Building a poured concrete basement foundation is one of the most critical phases in constructing a home. It provides the necessary support for the entire structure, ensuring stability and longevity. In this post, we’ll walk through the detailed process of how a poured concrete basement foundation is constructed, including some additional steps you may not have considered and clarifying any common misconceptions.
Surveying the Site and Determining Excavation Depth
The first step in building a poured concrete basement foundation is surveying the site to establish the building’s boundaries and the required excavation depths. This is typically done by a professional surveyor, who uses specialized equipment to mark the perimeter of the foundation. These boundary markers indicate where the foundation walls will be located, ensuring that the home’s footprint matches the design.
To determine how deep the excavation needs to go, excavators rely on a “cut sheet,” which details the precise depth of the foundation from the current grade level. This cut sheet is essential for ensuring that the excavation is accurate, as the foundation must be placed below the frost line to prevent issues with freezing and thawing, which could compromise the structural integrity.
Excavating the Site
With the boundary survey completed and the cut sheet in hand, the excavation can begin. Excavators typically start at one end of the site and methodically dig their way across, removing soil in manageable layers. The soil is either stored on-site, if space allows, or trucked off-site if there isn’t enough room. The latter option can be quite costly, as it often involves multiple dump truck loads, significantly increasing the project’s cost.
As the excavation reaches its final depth, the excavator will make several passes to ensure the bottom of the excavation is as level as possible. This is crucial for the next step, as the foundation footers—on which the foundation walls will rest—must sit on a level surface to ensure the stability and even distribution of the home’s weight.
Marking and Forming the Footers
Once the hole is excavated, the surveyor returns to the site to mark or “pin” the locations for the footers. These markers are typically placed at the corners and critical points along the future foundation walls. The pins guide the placement of the footers, which are the wide, shallow concrete bases that support the weight of the foundation walls and the home itself.
After the pins are in place, the footer boards are installed. Footers are usually wider than the foundation walls by several inches and vary in depth depending on the load they will bear. In areas where structural elements like beams or columns will be located, the footers are often made deeper and wider to provide additional support. The size and design of the footers are determined by the structural engineer, based on factors such as soil conditions and the overall weight of the structure.
Adding Rebar for Reinforcement
In many cases, rebar is added to the footers to provide extra strength and stability. Rebar (short for reinforcing bar) is made of steel and is used to help the concrete resist tension and cracking. Typically, two or three lines of rebar are laid parallel to each other along the length of the footers, with several inches of spacing between them. The size and number of rebar pieces depend on the specific structural requirements of the home. Adding rebar isn’t always required, but it’s a common practice for ensuring a strong foundation, particularly in larger or heavier homes.
Inspecting the Footers
Before pouring concrete, the footers must be inspected by either a structural engineer or a city building inspector. This step is crucial to ensure that everything has been installed correctly and that the footers meet all local building codes and engineering requirements. Without this inspection, the project cannot move forward to the concrete pour.
Pouring the Footer Concrete
Once the footers pass inspection, it’s time to pour the concrete. Depending on the design of the home, the type of concrete used can vary. Most residential projects use concrete with a strength rating between 20 and 30 MPa (megapascals). In areas with tight access or complex terrain, a concrete pump truck may be used to transport the concrete into the footers.
After pouring, the footers are left to set. Concrete begins to harden almost immediately, but it takes about 24 to 48 hours before any additional work can continue, depending on the weather and conditions.
Forming the Foundation Walls
With the footers set, the next step is to form the foundation walls. Foundation walls can be constructed using different materials, but for the majority of traditional homes, wood forms are used. In other cases, insulated concrete forms (ICF) or even 3D-printed forms may be employed, but these methods are less common in typical residential construction.
The forms are installed vertically on top of the footers. Typically, the first side of the wall is formed, followed by any necessary reinforcements or rebar inside the wall cavity. Then, the second side of the wall form is installed, creating a cavity between the forms where the concrete will be poured. This process can take several days or even weeks, depending on the size and complexity of the foundation. The forms are heavy, labor-intensive to install, and require careful alignment to ensure straight, plumb walls.
Pouring the Foundation Walls
Once the forms are installed, concrete is poured into the wall cavities. A pump truck is often used to pour the concrete efficiently and evenly, especially for larger foundations where many truckloads of concrete are required. After the concrete is poured, it needs time to cure. Although full curing takes approximately 28 days, the forms can be safely removed within a few days.
Waterproofing the Foundation
Waterproofing the foundation is a crucial step in protecting the basement from water infiltration. After the forms are removed and the concrete has set, any small cracks or holes in the walls are filled with a watertight cement product. Then, a layer of bituminous tar is applied to the exterior of the foundation walls to create a waterproof barrier.
Next, a dimpled plastic board—commonly known as dimple board—is installed around the foundation. This board helps direct any water that seeps through the soil away from the foundation walls and down to the drainage system.
Installing the Drainage System
To further protect the foundation, a drainage system called weeping tile is installed around the base of the foundation, next to the footers. The weeping tile is a perforated pipe that allows water to enter and be carried away from the foundation. The pipe is usually wrapped in a filter sock to prevent sediment from clogging it. On top of the weeping tile, clear gravel (usually 3/4 inch) is placed to further filter water and keep the system functioning properly. The collected water is directed into a storm drain or other drainage outlet, preventing water from pooling around the foundation.
Final Inspection and Backfilling
Once the waterproofing and drainage systems are in place, the foundation must undergo one final inspection to ensure everything is up to code. After receiving approval, the excavation can be backfilled. The soil that was originally removed is pushed back into place around the foundation, taking care not to damage the waterproofing materials.
Completion of the Foundation
With the foundation poured, waterproofed, and backfilled, the job is complete! This strong, durable base will support the entire house, ensuring it stays safe, dry, and stable for decades to come.
Building a poured concrete basement foundation is a multi-step process that requires careful planning, precision, and a skilled team of professionals. From surveying the land to pouring the final walls, each stage plays a crucial role in creating a solid and dependable home foundation. With the right approach, you can ensure your basement foundation is built to last.
Stay tuned for future posts where we’ll dive deeper into alternative foundation methods, additional waterproofing techniques, and more construction insights!