Phaedrus
You can walk on the roof but, stay on the rivet lines that transverse vertically the roofline. The max weight I've heard mention is around 200 to 250 lbs. You might consider using a staging area of sorts for long term working on the roof, such as a section of plywood to lie between those ribs. (Use old sections of rug under the board to protect the rooftop .)
Also, you should never step on the curved portions of your roof either the front or back as there is no support period underneath~!!
ciao