Hydrostatic Pressure: What It Is and What It Isn’t

Hydrostatic pressure is a term that is often used when excess moisture has created problems with concrete slabs and connected flooring systems. However, it is not as generic a term as people in the industry often think.

What Is Hydrostatic Pressure?

At its simplest, hydrostatic pressure is the pressure created by standing or resting (“static”) water (“hydro”). We’ve all heard the story of the little Dutch boy who saved a town from the force of standing water when a dam sprang a leak. That same relentless pressure can impact your concrete walls and floors too.

While concrete is a relatively solid material when dry, it isn’t technically solid in the same way that stainless steel or marble is. It isn’t impervious to moisture.

As concrete dries, water vapor from the original concrete mixture exits the slab, creating small capillary networks. These pathways remain open until properly sealed, and can be the path of least resistance when water pressure builds up against a concrete contact point. While newer high-strength concretes can resist higher levels of pressure than older mixtures, they still can be susceptible if cracks form or hydrostatic pressure builds high enough.

Hydrostatic pressure is a natural force that can move rocks, buckle walls, and cause havoc with your concrete, but it does not explain every instance of water intrusion in concrete.

What Causes Hydrostatic Pressure?

What Is Hydrostatic Pressure?
Normally this happens in basement floors or floors below water tables. The more water, the more pressure you’ll have at the bottom of the water table. Think of it like swimming; when you dive deeper, you feel more pressure on your ears. Same as hydrostatic pressure, the deeper your foundation and the wetter soil conditions, the higher the chance of moisture-related problems.

What Hydrostatic Pressure Isn’t

Hydrostatic pressure is a term often used rather generically to explain any moisture problem that occurs in a concrete slab, but more often than not, it isn’t the culprit. Several other sources of moisture could equally be responsible for water intrusion or moisture-related flooring issues.

Hydrostatic pressure, by nature, does not occur in slabs above grade. It does not even occur in every slab below the soil line. For a concrete slab to be impacted by hydrostatic pressure, it must be below the water table on the site or intrude into a natural water pathway. Water, underground as well as above, moves downhill under the pull of gravity, and so sites cut into a hillside stand a greater risk of having the “hillside side” be affected by hydrostatic pressure if adequate steps to redirect the water (and the subsequent pressure that might build up as it accumulates) are not taken. Poor drainage may cause water to collect against a concrete foundation but generally will not build up the volume to cause problems attributable to hydrostatic pressure.


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Only identifying the correct source of excess moisture will make proper remediation possible.

Other possibilities include:

  • Water Supply Sources
    Sprinklers, plumbing, city mains, and other water supply lines may be a source of moisture if they break or if a joint fails. If this occurs in a location with poor drainage or very dense soil, the water may end up in extended contact with the concrete and increase its internal moisture content, or relative humidity (RH). The pressure formed by a burst pipe is technically a type of hydraulic (or a mechanical force) pressure.
  • Inadequate Installation
    Educated WorkersObviously, we want to believe that every concrete and flooring professional is fully educated in his or her trade and uses the strictest standards to be sure each job is completed correctly. Unfortunately, the high level of flooring failure costs annually suggests that there is more to be learned. Some installation culprits that can result in excess concrete slab moisture may include a vapor retarder with insufficient or poor “perm” (permeability) rating, insufficient site evaluation or geotechnical survey to identify natural water sources, excessive troweling that prematurely seals the slab surface, or surface membranes applied before the slab was adequately dry.
  • Improperly Dried Concrete
    In cases where flooring failure is attributed to excess moisture, it is important to be sure that the slab was adequately dry before flooring was installed. Installers intent on meeting a construction deadline, or those who use surface-only test methods like calcium chloride testing, may not have had accurate moisture content data to base their adhesive or flooring material choices on. Only RH testing can adequately determine the true moisture conditions of a concrete slab and inform both schedule and flooring decisions.

Ultimately, if hydrostatic pressure is the culprit, the only way to correct it is to eliminate the pressure of standing water, a significant undertaking in any situation. However, accurate and comprehensive moisture testing and site evaluation can indicate the true source of concrete moisture intrusion to ensure proper and lasting remedies.

