A mobile roof safety device and method of use for arresting the fall of a roof worker off an elevated flat roof is described. The mobile device comprises a displaceable frame having a weighted base adapted to be positioned on a flat roof surface for restraining a load. At least one actuable surface gripping assembly having a pivoting arm to which a safety line is attached for the engagement of a gripping formation is secured to the weighted base. When a pulling force is applied on the pivot arm by a worker attached to the safety line secured to the pivot arm, the gripping formation is biased in the flat roof surface to arrest the fall of the worker.