Spray on foam roof coating

I saw some info a while back on a flat roof coating that was an expanding foam, probably polyurethane or whatever, that was white and had a pretty tough skin. I haven't seen anthing on it lately and wondered if the system has problems that have prevented widespead usage. I need to externally insulate a flat roof on a small comercial building, and was considering alternatives to the isoboard method. Anyone have any experience with it?
Thanks, Bill
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