base·ment (bâs’mənt) n.
- The substructure or foundation of a building either partly or below grade. .
- The lowest habitable story of a building, usually below ground level.
- The lower part of the wall or walls of any building.
- The substructure of a column or arch. Frequently, the applicable building code specifies that only one floor level shall be classified as a basement.
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 water line entry point in areas where there is a frost line, the electrical distribution system, and cable television distribution point.
For most of its early history, the basement took one of two forms. It could be little more than a cellar, or it could be a section of a building containing rooms and spaces similar to those of the rest of the structure, as in the case of basement flats and basement offices.
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.
Types of Basements
- Types of basement by structural design
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 frost line.
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 WWII.
Except for Britain, 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.
A crawlspace (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. While this cannot be used as living space, it can be used as storage, often for infrequently used items. These can be placed directly on the dirt, but it is more desirable to finish with either plastic or some sort of wood or concrete flooring. These are rarely included in houses as standard due to health and safety issues. However, they are frequently added on by home owners installing a small door into the foundations.
Design and Structural Considerations
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 frost line. 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, 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 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 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, 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. It is recommended that the basement walls be insulated to the frost line. 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. 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:
- Tanking – Systems that bond to the basement structure and physically hold back groundwater.
- Cavity Drainage – Dimpled plastic membranes are used to line the floors and walls of the basement, creating a “drained cavity.” Any water entering this drained cavity is diverted to a sump pump and pumped away from the basement.
- Exterior Foundation Drain – Installing an exterior foundation drain that will drain away by gravity is the most effective means to waterproof a basement. An exterior system allows water to flow away from the basement without using pumps or electricity. An exterior drain also allows for the installation of a waterproof membrane to the foundation walls.
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:
- Gravity system
- No pumps or electric required
- Membrane applied to exterior walls to prevent dampness, mold, moisture, and soil gassed from entering the home
- Permanent solution
- The Unfinished Basement
This first unfinished design, although typically larger, remains common today, usually containing within it a water heater, various pipes running along the ceiling and downwards to the floor, and sometimes a workbench, a freezer or refrigerator, or a washer/dryer set. Boxes of various materials, and objects unneeded in the rest of the house, are also often stored here; in this regard, the unfinished basement takes the place of the more traditional attic. The basement can contain all of these objects and still retain the title of “unfinished,” as they are either mostly or entirely functional in purpose.
- Partially-finished Basement
There is another class of objects, partly on the way to the appellation of “finished,” but not completely so, which are often found in this variety of basement. These include weight sets and other exercise equipment, the boom boxes or entertainment systems used during exercise, musical instruments (which are not in storage; an assembled drum set would be the most easily identified of these), foosball tables, chairs, couches and entertainment appliances of lesser quality than those in the rest of the house, sports pennants and/or other types of posters which are attached to the wall, and loose carpet which has been placed over the concrete floor.
As the description suggests, this type of basement, which 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.
Toilets and showers can exist in this variety of basement, as many basements allow for this in their construction.
- Finished Basements
It is increasingly common for a homeowner to finish a basement by putting in walls, flooring, ceiling, paint, and furniture. Usually homes are constructed with unfinished basements, which are later finished by the homeowner. Finished basements may contain extra bedrooms, an entertainment room, or even a separate apartment for a tenant.
Before finishing a basement, one may consider waterproofing the basement. Basement waterproofing is very important because you do not want to get water in your basement after you have finished your basement. Due to heavy rain or other unfortunate events, a basement can be flooded and may cause the content of the basement to get ruined.