The Difference Between Block Foundations and Poured Foundations
Dec 16, 2021
Foundations are essential to the structural integrity of homes and buildings, as they provide strength, balance, and protection from water and rot. When constructing the foundation, first the concrete footings—the initial point of contact between the ground and the building—are formed. Once those are secure, foundation walls are built on top, helping to distribute a building’s weight evenly and protect the structure from shifts in soil and moisture that weaken the ground underneath.
There are several types of foundations, each with their own characteristics and advantages. The most common are concrete block and poured concrete foundations.
What is a concrete block foundation?
Concrete block foundations, or Cinder Masonry Units (CMUs), are made from stacked cinder blocks. Using the CMU method, the footings are coated with mortar—a mixture of sand, water, and fillers to bind masonry—then cinder blocks are layered evenly on top. The process is repeated until a foundation wall is formed.
Because cinder blocks are hollow, block foundations are often reinforced with rebar––vertical steel bars that strengthen concrete. Once the rebar is threaded through the cinder blocks, the surrounding gaps are filled with concrete. Rebar isn’t necessary for structures like garden walls, but is often used for block foundations that support heavy structures.
Advantages of concrete block foundations
- A straightforward process. Laying walls one block at a time helps avoid mistakes, and special equipment and machinery aren’t required.
- Easier to transport. Cinder blocks are lighter than poured concrete, making them easy to move from one place to another.
- Provides compressive strength. Block foundations, especially ones reinforced with rebar, can bear heavy vertical loads.
- Not reliant on concrete. Some block foundations use concrete to fill in cinder blocks and rebar, although the concrete doesn’t need to be precisely poured and cured. Concrete is prone to cracking and air pockets, which can compromise a foundation’s integrity.
Disadvantages of concrete block foundations
- Prone to leaks. Gaps in the mortar between cinder blocks can make concrete block foundations vulnerable to water damage.
- Lack lateral strength. Block walls without rebar may bow from groundwater or shifts in the soil (lateral pressure).
What is a poured concrete foundation?
Poured concrete foundations use wooden wall forms built on top of foundation footings. These forms act like molds, shaping foundation walls. These wooden walls are built around steel rebar supports, both vertical and horizontal, which strengthens the foundation walls. Next, concrete mixed on-site is poured into the wall forms. Once it cures, or solidifies, the wall forms are removed, leaving concrete shaped like a wall that holds back the earth surrounding the home.
Advantages of poured concrete foundations
- A predictable process. Poured walls take less time than concrete block foundations to build since most of the work is done with machinery.
- Not prone to leaks. Solid concrete walls are less prone to water damage.
- High lateral strength. Solid concrete walls can withstand pressure from all sides. Poured concrete walls don’t have multiple openings like block foundations, which can increase the likelihood of a joint weakening over time.
Disadvantages of poured concrete foundations
- Requires specialized equipment. Pouring concrete and inserting wooden forms requires a lot of preparation, and bulldozers, soil compactors, and cranes may be needed to move heavy loads. Also, the concrete-pouring process, which takes place in one step, may create weak spots or air pockets.
- Harder to transport. Large wooden forms are harder to move and might require cranes to install, which can increase building and labor costs.
How to determine which foundation is right for you
Both types of foundation are popular, so deciding between the two really comes down to factors like moisture, budget, and timeline. Poured concrete tends to be less vulnerable to water, though it does require special equipment, more preparation, and potentially more money. Block foundations are simpler to build, but might be prone to leaks if they’re not mortared evenly. For either poured or block walls, using rebar is an effective way to bolster lateral strength.