I weigh 220 pounds and go up there all the time. Just don't put all your weight on one foot and jump up and down on that foot. I stay away from the sides of the vents, etc. I also feel with my feet when up there and walking around and if a spot seems to be soft when I am starting to step on it, I move to a different spot. You will eventually begin to know where the bracing is under the roof. I am up there several times a year -- to clean it and wax it and any other preventive maintance that needs to be done up there.
Just go easy and be aware of slickness when it is wet up there -- also, don't go near the edges -- stay in the middle if possible or the middle 1/2; ie, 1/4 on each side.
Dave and Karen -- '02 Ultimate Advantage 36 C, 350 HP Cummins, Allison 3060 Tranny
'13 Ford Edge, InvisiBrake