what is the diff between helmets?
sounds like a flat roof, have you considered a more permanent solution? copper is much cheaper than one may realize for a flat roof and has proven itself to last centuries. worth a couple of bucks more for sure. here's a link below about flat-locked copper roofing. good luck! p.s. the site below is run by joe jenkins, author of the slate roof bible, he and his company are located in PA.