In all nonresidential zones, outdoor storage is only permitted in the side and rear yards, as herein regulated. No article, equipment, vehicle, supplies or materials shall be kept, stored or displayed outside the confines of any building unless and until the same is screened by special planting or fencing, as pursuant to Chapter 22, Fences, and as approved by the Planning Board, and maintained in good condition so that it shall not be visible from any adjacent property or public street. Any fence required to screen the outdoor storage of flammable material otherwise permitted by this chapter shall not be closer than 20 feet to any property line on the lot upon which it is erected.