Here are some notes from Glenn on equipment designed to keep you on the upper side of the roof...
Don't worry about the height. A fall from 20 feet is not a lot worse than a fall from 30 feet...either could kill you. Just be careful. Work 100% tied off if you feel better about it- a bit slower but safer. Safety harnesses are available for around $100.
(http://ecx.images-amazon.com/images/I/51RNWAMW4EL._SL500_AA280_.jpg)
Complete kit - $129 http://www.amazon.com/gp/product/B00005Q7A6/ref=pd_cp_hi_0?pf_rd_p=277661601&pf_rd_s=center-41&pf_rd_t=201&pf_rd_i=B0007KQXVI&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=02N83KGBPJP3X0TWZHF5
Just a harness is really cheap -
http://www.amazon.com/MSA-SAFETY-WORKS-10072487-HARNESS/dp/B000S8647W/ref=pd_bbs_sr_3?ie=UTF8&s=electronics&qid=1214351611&sr=8-3
or kit with fall cushion lanyard which is best
http://www.amazon.com/gp/product/B0007KQXVI/ref=pd_cp_e_0?pf_rd_p=277661601&pf_rd_s=center-41&pf_rd_t=201&pf_rd_i=B000S8647W&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=005C1T8JQ63F38F7QFEE
The big kit is best as it has the adjustable safety rope grab, roof anchor and safety lanyard -- it keeps you from snapping your head off when you hit the end of the rope..it must be replaced after a fall to properly cushion you the next time.
The rope grab will let you easily adjust slack to work quite a large area without retying the safety rope.