What Is a Mat Foundation?

Mary McMahon

A mat foundation is a type of shallow foundation made by pouring concrete over a reinforcing mat of material, usually rebar. This design is also known as a raft foundation in some regions. A mat foundation distributes the weight of the structure very close to the surface, rather than deep into the ground, and it is suitable for a range of environments. If an engineer thinks that a mat foundation is appropriate or necessary for a structure, he or she can design the specifications to meet the need and can supervise the creation of the mat and the pouring of the foundation.

Batter boards may be used to mark the corners of the area before the foundation is poured.
Batter boards may be used to mark the corners of the area before the foundation is poured.

The depth of this style of foundation can vary and will depend on the specifications of the project. Engineers can find mat foundations helpful when they need space under a structure for underground facilities or when they build on potentially unstable soils. The foundation starts with a tight mat made from rebar to create a lattice of support. Some designs include plastic blocks for weight distribution that will also cut down on the amount of concrete needed.

Fluctuations in temperature, ground water, and air quality can affect the overall strength of poured concrete.
Fluctuations in temperature, ground water, and air quality can affect the overall strength of poured concrete.

Construction workers might require several days to set up and secure all of the rebar. Tensioner bars can be placed inside the mat to keep it stable. The workers pour concrete continuously over the foundation until the mat is entirely covered, and they use tools to vibrate it into place to eliminate air bubbles and any other issues that arise, before finishing off the top and allowing the foundation to set. After the foundation is finished curing, building can progress.

Building a mat foundation for a small area like a garden shed can usually be done without the help of an engineer or a foundation specialist.
Building a mat foundation for a small area like a garden shed can usually be done without the help of an engineer or a foundation specialist.

The finished mat foundation should distribute the weight evenly, without specific load-bearing pillars that send the weight of the structure deep underground. It can cover a large area and will remain stable and solid in a variety of conditions. Mat foundations can be used for residential and commercial construction. Foundation work such as repairs can be challenging with this design because it is a flat, closed area of concrete without easy access points.

Engineers and foundation specialists typically are needed to design a mat foundation unless the area is very small, such as a foundation under a shed. They review the work site and the building specifications to determine what kinds of loads will be present and how the soil under the building might behave. This information is valuable for the stage of the process when they design the foundation, including the configuration of supports in the mat to distribute weight and maintain tension across the finished foundation.

A mat foundation typically uses rebar to strengthen the concrete and better distribute the weight of the structure.
A mat foundation typically uses rebar to strengthen the concrete and better distribute the weight of the structure.

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