The blizzard that swept through the Northeast dumped snow on rooftops, threatening to cause some to collapse. An insurance company spokesman advised that only professionals remove serious snow buildup from roofs, but some contractors said the job can be risky. "On a flat roof, it's doable. If you're on a pitch, it's dangerous. You're talking about a very slippery surface," said Robert Bryant of A Plus Roofing and Siding in Mount Laurel, N.J.