Outdoor storage of lumber, building materials, contractors' equipment, trucks, vans, buses, passenger motor vehicles or any combination thereof, whether new, used or in use, for sale or for hire, provided that:
(a) Outdoor storage operations shall be enclosed, except for entrances and exits, by a locking fence as follows:
[1] For enclosing a towing storage yard or parking lot for private passenger vehicles, such fence shall not be less than four feet in height.
[2] For enclosing any other outdoor storage permitted above, such fence shall be six feet in height and of solid construction.
(b) Storage of lumber and other similar combustible material shall not be nearer than 15 feet to any side or rear lot line bordering on a residential district.