20+ years as a contractor and home builder just for information on why I state these things.
Not trying to scare anyone out of anything but this time of year there are way too many deaths and falls from roofs just because of a time of the year that should be fun, festive, and happy for all. Not a time for mourning because of a dumb accident.
Take your time, think twice, and be safe and careful when working on a roof and make sure your roof can handle whatever you are going to add up there.