Waterproofing is defined as the treatment of a surface or structure to prevent the passage of water under hydrostatic pressure. Water exerts a pressure of 450 kg/m (62.4 lb/ft) of depth. Therefore, water lying against a barrier exerts a steadily increasing pressure as the depth increases. A waterproofing treatment must keep the water from penetrating into the building interior.

Waterproofing is used:

• To protect floors and walls below grade of buildings, tunnels, and similar structures from groundwater.
• To protect spaces beneath roofs and plaza decks.
• To isolate wet spaces such as kitchens, showers, and mechanical equipment rooms from other areas of buildings.
• On bridge and parking garage decks to protect against deterioration from de-icing salts and to help minimize the negative effects of thermal expansion of the structural elements and topping materials.
• To keep water from leaking from pools, planters, lagoons, irrigation trenches and dams, or into basements and other underground structures.

Waterproofing materials must perform for an extended period of time, usually for the life of the structure. It is expensive, if not impossible, to excavate around the foundation walls or to remove a reinforced concrete floor slab to repair or replace the waterproofing material. And as a result, unlike a roof membrane, a waterproofing membrane must perform completely trouble and maintenance-free for a long period of time.

The waterproofing is usually in a constantly wet environment, but there may be relative dry periods. Waterproofing is usually in continuous contact with groundwater; or is retaining water; such as in a planter or pool.

The waterproofing material must be compatible with both the soil and the substrate to which it is applied. It must resist environmental contaminants, such as acids or alkalides, and other contaminants. Some waterproofing materials are affected by soil salts, and others are affected by oils that could be spilled onto floors in mechanical equipment rooms. These contaminants are much different from those to which roof membranes are exposed.

A waterproofing material must withstand construction activity and must remain in place and intact until excavations can be backfilled or the protective wearing course can be applied. During backfill placement, rocks, construction debris, and other sharp objects may be dumped against the waterproofing material. However, the greatest threat to waterproofing often comes from other construction trades. Even though horizontal surfaces are usually covered with a protection course, other trades often use the waterproofed surface as a staging area or for access to adjacent work areas.

Below-grade concrete and masonry structures may experience settlement and/or shrinkage as the substrate materials cure. Horizontal plaza decks experience thermal movement and load deflection. It is necessary for the waterproofing material to be able to bridge small cracks, and expand and contract to some degree without rupture or failure.

Waterproofing materials are usually adhered directly to structural decks or wall surfaces and not usually exposed to direct ultraviolet radiation because they are most often buried in the ground, covered with a plaza deck surface, or installed inside the building.

After installation, waterproofing materials are usually covered with a protection course and backfilled with a permanent protection slab. People should not walk directly on the waterproofing membrane.

The most effective way to waterproof walls and floors placed against earth is to remove the water from the earth prior to it reaching the wall or floor. The waterproofing of any building can be greatly improved if the hydrostatic pressure against the waterproofing material can be reduced or eliminated entirely.