A slab on grade, also known as a floating slab, is a construction method whereby a reinforced concrete slab for a house or building is placed directly on the ground, thus serving as the structural foundation of the superstructure. These types of foundations are especially renowned for their durability and cost efficiency compared to other structural foundations.

Slabs-on-grade are constructed by pouring the concrete into a mold (formwork) with the appropriate reinforcement configuration at the foundation level, leaving no openings between the ground surface and the structure. This type of construction is commonly found in warmer climates, where ground freezing and thawing are not a concern.

Common Causes of Slab Leaks

While slab-on-grade foundations are a common and efficient construction practice, one of their main drawbacks is the inability to easily access utilities installed below the structure. During installation, the plumbing is laid out prior to pouring the concrete. Once the slab is poured, the piping is encased either in the soil beneath the slab or, in some cases, within the slab itself.

As a result, cracks, leaks, bursts or other piping defects can often occur undetected until physically manifesting on the surface of the slab. (Another home hazard is lead pipes. Learn more in How to Tell If Your Home Has Lead Pipes and How to Replace Them.)

Some of the most common causes of leaks in slab-on-grade foundations include:

  • Corrosion – Piping deterioration due to corrosion is typically associated with metal pipes. For example, copper pipes can react with certain chemicals in the concrete or soil, causing the pipe to slowly degrade over time until failure eventually occurs.

  • Faulty construction – Improper piping installation practices, particularly at the joints, can cause leaks which can worsen if undetected.

  • Construction damage – Piping damage can also occur during installation or other construction activities, such as pouring. While the initial damage may not be enough to alert inspectors, wear and tear from regular use can erode these weak spots until failure occurs at that location.

  • Soil settlement – The weight of the slab, and the structure it supports, can result in the natural settlement of the ground surface. This vertical soil movement, if significant enough, can place excessive pressure on the pipe, which can lead to a buildup of internal stresses and the formation of cracks.

Early Signs of Slab-on-Grade Leaks

Leaks in a slab on grade, if left unattended to, can be detrimental to the structural integrity of the slab and the supported house or building. Understanding the signs of slab leaks can help preserve the structure and minimize the costs associated with remedial work. Some of the most common indicators of leaks in a slab on grade include:

Slab Cracking

Cracking in the slab itself can be indicative of foundation failure due to leaking under or within the slab. As water continuously escapes and saturates the surrounding soil, its strength properties change, resulting in soil movement (settlement or heaving) which can cause the slab to become overstressed and lead to the development of cracks.

Cracks in the slab can also form if piping leaks cause water to penetrate the slab itself. This water can come into contact with reinforcing steel in the slab, causing it to corrode. As the reinforcement corrodes, the corrosion products cause the steel to expand, creating tensile stresses in the concrete, which can initiate cracking.

Water Ponding

Ponding or pooling of water on the surface of the slab is almost always a sure sign of leaking. While concrete may seem like a solid material, on a macroscopic level, it is porous, i.e., it consists of tiny spaces or holes which allow liquids or gases to pass through.

As water accumulates on the underside of the foundation, it will look for a way to escape, usually permeating through the concrete slab and manifesting as excessive moisture of the surface. Water that does not immediately penetrate the slab may be found near walls outside the building structure.

Warped or Uneven Floors

When piping leaks saturate the surrounding soil, the resulting heaving (expansion and upward movement of the soil) can cause the floor to become uneven at specific locations. Uneven flooring is usually characterized by:

  • High and low areas on the flooring surface
  • Rotated, bowed or separated walls
  • Doors or windows that do not close or have become separated from the building

Slabs that are constructed on clay soils are particularly susceptible to warping since clays expand significantly more than other soils when exposed to moisture. As concrete is weak in tension, the resulting buildup of tensile stresses in the slab from heaving can eventually lead to the formation of visible cracks on the surface.

Mold and Mildew

Moisture buildup can encourage the growth of mold and mildew on the building’s interior. Moisture that makes its way through the slab can become absorbed by materials such as carpet and drywall. This excessive moisture creates an ideal environment for mold and mildew to thrive.

Usually accompanying mold is the ensuing smell. Musty or pungent odors can be indicative of mold growth, especially in areas that are not readily visible such as the underside of carpets and drywall interiors. (Trenchless solutions can be ideal for home repairs. For more, see Is Trenchless Rehabilitation Right for Your Home?)

Hot Spots

Hot spots can develop in slab-on-grade foundations from leaks in hot water pipes. Hot or warm spots are caused by the conduction of heat by the concrete slab. The heat from the water travels through the slab, to the point where it can be felt by touch.

These spots are especially pronounced on tile or wooden floor finishes, which tend to conduct heat better than carpet. Hot spots can usually be detected before the formation of any other visible defects.

What We've Learned

Slab-on-grade foundations are a critical structural element in many houses and building structures. Early detection of leaks occurring under or within the slab is, therefore, crucial to ensure the structural integrity of the foundation and the supported superstructure.

If left undetected, leaks can result in significant structural damage which can be both hazardous and costly.