Last updated on June 8th, 2021

50 Comments

  1. Catherine in Florida says:

    Hi Jason,

    I have a 22 yr old monolithic slab home in central Florida. I replaced my carpet with porcelain tile that had to be removed 6 months after being installed due to an installation issue. When the tile was removed about 1/3 of the concrete flooring in that room had a sheen of standing water which promptly evaporated. We have installed French drains, which are draining well, along the perimeter of the house and sealed the side of the slab down to the footer over pour. However even after running a dehumidifier and leaving the slab exposed for the past 6 months the rh moisture reading in the problem area remains at 99%. We have brought in 3 different plumbers and slab leak specialists who assure us that there is not a plumbing problem. There is no roof problem and the walls show no excess moisture readings. We brought in a foundation specialist who has suggested that injection of PolyRenewal under the slab in that room might solve the moisture issue. Any input you could offer would be greatly appreciated!

    • Jason Spangler says:

      Catherine:

      Thanks for the questions and sorry you are having issues. My guess would be, given the evaluations you have had from various professionals, is that you don’t have an intact vapor retarder under the slab. This would allow moisture vapor from the soil to infiltrate into the slab. The most common way to deal with this is to put some type of moisture mitigation product on the surface of the concrete to lessen the amount of moisture that can actually interact with the floor finish. If you are going back with porcelain, you might also investigate uncoupling membrane.

  2. Carolyn says:

    HiJason, I own a townhouse built in the early 80’s on a slab above grade, no basement. We are in Annapolis, and our front is a small but steep hill, and our yard backs up to a fairly large hill in the back yard. Recently, we’ve noticed our dining room (center interior of 1st floor) rug being damp one day, then wet a day or two later in one particular spot close to the Kitchen fridge and dishwasher. Monitored both appliances and no leaks. Yesterday we thought we had no additional dampness until last evening when I noticed another small damp spot about 2-3 feet away from the spot first identified. It appears water is seeping through the cracks between my relatively new luxury vinyl plank floor boards. What could be causing this and what do I do next, call a specialist or my insurance company? Thanks, in advance, for your advice!

    • Jason Spangler says:

      Carolyn:

      Thanks for the questions. If you had the floor installed by a professional, I would start there and see if they can determine the problem. Did they test the moisture condition of the concrete prior to installation? You may also want to rule out any plumbing leaks by calling in a leak detection specialists. If no leaks in the plumbing then I would probably look to a geotechnical engineer. Good luck.

  3. It was interesting when you explained that concrete may get increased relative humidity if it’s left in contact with water for a long time. My husband and I are interested in having a small custom home built so we can have a comfortable yet minimal place to retire. thanks or teaching me why basement waterproofing should be an important part of the home-building process!

  4. Carol Wheeler says:

    Our 4 metre high stone retaining wall collapsed in December during an orange warning storm and unusually high rainfall on the weekend and for a week before. The wall had drainage holes every 1.5 metres. The insurance expert asked to do a visit by video and has said that the collapse is due to hydrostatic pressure due to lack of drainage. The wall is probably 150 years old and we had no previous problems. Is this really a diagnosis that can be made on a 5 minute video call and wouldn’t we have had some warning signs?
    Thanks, Carol based in South West France!

    • Jason Spangler says:

      Carol:

      Thanks for the question. In my opinion, nothing is ever impossible, but a five minute video call for a proper diagnosis of this nature is very improbable. I believe I would ask for a second opinion or find someone on my own that would give me a more thorough evaluation to potentially dispute the “experts” claim. Good luck.

  5. Jennifer Watkins says:

    Can hydrostatic pressure happen to a house with a basement on top of a hill. From what I am leaning the issue normally happens to homes below other homes or on a hillside. But if you are at the top of a hill wouldn’t the water run down. My parents basement just flooded and the restoration people are saying hydrostatic pressure. I am just not understanding how that is possible on top of a hill?

    • Jason Spangler says:

      Jennifer:

      Thanks for the question. Anytime you have a building elevation that has the potential for water to reach a level higher on the outside than on the inside (basements) hydrostatic pressure is possible. The backfilled soil is moist and this can cause a pressure differential between the moisture against the wall and the moisture below the floor. Now, I’m not saying specifically that is what your parents’ issue is, but it is possible.

  6. Kris Bridgewater says:

    Hello. I have a basement floor that water comes up through where they are cracks. Basement had a drainage tile put in around the outer walls. Holes drill into the blocks and sump system installed. My question is. How is water still able to come up from the floor if the pressure was taken off by the tile and sump.
    The floor was poured right on the mud. No nothing between the two.
    Way to many years of water coming up that way is still easier for it do so? Then go through the mud to the tile?
    Thanks

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