I have done some roofing but never used harness but also none of them were very steep. I would advise against using the chimney as an anchor point. Because it could give or break and ur harness is useless and you have a 400lb brick falling off the roof. I would either do what your buddy suggested or go to the opposite side of the house and tie rope to a large tree and vehicle and then bring it over roof and down and tie it to tree or vehicle and then use this as your main line and use an ascender and clip in to that.
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