Second, various other construction materials can be used to upgrade
existing walls. These materials include wood, dirt, sand, gravel, brick, and
concrete blocks. To upgrade hollow block walls, materials such as dirt,
sand, and gravel should be added to a fill height of 6 feet (1.8 m) or more.
Other materials such as brick or concrete blocks could be added either to the
inside or the outside of existing weak walls to defeat a projectile threat.
It appears feasible to upgrade walls with brick or block on the outside
because: (1) footers to support the added weight of the new wall can be
installed more easily on the outside than the inside; (2) usually fewer
obstructions such as pipes, receptacles, vents, or doors are found on the
outside of a building rather than the inside; (3) usable work space is not
reduced; (4) interference with daily operations of the protected areas is
minimal; and (5) the upgraded walls sometimes can improve the appearance of
the structure. Conversely, it is possible that the existing structure might
be ornate or unusually decorative, in which case upgrading it with brick or
blocks could cause some degradation of its appearance.