You've just dug out your car and home, now it's time to check the roof.  Governor Patrick and the Mass Emergency Management Agency are issuing a warning that Monday's wintry mix of sleet, freezing rain and rain could make for some dangerous conditions for some homes and businesses.  MEMA says the fluffy snow piled on roofs can act as a sponge, absorbing today's moisture, adding weight and stress to structures.  MEMA says flat, commercial roofs are most susceptible to collapse if they're not draining properly.