Foundation comparison: conventional footing vs. post-tension slab

    The two most common types of foundations found in central Oklahoma are conventional footing and stem wall ("conventional" for short) and post-tension slab and foundation ("post-tension" for short).

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    There are pros and cons to both approaches, so be sure to ask plenty of questions when buying or building a house.

    Here are some things to look out for.

    Overview of conventional foundation

    With a conventional foundation, you have a trench in the ground at least 18 inches deep and anywhere from 18 inches to two feet or more wide.

    Two to four bars of reinforcing steel, commonly called rebar, are suspended in the trench and then it's filled with concrete to form a continuous solid beam of reinforced concrete around the perimeter of the home. This beam is called the footing or footer, and it creates the foundation of your home.

    Next, a formed concrete wall, called a stem wall, is poured on top of the footing. The stem wall typically isn't reinforced with steel because it's not subjected to the movement or expansion of the soil underneath like the footing is.

    After that, the under-slab plumbing gets added and then the interior area of the concrete floor, commonly referred to as the slab, is poured. The concrete slab isn't physically attached to the stem wall, so it's commonly known as a floating slab.

    Pros of conventional foundation

    • - Simple construction.
    • - Many contractors available to build it.
    • - Construction specifications are covered in the Residential Building Code.
    • - It has been in wide use for many years, so performance is well understood.

    Cons of conventional foundation

    • - A long, straight stretch of foundation and stem wall may tilt outward slightly, which can lead to cracks in the brick or drywall and gaps between the floor and wall inside the house.
    • - Offers no real structural support except for the outside perimeter. This means the floor slab itself isn't really a structural part of the home, which requires planning in advance to route ceiling and roof loads to the outside walls. This is very difficult to do when stick framing a house, as opposed to truss framing, which typically places all roof and ceiling loads on the perimeter walls.
    • - Since the slab is floating, it is subject to vertical forces from the underlying soil that can lift parts of the slab and create nuisance cracks in tile.

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    Overview of post-tension foundation

    With a post-tension foundation, the contractor digs the perimeter trench in much the same way as in the conventional foundation. The contractor places form boards around the perimeter to define the shape of the house and the plumber installs the under-slab plumbing before any steel is added or concrete poured.

    Rebar and fill sand are used in a similar way as conventional foundation, but a post-tension foundation also includes steel cables that form a grid pattern across the entire slab. They cables are encased on special sleeves that prevent them from binding the to concrete when it's poured. The concrete for the footing, stem wall, and slab is poured at one time, which is called a monolithic pour.

    Once the concrete has cured, a hydraulic cable tensioner is used to pull about 20,000 pounds of tension on the grid of cables, which are then anchored into the concrete. The force of the cables puts the entire slab and foundation under a continuous compression load, which means the slab and foundation becomes effectively one unit

    Pros of post-tension foundation

    • - Very stable; it rarely moves around enough to cause any cracks in the brick or drywall.
    • - Designed by a licensed structural engineer for the exact house and site.
    • - No slab cracks to cause cracks in tile or squeaks in wood flooring.
    • - Very tolerant of ceiling and roof loads placed on the slab. This means there is a lot more flexibility when framing any ceiling and roof system.

    Cons of post-tension foundation

    • - Cost is higher than a conventional system due to the steel cables and additional labor to install and tension them.
    • - Complexity, which means fewer contractors capable of installing a post-tension foundation and slab system in central Oklahoma. Because there critical elements for a good installation, you have to be careful about who you hire to do a post-tension job.
    • - More time required because there is simply more work that goes into post-tension.

    So which foundation is the better option? They're both good options. As long as the pros and cons are well understood and taken into account by the builder, either type of foundation should serve its purpose well into the future.

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