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

Exploring Questions

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.

Hydrostatic Pressure: What It Is

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.

Geyser EruptingWhile 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 It 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.

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.

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Jason Spangler

Jason has 20+ years' experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

19 Comments

  1. Larry Hall says:

    Jason,
    I own a home in Florida, built on a slab in 1981. I bought the house 4 years ago. About one month ago I notice a small puddle of water in my family room sitting in the grout line of the tile floor. I dried it up and the next day it came back. It does not grow; it spreads about 2 inches along the grout line and stops. It is as if the surface tension does not let it flow onto the tile, or that there is not enough pressure to make it run.

    A leak professional told me that I had no leak after doing his tests. He thought it was the slab sweating. He pointed out the efflorescence on several other grout lines, telling me it was an indicator that the sweating had actually been going on for a while.

    This led me to your posts on the Wagner Meters website. My question to you is: what type of trade professional do I contact to help me solve this problem? I do not know where to start.

    Thanks for any help you can give me. And Merry Christmas.

  2. Andrew Larson says:

    Hi Jason,

    Thank-you for the informative posting. I have a question on a hydrostatic diagnosis and trench/dry-well fix.

    My scenario: (Pac. NW/Seattle) below grade concrete basement (circa 1941) – clean-looking foundation floor/walls when demo’d. The house sits at a high spot, with gentle grade away from all sides, esp. the SW area. A great finished basement was constructed in the space.

    Over this current very rainy fall and winter, notice the SW corner only (spot that gets the most weather here) with some significant seeping in at slab/walls intersection floor level. Causes carpet and floor-molding to be pulled. Isolated (luckily) to that corner.

    That location in the SW corner is adjacent to and below a window well (window down from the corner of the house by 24″ or so) that had a bad drain pipe out of it routed towards the backyard, that would fill-in with standing water, during heavy rain periods.

    Conclusion was that standing water in window well was very possibly contributing to too much water presence in and around that SW corner area overall, possibly creating h-static pressure down below. Further evidence on too much water was highlighted by water backing INTO that window-well via poorly performing drain pipe (from the yard), vs. evacuating out (which was the original design, I’m sure) – and that it needed to be fixed.

    (I at one point shop-vac’d 100 gal. out of it.. it kept back-filling up from that drain pipe that terminated somewhere in the backyard, each time I’d fill the 20-gal. vac up ..more water would run in from that drain-pipe.)

    After several months’ inspection and conjecture, best course and fix seemed to be trenching a properly sloped drain-pipe (perf’d pipe w/ gravel etc.) from that SW window well to a significant 8′ + deep dry well 18′ – 20′ away from the house. Excellent local co. did the work, and that window well has performed perfectly in heavy rains since (no standing water at all).

    However, still seeing some ~ reduced but some, continued seeping in that basement SW corner (which is exposed still) after the recent heavy rains.

    Is it possible that down that far (7′ – 8′) the hydrostatic water effect is still present to some degree, and will exhibit residual presence/seepage in through that corner/slab intersection for awhile, albeit at reducing levels, until the new trench/dry well (even though it’s deep, it is still above the slab level of the basement) has had time to make its full impact and effect?

    Or, should that type of solution, if it is the correct answer, have an immediate all-encompassing effect and eliminate further seepage instantly?

    Thank-you for your thoughts and reading this,
    -Drew

  3. Tom Walentowski says:

    My basement will flood from under the ground when the creek which is located 150 feet from my house is full of water. The last time my basement flooded I had 5 sump pumps running and the the basement still ended up with 20 inches of water in it. I have had several people out who have not been able to help I purchased a system from one company that put additional sump pumps and crocks in but they have not help when we get the worst case situation. I am told that the bedrock in my area is high and the creek is somehow feeding the bedrock and when the creek is full the water backs up into my basement. apparently my basement floor is only a foot or two above the bedrock. I need to find someone who will know the type of situation I am in who can give me some answers. All I hear from people is the same comment ever time “I have never seen anything like this before”. any advice would be appreciated.

    • Jason Spangler says:

      Tom:

      Thanks for the question. I hate to be a broken record, but you have me stumped on this one. Reaching for straws, I could only question whether setting up a perimeter drainage system around the house, deep enough that it would intercept the water prior to infiltration wouldn’t work? I would have some type of soil engineer evaluate and give recommendations.

      Thanks,

      Jason

  4. Ty says:

    Hi,
    Wondering if you could propose a hypothesis. I have a below grade slab in 1984 built colonial. Never, ever had standing water on floor or any type of water. There is a sump basin in corner that rises and falls due to melting snow, etc., never had a need to pump. Did see some efflorescence here and there at junction of walls and floor, but again no actual water. I refinished part of the basement and put vinyl plank down over the concrete. The concrete had some old paint from previous owner and I scraped all loose and flaking prior to laying floor. The specs on floor said NOT to put down a vapor barrier or 6 mil sheet etc. Install was done in summer and a few months ago, I noticed water coming through the gaps/cracks in the vinyl floor in different areas so I know that the water vapor that must of evaporated in the past is now getting trapped. I’m planning on pulling up the vinyl, letting it dry an then reinstall but I need to address the water. Some ideas are to concrete sand the paint, etch, then use something like Xypex, RedGuard, HydroBan, etc., then put DMX Airflow before installing the vinyl. Main question is whether the water getting trapped is due to hydrostatic pressure or weeping. Not sure I understand the difference and some of the waterproofing products are specific as to yes or no for hydrostatic pressure.
    Thanks for you time.

