I bought one of those steps Knappster posted the link to. It bent the first time I used it. Upon further inspection, I found it was rated for 150lbs (found nowhere on the site I bought it from, in tiny print on the package). I am 220lbs. I don't remember what I was lifting, but add that weight to mine, plus the stress of being off center while placing the items on the roof.

I think the step is a great concept, but don't buy one that is tube metal, as I did. Not worth the money. I threw the step away so I don't remember the brand, but it folded up and looked just like the one in the link.