For uses denoted with a “U” in Section 42-191 (Use Group IX – general use allowances), a use may be open or enclosed except that:

1. building materials or contractors’ yard, including sales, storage, or handling of building materials, may be open or enclosed provided that any yard in which such use is conducted is completely enclosed on all sides by a solid opaque fence or wall (including opaque solid entrance and exit gates) of suitable uniform material and color, at least eight feet in height and constructed in accordance with rules and regulations to be promulgated by the Commissioner of Buildings;
2. boat storage may be conducted outside a completely enclosed building only if located at a distance greater than 100 feet from a Residence District boundary; and
3. public parking garages may be open or enclosed, provided that no portion of such use shall be located on a roof other than a roof which is immediately above a cellar or basement.