Home & Garden Columns

About the House: Hydrostatic Pressure And Why Your Basement Leaks

By Matt Cantor
Thursday June 04, 2009 - 06:58:00 AM

It might appear to be over-reaching to attempt a discussion of something that sounds as high-handed as hydrostatic pressure in a lay essay, but if you’ll bear with me, you’ll quickly see how this is both relevant and conceptually accessible to just about everyone. 

If you rent, it might have a substantial effect on your life, but if you own a home, hydrostatic pressure could turn out to be of vital interest, as it can and may affect your pocketbook, your health and the livability and value of your home. 

Why are some basements dry and some sopping damp every winter? Why do some homes gradually split or distort until they demand new foundations at great expense? Hydrostatic pressure may not be the only answer to these questions, but it’s a big part and often the central answer. 

Let’s start with a basement on a hillside, something we see a lot of around here. Many of our hillside houses began with the stair-stepping of the hillside. By hacking away a wall and flattening a bit of hillside, one creates a little more space in the boxy world of houses. As soon as we start doing that, we invite nature to mess with us. In a funny sort of way, you’ve cut into a pipe, a conduit in which water flows through the layered plates of clay or the interstices of a rocky matrix that make up the ground below your feet. Since water naturally flows through this agglomeration, it’s normal for it to start flowing right into the space you’ve carved out for yourself.  

If you then pour a concrete wall and floor in this area, you invite this water to push up against your new barrier and to try, at the very least, to get into your new bedroom, office or garage. Water hitting a barrier like this can back up and rise as it fills the cavity, either in the soil or in the void behind the wall, until there is considerable volume behind this wall. Water fills the soil or rock much as it fills the space behind a dam, adding weight and generating thrust behind the wall. The taller the body of wet material, the greater the force, especially at the bottom of the volume. This is hydro (water) static (standing) pressure, the force of water standing behind something. Hydrostatic pressure destroys dams, moves earth and rock and drives many of the processes that give us the natural world around us. If it could laugh at our silly little houses, it would. 

Hydrostatic pressure can move your foundation or buckle a retaining wall until there is nothing to do but accept the line of credit you keep resisting. But it does something equally annoying and far more pervasive. It pushes water right through solids. Concrete, like all matter, isn’t really solid. It’s a weave of molecules chosen for its strength and the simplicity of its manufacture.  

Low-strength concretes, like those that support most of our older housing stock, are highly porous. Given a sufficient “head” or load of water behind them, they will weep until the opposing side is damp. Add some fungi and they can be slimy, smelly and unsuitable for habitation. This all varies quite a bit and depends on the recipe used in cooking the concrete.  

High-strength concretes can physically resist a high load (especially when properly reinforced) but also retard water intrusion. The “weave” of the concrete molecular fabric changes as we modify the recipe, and we now know a lot more about how to get concrete to catch and hold water. But the take-home message is that your concrete in your old house doesn’t know this trick, though its talent may still be sufficient, as you will soon see. 

Since increased hydrostatic pressure allows water to weep through concrete that would otherwise resisted dampness, we have a clue as to how we can keep the basement dry. Eliminate the pressure. Most basements, even those with lower strength (read high porosity) concrete can stay relatively dry if we can prevent water from welling up behind the surface. This is what drainage is really good at.  

A drainage system (some of these are called French Drains, though the term is more colloquial than scientific) is not designed to keep the back side of your foundation or retaining wall dry, it’s designed to prevent water from pushing against these surfaces by directing the lion’s share away to a more suitable locale. 

By trenching behind (this is usually uphill of the structure or basement) the problem area, you can allow water to find its way out and down to a place that it will do no harm (at least to you). By the way, sump pumps don’t usually do this adequately. They may serve well as a mechanism of discharge when you can’t get water to flow downhill to an allowable location, but a sump is usually not going to catch the mass of water that is flowing through the soil or rock to assail your building.  

For this you need a trench, which is, ultimately, a broad, three-dimensional thing. It has length, breadth and height (unlike a sump, which is at a confined location in space) so, if adequately sized and placed, it can really do the job of reducing that hydrostatic pressure to something like zero. Then the job of keeping the inside dry is a much smaller one and it is much more likely (note the language…more likely) that you can achieve your goals with simple methods such as sealants. 

If you are undertaking such a project, keep in mind that there is an array of other tools to help you meet your goals and that drainage, alone, is not usually adequate unless your only goal is to diminish movement.  

For the drainage system, these tools can include:  

• Sheet drainage material (such as Amerdrain): Like it sounds, a sheet of material that carries water down along a surface to a place where it can join other drainage. 

• Moisture retarders/Moisture barriers (depending on your confidence level): These mount on the exteriors of foundation, basement or retaining walls to prevent water from getting to the porous concrete surface. Proper installation is critical. 

• Field drains: These small, usually round, drainage entry points are often placed so that surface waters will not find them. They only work if they’re right where the water goes. 

• Channel Drains: A very nice choice for the bottom of a driveway but other locations can also benefit from this clever, age-old method, in which a length of grill-covered drainage material is recessed into the ground surface material (e.g. concrete) where it can capture a sheet of flowing water. 

If you’ve managed to control hydrostatic pressure and divert water effectively, you can often finish the job by sealing concrete on the inside of a basement or other below ground surface. A range of sealants exist, including Drylok, Thoroseal, Aquafin and others.  

Surface preparation is what really matters when doing this and following the instructions carefully (RTFM, right?). Old concrete with lots of cracks may be an uphill fight and some below ground spaces are never going to be suitable for living without huge structural upgrades. But you always make things better if you take your time, get good advice and respect the power of that most formidable of opponents. 



Got a question about home repairs and inspections? Send them to Matt Cantor at mgcantor@pacbell.net.