At 14:00 hrs we were dispatched to a structure fire on Dusty Rd... Units were advised that it was an old trailer with fire under the floor... Engine 85-4 arrived on scene and found a small fire under the floor... Firefighters extinguished and did overhaul to the flooring...The fire started when demolotion crew was dismantiling mobile home and a spark started the fire... Under control at 1420 and units returning at 1446...Units on scene:85-8, 85-3,85-4,Rescue 85,Tanker 85,and DSFP.