Any building which houses a nonconforming use and which is damaged or destroyed by less than 50% of its sound value or floor area, exclusive of the foundation, from any cause, may be restored, but not enlarged, and the nonconforming use reinstated within six months of such damage. If the restoration of such building is not completed within the said six-month period, the physical operation of the nonconforming use of such building shall be deemed to have ceased, unless such nonconforming use shall have been carried on without interruption in the undamaged portion of such building.