A basement is one or more floors of a building that are either completely or partially below the ground floor. Slab-on-grade buildings do not have basements. Basements are typically used as a utility space for a building where such items as the furnace, water heater, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable television distribution point.
In British English the word 'basement' is used for underground floors of, for example, department stores, but is rarely used for a space below a house, and the word cellar is used to apply to any such large underground room. Subcellar is a cellar that lies further underneath.
However, beginning with the development of large, mid-priced suburban homes in the 1950s, the basement, as a space in its own right, gradually took hold. Initially, it was typically a large, concrete-floored space, accessed by indoor stairs, and with exposed columns and beams along the walls and ceilings, or sometimes, walls of poured concrete or concrete cinder block.
A "walk-out" or "daylight" 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 the 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 must still be below the frostline.
In a "look-out" basement, the basement walls extend sufficiently above ground level that some of the basement windows are above ground level. Where the site slopes gently and is 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, necessitating 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 stairwell with angled “basement doors” or "bulkhead doors" to keep rain water from accumulating in the stairwell.
When initially built, the main floor joists are often exposed and the walls and floors concrete (with insulation, where appropriate). Unfinished basements allow for easy access to the main floor for renovation 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.
A cellar is a type of basement, primarily used for the storage of food and drink (especially wine) for use throughout the year. A cellar is intended to remain at a constant cool (not freezing) temperature all year round. Cellars are more common in older houses than in modern houses, and were important shelters from air raids during World War II. In parts of the U.S. that are prone to tornadoes (See: Tornado Alley), cellars still serve as shelter in the event of a direct hit on the house from a tornado or other storm damage caused by strong winds.
Except for Britain, Australia and New Zealand, cellars are popular in most western countries. In Britain, people tend to store food and drink in a garage, if at all. However, the majority of continental Europeans have cellars. In North America, cellars usually are found in rural or older homes on the coasts and in the South. However, "full" basements are commonplace in new houses in the US Midwest and other areas subject to tornado activity or requiring foundations below the frost line.
Cellar can also be used in American sports terminology to describe the bottom team in an athletic division.
A crawl space (as the name suggests) is a type of basement in which one cannot stand up — the height may be as little as a foot, and the surface is often soil. They offer a convenient access to pipes, substructures and a variety of other areas that may be difficult or expensive to access otherwise. While a crawlspace cannot be used as living space, it can be used as storage, often for infrequently used items. Care must be taken in doing so, however, as water from the damp earth, humidity entering from crawlspace vents, and moisture seeping through porous concrete will create a perfect environment for mold, mildew to form on any surface in the crawlspace, especially cardboard boxes, wood floors and surfaces, drywall and some types of insulation.
Health and safety issues must be considered when installing a crawl space. As air warms in a home, it rises and leaves through the upper regions of the house, much in the same way that air moves through a chimney. This phenomenon, called the "stack effect", causes the home to suck air up from the crawlspace into the main area of the home. Mold spores and fecal material from dust mites in the crawlspace come up with the air, agitating breathing problems such as asthma and creating a variety of health-related problems.
These can be placed directly on the dirt, but it is more desirable to finish with a plastic vapor barrier that will not support mold growth or allow humidity from the earth into the basement This helps to insulate the crawlspace and discourages the habitation of insects and vermin by breaking the ecological chain by which the insects feed off the mold and vermin feed on the insects as well as creating a physical inorganic barrier that deters their entrance into the space. Almost unheard of in the 1990s, these barriers have become increasingly popular in recent years.
Structurally, for houses, the basement walls typically form the foundation. In warmer climates, houses sometimes do not have basements because they are not necessary (although many still prefer them.) In colder climates, the foundation must be below the frostline. Unless constructed in very cold climates, the frost line is not so deep as to justify an entire level below the ground, although it is usually deep enough that a basement is the assumed standard. In places with odd stratified soil substrata or high water tables, such as most of Texas, Oklahoma, Arkansas, Tennessee, Mississippi, Alabama, Georgia, Louisiana, and Florida, basements are usually not financially feasible unless the building is a large apartment or commercial structure.
Some designs elect to simply leave a crawl space 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 Canada and the northern United States were typically only 7 feet 10 inches (2.39 m) in height, rather than the standard full 8 feet (2.44 m) 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 cost of the additional depth of excavation is usually quite expensive. Thus, houses almost certainly never have multi-story basements though 9' basements heights are a frequent choice among new home buyers. For large office or apartment buildings in prime locations, the cost of land may justify multi-story basement parking garages.
The concrete floor in most basements is structurally not part of the foundation; only the basement walls are. If there are posts supporting a main floor beam to form a post and beam system, 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. Load-bearing wood-stud walls will rest directly on the concrete floor. Under the concrete floor is typically gravel or crushed stone to facilitate draining. The floor is typically four inches (100 mm) thick and rest on top of the foundation footings. The floor itself is typically sloped towards a drain point, in case of leaks.
Since warm air 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 may be required. A well-defined central heating system may minimize this requirement. Heating ducts typically run in the ceiling of the basement (since there is not an empty floor below to run the ducts). Ducts extending from the ceiling down to the floor help heat the cold floors of the basement. Older or cheaper systems may simply have the heating vent in the ceiling of the basement.
The finished floor is typically raised off the concrete basement floor though modern laminate flooring is typically placed on concrete floors in Canada with a thin foam underlay. 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, and to prevent water supply pipes, drains, etc. from freezing and bursting in winter. It is recommended that the basement walls be insulated to the frostline. In Canada, the walls of finished basements typically are insulated to the floor with vapor barrier(s) to prevent moisture transmission.
Basement floor drains need to be filled regularly to prevent the trap from drying out and sewer gas from 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 devices in all basement drains may be mandated by code and definitely are recommended even if not mandated.
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.
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. There should be drains in the window well, connected to the foundation drains.
If the water table, outside the basement, is above the height of the basement floor, then the foundation drains or the weeping tiles outside the footings may be insufficient to keep the 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. Downspouts should not be connected to the foundation draintiles. If the draintiles become clogged by leaves or debis from the rain gutters, the roof water would cause basement flooding through the draintile. 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 draining 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.
Where drainage is inadequate, waterproofing maybe needed. There are numerous ways to waterproof a basement, but most systems fall into one of three categories:
The waterproofing system can be applied to the inside or the outside walls of a basement. When waterproofing existing basements it is much cheaper to waterproof the basement on the inside. Waterproofing on the outside requires the expense of excavation, but does offer a number of advantages for a homeowner over the long term. Among them are:
As the description suggests, this type of basement, which also might be called “half-finished,” is likely used by teenagers and children. The entire family might utilize a work-out area. It is also common to have a secondary (or primary) home office in a partially-finished basement, as well as a workbench and/or a space for laundry appliances.
Toilets and showers sometimes exist in this variety of basement, as many North American basements are designed to allow for their installation.
In Canada, historically the basement area was excluded from advertised 'square footage'. For example, a "2,000 square foot bungalow" would in reality have of floor space. More recently, finished space has become increasingly acceptable as a measure which includes the developed basement areas of a home. However, some jurisdictions may require some type of egress (through either egress style windows or, in the case of a walk-out basement, a door) to include the basement square footage, due to fire code requirements.