For houses, structurally, the basement walls typically form the foundations. In warmer climates, houses often do not have basements. In colder climates, the foundation must be below the frost line. Unless constructed in very cold climates, the frostline is not so deep as to justify an entire level below ground. Some designs elect to simply leave a crawlspace under the house rather than a full basement. Most other designs justify further excavations to create a full height basement sufficient for another level of living space. Even so, basements in North America are typically only 7 feet 10 inches in height rather than the standard full 8 feet of the main floors. Older homes may have even lower basement heights as the basement walls were concrete block and thus could be customized to any height. Modern builders offer higher basements as an option.
The concrete floor in most basements is structurally not part of the foundation; only the basement walls are. If there are posts supporting a mainfloor beam, these posts typically go right through the basement floor to a footing underneath the basement floor. It is the footing that supports the post and the footing is part of the house foundation. Loadbearing wood stud walls will rest directly on the concrete floor. Underneath the concrete floor is typically gravel or crushed stone to facilitate drainage. The floor is typically 4 inches thick and rests on top of the foundation footings. The floor itself is typically sloped towards a drain point in case of leaks.
Basement floor drains needs to be filled regularly to prevent the trap from drying out and sewer gas escaping into the basement. The drain trap can be topped up automatically by the condensation from air conditioners or high-efficiency furnaces. A small tube from another downpipe is sometimes used to keep the trap from drying out. Some advocate the use of special "radon gas" traps. In areas where storm and sanitary sewers are combined and there is the risk of sewage backing up, backflow prevention valves in all basement drains is recommended.
A "walk out" basement is a modern architectural form where the house is situated on a slope and part of the basement is above ground. Occupants can "walk out" at that point without having to use stairs. For example, if the ground slopes downwards towards the back of the house, the basement is at or above grade (ground level) at the back of the house. It is a modern design because of the added complexity of uneven foundations: where the basement is above grade, the foundation is deeper at that point and still must be below the frostline.
A "look out" basement is where the basement walls extend sufficiently above ground level that some of the basement windows are above grade. Where the site slopes only gently and insufficient for a walk out basement, a look out basement will result. Sometimes, a look out basement is deliberately constructed even on a flat site. The advantage is that the basement windows are all above grade. The disadvantage is that the main floor entry is above grade as well requiring steps to get up to the main floor. The "raised" bungalow design solves this by lowering the entry half-way between the main floor and basement to make a dramatic, high-ceiling foyer. It is a very economical design because the basement is shallower and excavation costs are minimized.
A "walk up" basement is any basement that has an exterior entrance via a stairwell. Some designs cover the stair well with angled "basement doors" to keep rain water from accumulating in the stairwell.
Other than with walk out or look out basements, windows in basements require a window well and are below grade. Clear window well covers may be required to keep the window wells from accumulating rain water.
If the water table outside the basement is above the height of the basement floor, then the foundation drains or "weeping tiles" outside the footings may be insufficient to keep basement dry. A sump pump may be required. It can be located anywhere and is simply in a well that is deeper than the basement floor.
Even with functioning sump pumps or low water tables, basements may become wet after rainfall due to improper drainage. The ground next to the basement must be graded such that water flows away from the basement wall. Downspouts from roof gutters should drain freely into the storm sewer or directed away from the house. Damp-proofing or waterproofing materials are typically applied to outside of the basement wall. It is virtually impossible to make a concrete wall waterproof over the long run so drainage is the key. There are drainage membranes that can be applied to the outside of the basement that create channels for water against the basement wall to flow to the foundation drains.
Since heat rises, basements are typically cooler than the rest of the house. In summer, this makes basements damp due to the higher relative humidity. Dehumidifiers are recommended. In winter, additional heating such as a fireplace or baseboard heaters are required. The finished floor is typically raised off the concrete basement floor. Radiant heating systems may be embedded right within the concrete floor. Even if unfinished and unoccupied, basements are heated in order to ensure relative warmth of the floor above. It is recommended that the basement walls be insulated to the frost line.
The furnace, water heater and central air conditioning system of the house is typically located in the basement. The electrical distribution system, telephone switch, cable television distribution point and alarm systems are also typically located in the basement. The main water cut-off valve is usually in the basement. Basements often have "clean outs" for the sanitary and storm sewers where these pipes can be accessed. The storm sewer access is only needed where the weeping tiles drain into the storm sewers.
When initially built, basements are typically unfinished. The main floor joists are exposed and the walls and floors are concrete (with insulation where appropriate). Unfinished basements allow for easy access to the main floor for renovations to the main floor. Finishing the basement can add significant floor space to a house (doubling it in the case of a bungalow) and is a major renovation project.