that model is considered lightweight and as such has lighter materials used in the construction process. The roof is prob. 1/4 plywood with a sandwich of styrofoam for insulation and is not what would be considered a full walk-on roof. The crunch noises are normal. You take a 180-200 lb man and they try to walk on a roof made out of that material and you risk damage to the roof. Suggest putting a small sheet of plywood down to evenly distribute your weight if you are working on the roof, or rent some scaffolding. Unless roof has water damage there is no need to rebuild anything