An essential part of any building structure, foundations, and footings have one main purpose: to distribute the weight of the building. Often built underground, foundations work by transferring the weight of load-bearing walls directly to the earth below. Footings in construction help distribute the weight evenly across the structure, sometimes connected to a stem wall so that the building doesn’t sink into the ground.
In this guide, we explain the various types of building foundations—both shallow and deep—and how they are used in different kinds of construction projects.
Shallow Foundations vs. Deep Foundations
Commonly used for smaller and lighter buildings, shallow foundations typically have more width than depth. This makes them both cheaper and faster to build.
Lighter projects like residential homes and wooden structures can make use of this type of house foundation. When the lowest part of the finished structure extends less than six feet deep—and the earth below it has the sufficient weight-bearing capacity—a shallow foundation may fit the bill.
Heavy buildings like skyscrapers and shopping centers, on the other hand, typically require deep foundations. Deep foundations provide a stable base in soft or weak soil that cannot support the weight of the building. As a general rule, the taller the building, the deeper the foundation it requires.
The strength of deep foundations makes them useful even in water environments, where they provide strong support for bridges, piers, and dams.
Types of Shallow Foundations
Shallow foundations come in a variety of types. The ideal choice depends on the specific structural characteristics and soil conditions of your building construction.
1. Mat (Raft) Foundation
Consisting of a single large continuous rectangular or circular slab under a building, the mat (or raft) foundation carries and distributes an entire load of a structure. Raft foundations can support a number of columns and walls at once and spread the load out under the entire footprint of the building.
When the soil layer beneath a building has low stability or bearing capacity, mat foundations can reduce differential settlement. Mat foundations are common in commercial building projects and in areas where basements are popular.
- What it looks like: Large continuous rectangular or circular concrete slab
- Often used for: Basements or commercial buildings
2. Spread Footing
Footings are the bottom part of a foundation that actually touches the ground. They help transfer the weight of the building directly to the soil, as well as prevent moisture from seeping into the structure, which can cause mold and mildew problems. Different types of foundations use different types of footings: concrete footings are common, but stone, brick, and wood footings also exist.
A type of shallow foundation known as spread footing (or pad footing) has a wider base than the top. This design helps spread the weight of the structure to a larger area, creating greater stability. Spread footing comes in various sizes and shapes, including square, round, and rectangular.
Common uses of spread footings in construction include supporting individual columns of a building or piers of a bridge. Residential home foundations also frequently employ spread footings as a cost-effective way to increase stability.
The advantages of spread footing include cost savings, easier construction of basements, and decreased risk of foundation cracking or collapse. On the downside, spread footings are limited to certain types of soil and settlement can be a significant issue.
- What it looks like: The base is made wider than the top
- Often used for: Residential buildings, walls, or masonry columns
Depending on the type of construction project at hand, you may see the following types of spread footings:
- Isolated footings: With isolated footing (or “isolated column footing”), each column has a separate footing in order to distribute its load uniformly over the soil.
- Combined footings: In this type of footing, a single base supports two or more columns together.
- Continuous footings: With this type of footing, more than two columns in a row share a base. You may use continuous footing when you have soil with low load-bearing capacity and/or columns spaced too close together for individual footings.
- Grillage footing: Grillage foundations provide stability when excessive column weight—such as in high-rise buildings—and weak soil prevent the construction of a deep foundation.
- Raft footing: Raft footing provides common footing to multiple columns, ensuring uniform weight distribution when the column load is excessive or the load-bearing capacity of the soil is low.
- Strip footing: Strip footing runs beneath load-bearing walls to help maintain the stability of a structure.
3. Basement Foundation
Basement foundations serve the same purpose as regular foundations: to distribute a building’s weight evenly and protect it from moisture and water. Unlike typical foundations, however, basement foundation walls are fully submerged in the soil to create working or storage space below the ground level.
Because basements are typically completely below ground level, they can take longer to build than standard foundations. They often require heavy-duty equipment like excavators, cranes, and graders to dig and move soil. Despite the difference in depth, basement foundations are constructed in the same way as regular foundations, using poured concrete, concrete blocks, or precast concrete slabs.
- What it looks like: Concrete walls consisting of footings
- Often used for: Residential buildings
Depending on the area you’re working in and your client’s requests, you may encounter these two types of basement foundations:
- Full basement: Full basements are submerged on a level plane that covers the entire building perimeter. You can opt to leave a basement unfinished or install insulation, drywall, and flooring to create living space below ground.
- Daylight: If a house rests on a slope, a daylight basement foundation makes more sense. As the name suggests, one side of the basement sits above ground (letting in daylight) while the other side is completely submerged. Daylight basements are ideal for homeowners who want walkout basements that allow them to access yards or patios.
4. Crawl Space Foundation
Typically elevated a few feet off the ground, crawl space foundations are deeper than standard foundations but more shallow than basement foundations. These foundations leave a small protected space of about three to four feet high under the house, providing access to drainage pipes, plumbing, and other elements running beneath the house, but not enough room for most people to stand.
In addition to making it easier to reach plumbing and piping, crawl spaces keep houses cooler in warmer climates by allowing air to move freely underneath the structure. Although crawl space foundations tend to cost less to build than other basement foundations, you will also need to consider insulation and moisture control. Moisture control may mean installing a vapor barrier, a sump pump, or both.