  5. Kim says:

    Thank you for this informative article. I am currently having trouble finding home owner’s insurance that will cover hydrostatic pressure cracks; however, I am trying to examine the potential risks for one. It is my understanding that there was a repaired crack before I ever bought my house (could not be inspected because there is drywall up in my basement). It was supposedly fixed and then the basement was professionally waterproofed with a lifetime transferrable warranty. I know it’s impossible to know for sure what the future will hold, but I didn’t know if you could provide any insight as to whether it is worth paying considerably more of a premium to have that coverage.

    Thank you!

    • Jason Spangler says:

      Kim,

      Unfortunately, this is a question that could only be answered by a crystal ball. I guess I would have to evaluate, personally, by what the damage could be if there was a problem (how much would it cost to fix) and then how long would it take me to save that much if I took the premium difference between an insurance with that rider and one without.

  6. Renaud Martowski says:

    Hi,
    It’s spring thaw season. I have a house with a crawl space in a rural area(country).
    I dug to 8 inch holes about with a three foot depth. There was water in the bottom of the hole.
    With the snow melting and rain, the water level has risen to the top of the hole.
    Will hydrostatic pressure force the water out of the hole and into my crawl space ?
    I am afraid that the water will soften the sandy earth in my crawl space and the cement pads which hold the beans and joists, will sink and force the house to collapse.
    Any info relevant to my situation would be appreciated.

    Thanks,

    Renaud Martowski

    • Jason Spangler says:

      Renaud, thanks for the question. I would consult an engineer that would have specific knowledge of your area, including soil types. They should be able to help in this situation.

  7. Joanne Aliber says:

    Hi
    I have a 3 story townhouse built in 1985. It is outside Boston. The basement is unfinihsed, below grade and opens to a common hallway corridor that is shared by the other 9 units, all of which are connected in the row.
    The Association has a history of water problems – poor gutter drainage, roof leaks, bad storm drain construction, clogged drywells. The foundation outside the basement of my unit has cracks, which have allowed water to enter into the corridor and travel under the walls and doorway into the basement. This problem went on for years until finally I filed a legal complaint against the Association; two years ago they brought in CrackX to plug the leak holes. I had to mold remediate the basement, reseal the floor and essentially I thought this was done. Then, just a few weeks ago, the water started again. It started coming across the hallway corridor and under the wall. CrackX came again and re-treated the work they had done 2 years ago as it was under warranty. But even since, without any heavy rains this week, there are puddles again in the middle of the basement floor.
    The association property manager is inexperienced and blames everything on rain. I am convinced this is all from hydrostatic pressure this is due to cracks in the exterior foundation. (I went through this once before in another house). I am pretty convinced the only solution would be in interior basement perimeter drain, but I don’t think that cost should fall on me. Is there a way to image the foundation to find out where the water is coming in and building up from? I fear the Association will again deny any responsibility, as they did for years, and then I would have to again file a legal complaint, and essentially when the house is sold, potentially not realize full market value due to the ongoing water problem.
    Can you recommend any Massachusetts firms to help me evaluate this problem?

    • Jason Spangler says:

      Joanne:

      Thanks for the email. Unfortunately, I have no immediate knowledge of someone in your area that could help. I would recommend starting with a Geotechnical Engineer (Google can probably find a few) and you may also need to get a Structural Engineer involved. I hope this helps.

      Thanks,

      Jason

  8. Kathy Rice says:

    Hi Jason

    We moved into a new home in June of last year. We immediately noticed hi RH 80%+ in the crawl space. We found a leak in a drain line and fixed it, but RH didn’t drop off until winter. Then we found mold in February. We had it remediated now the RH is back above 90 most times. The builder hired a structural inspector. Who has done some testing and found that downspout and runoff water are coming in the crawl space. The builder said at first that hydrostatic pressure was pushing the water in under the footer. There is a sealer on the outside of the block and no dampness on them. The builder used forma drain for the footer, with no escape for water from inside. He did not tie the inside to the outside drain either. We have dug out several holes beside the footer and looks like some gravel below the footer, but builder is swearing hydro pressure. Would you have any advice for us. We installed a sump pump and it comes in several hundred gallon at a time. We would like to stop it from coming in. Any advice?

    • Jason Spangler says:

      Kathy:

      Sorry to hear about your issues. The best advice I can give you would be to hire a geotechnical engineer and see if they can be of assistance. Some in this field are also construction orientated and may be able to identify the problem and offer solutions. I hope this helps.

      Regards,

      Jason

  9. Megan says:

    Hello Jason,
    I was wondering if there was any procedure for measuring hydrostatic pressure? Specifically, I was wondering if there was a way to measure the hydrostatic pressure on a floor in PSI (or equivalent that I can convert to).
    Would love your input.
    Thanks,
    Megan

    • Jason Spangler says:

      Megan:

      Thanks for the comment. Unfortunately, I am not aware of an instrument that records this information.

      Regards,

      Jason

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