Personally, I don't care for ladders going up the roof - I don't need to be up there often enough to worry about it. Besides, I thought "walkable" meant you could walk around one the roof without any real concerns since it can take the weight per square foot of human feet. You can still get on a non-walkable roof for inspections / repairs and what-not just fine, you just have to be careful since that much pressure in one point (people feet) can damage the roof. I was taught some cardboard and a sheet of plywood laid down where you need to work would spread the load out fine. I put Maxair vents on my non-walkable roof myself with no issues.