- What it looks like: Elevated concrete walls with footings
- Often used for: Sloped lots or residential buildings
5. Concrete Slab Foundation
The most common type of foundation, slab foundations consist of a 6- to 8-inch concrete pad underneath a structure. The load your foundation needs to bear determines the thickness of the slab. Although less suited to extremely cold climates that get a lot of freezing temperatures and snow and earthquake-prone areas, slab foundations tend to be relatively cheap, easy to build, low maintenance, and energy efficient.
Constructing a simple slab foundation involves pouring concrete directly onto soil prepared with sand or gravel to assist with drainage. Other slab foundations have additional components like supporting concrete feet or insulating foam.
- What it looks like: 6- to 8-inch-thick slabs of concrete
- Often used for: Residential or commercial buildings
Slab foundations come in three common types:
- T-shaped: A traditional foundation in colder climates, this design sits on t-shaped feet placed in the ground below the frost line. Foundation walls go on top, followed by a slab between the walls. Requiring three separate concrete pours, t-shaped foundations take longer to finish and tend to cost more. However, they also provide more structural integrity and greater support for load-bearing walls.
- Slab-on-grade: The simplest type of foundation, slab-on-grade consists of a single layer of concrete poured directly onto prepared soil. Although you can insulate slab-on-grade foundations, they work best in climates where the ground does not freeze.
- Frost protected: Designed for colder climates, frost-protected foundations use polystyrene sheets to insulate the structure against frost heave. They have a shallower depth than T-shaped foundations and only require one pour, making them easier and quicker to build.
Types of Deep Foundations
Deep foundations extend much further into the ground than shallow foundations, allowing us to build skyscrapers, high-rise apartments, and other tall buildings. The size and type of deep foundation depend on building weight, soil conditions, groundwater level, total cost, and project timeframe.
6. Pile Foundation
Pile foundations consist of long, slender cylinders made of strong material. When pushed into the stable ground deep below the surface, pile foundations transfer the load to the desired depth and support the structure above.
A number of specific ground conditions call for pile foundations, including a layer of weak soil at the surface, high groundwater levels, deep drainage systems, or soil that is impossible to excavate to the desired depth. Pile foundations are also used on structures with heavy, concentrated loads, such as water tanks, bridges, or high-rise buildings.
Common materials used to create pile foundations include concrete, steel, and timber—and each type has disadvantages and advantages. Steel can take heavier loads and reach greater depths, but may cost more and corrode over time. Wood foundations are economical and easy to install but can’t always penetrate hard soil.
- What it looks like: Long and slender cylinder columns
- Often used for: Buildings near seashores or river beds
There are five types of pile foundations, which are classified based on their uses and functions:
- Sheet piles: Offering lateral rather than vertical structural support, sheet piles resist pressure from external sources like water and loose dirt. Builders may use them to construct retaining walls, protect from riverbank erosion, isolate the foundation from adjacent soil, and confine the soil to increase its load-bearing capacity.
- Load-bearing piles: This kind of pile transfers the load from a vertical structure into a stronger layer of soil deeper below the surface.
- End-bearing piles: With this type of pile, the bottom end rests on a particularly strong layer of soil or rock. Loads pass through the pile, bypassing weaker layers of soil and safely transferring the forces to stronger layers underground.
- Friction piles: This type of pile also transfers the load of the building above to the soil below, but it works on a different principle. The frictional force between the surface of the pile and the soil surrounding the pile transfers the forces to the soil.
- Soil-compactor piles: Unlike other types of pile foundations, soil-compactor piles do not bear any direct loads themselves. Instead, they are inserted into the ground at close intervals to increase the stability and bearing capacity of the soil.
7. Caisson Foundations
Caisson foundations consist of prefabricated hollow boxes or cylinders that serve as watertight retaining structures. Also known as pier foundations, caissons provide underwater support for bridge piers, concrete dams, or ship repairs.
Caisson foundations can be built at or near the surface of the ground and then sunk to the desired depth. To create this kind of deep foundation, engineers auger a hole in the ground, excavate or dredge material from within the caisson, and then fill it with concrete. Caissons can be drilled into bedrock or other suitably stable soil layers below the water surface.
- What it looks like: Long and slender cylindrical columns
- Often used for: Hillside homes, freeway overpasses, or beneath rivers
Caisson foundations come in various types:
- Open caissons: Generally used in the formation of a pier, open caissons have an open top and bottom. After being sunk into the water, these caissons are pumped dry and then filled with concrete.
- Pneumatic caissons: Mainly used for underwater construction, pneumatic caissons have a pressurized working chamber that prevents mud and water from entering.
- Monolithic caissons: These large single-column caissons are made of reinforced concrete.
- Sump caissons: Often used by offshore oil drillers to recirculate contaminated water, sump caissons have the ability to pump water from below.
- Box caissons: Typically made of heavy timber, these watertight structures are closed at the bottom and open at the top. They are typically floated to their intended location and then sunk into place with a masonry pier.
8. Buoyant Foundation
Designed to support heavy loads on soft soil surfaces, buoyant foundations act as floating substructures. Whether you know them as floating raft foundations, hollow box foundations, or compensated foundations, the purpose remains the same: to reduce the load intensity over the soil.
Buoyant foundations balance the weight of excavated soil with a structure of the same weight, reducing settlement in soft or weak soil. Also useful for home building in flood-prone areas, buoyant foundations elevate structures above high water levels.
- What it looks like: Blocks or pillars underneath the house
- Often used for: Buildings that are in high-risk flooding areas
Choose The Right Type of Foundation
Choosing the right type of foundation involves having a good grasp of your project specifics. You will need to consider the building construction type, soil conditions, local climate, necessary materials, cost, and any other relevant variables such as high water tables or nearby bodies of water.
