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How to Waterproof a Basement Inside

Jul 25, 2014 • By Matthew Stock.

Why Cracks in Basement Floors Can’t be Fixed by Urethane Injection

It seems logical that if you want to keep something dry, you wrap it in something waterproof, right?  That’s why your morning newspaper is delivered in a plastic bag and we wrap ourselves in rain jackets when we go out in inclement weather.

The “wrapping” principle can apply to keeping a basement dry, too.  One very effective method of basement waterproofing is to “wrap” a basement, either whole or in part, in an exterior waterproofing membrane.  Of course, such a membrane is created by coating a wall with viscous asphalt-modified polyurethane and not covering it with sheet goods, but  when the membrane cures it forms a wrapping on the exterior nonetheless.

Of course, an exterior waterproofing membrane is only good for repairing or preventing certain kinds of basement seepage, including seepage through cracked mortar joints (in masonry walls), seepage through porous concrete or masonry units or over the top of a concrete foundation wall.   Other types of seepage require that we waterproof a basement inside.

Seepage Problems that Require Waterproofing Inside

Basement seepage occurs for several reasons and from a number of sources.  Regardless of how it enters the basement, water that ends up there starts out in the ground surrounding the foundation.

Water exists in the ground just about everywhere although in some places it may be located very deep below the surface.  The level of ground water, called the “water table,” is more or less stable depending on the geographic features of the area but can be raised or lowered in times of heavy rains and drought respectively.

Ground water under the foundation creates hydrostatic pressure that exerts force upward against the basement floor.  Water in soil around the foundation creates lateral or sideways pressure that exerts force against walls.  Either of these forms of hydraulic pressure can cause cracks and force water into the basement and pressure increases as more water is absorbed into the soil.

Basement Floor Cracks — Basement floors are essentially a flat covering that has no structural purpose in the home’s foundation – a basement with a dirt floor is just as strong as a foundation with a poured concrete floor.  Accordingly, basement floors are pretty thin, averaging about four inches of poured concrete.

With the floor being so thin it is common that, when hydrostatic pressure beneath the floor increases because the soil has absorbed more water, the floor can crack in one or multiple places.  Since the floor is non-structural these cracks don’t represent any danger to the foundation but they are often a source of seepage, particularly when the water table rises due to heavy or sustained rains.

Wall Cracks – A crack in a poured concrete foundation wall is the most common source of water in the basement.  Although poured concrete foundation walls are fairly thick at eight to ten inches, lateral pressure from oversaturated soil surrounding the foundation can exert enough force to create nonstructural cracks in the wall.  Wall cracks can also form when a foundation settles or sinks.

Non-structural wall cracks like these can be identified by their random occurrence and their width, usually less than one-eighth inch.  Structural cracks, which indicate a more serious problem with the foundation, are generally wider than one-eighth inch and occur in a pattern.  A wall with structural damage will have a vertical crack in the center and two angled cracks across the upper corners.  Not visible from inside are vertical racks at each corner where the wall has begun to separate from the adjacent ones.

Cove Joint – When a foundation is built, the first step is to pour a footing, a wide slab of concrete that describes the perimeter of the foundation and helps to distribute its weight.  When that cures, the foundation walls are either poured in forms or built by masons on top of the footing.  In either case, there is no bond between the footing and wall so a tiny gap remains between them.

When the foundation is complete the basement floor is poured, which also does not bond to the walls leaving another very small gap.  This is called the cove joint and when hydrostatic pressure builds under the foundation water can be forced through it into the basement.

All of these sources of seepage can be repaired by interior methods.  Here’s how to waterproof a basement inside:

How to Waterproof a Basement Inside

Waterproofing a basement in the inside is a very popular option because it is done without major exterior excavation and, in some cases, can be done very quickly and economically.  This also makes it a tempting opportunity for do-it-yourselfers who, regardless of their skill level, usually fail to stop the seepage because they do not have access to the proper materials or lack the knowledge to install permanent improvements.

Hydraulic cement and caulk are popular choices for DIY attempts at waterproofing a basement on the inside.  The problem is that neither works; using them to stop seepage only delays the inevitable call to the basement waterproofing professional.

So, what will the professional do?  It depends on the source of seepage.  In the case of that most common of problems, a non-structural crack in a poured concrete wall, there’s a great and simple solution.

Interior Crack Repair – The professional’s approach to fixing a seeping wall crack permanently is to fill and seal it and ensure that it doesn’t reopen.

The repair work begins with cleaning the crack thoroughly with a wire brush to remove debris, aggregate and failed attempts at DIY fixes. 

Once the crack is clean, the technician installs plastic injection ports at regular intervals along the length of the crack.  When the ports are in place, he applies a sealer coat of fast-drying epoxy to cover the crack and hold the ports.

After the epoxy cures, the technician injects the crack, starting at the top, with expanding polyurethane, which fills and seals the crack all the way to the outside soil.  In fact, the pressure of the soil outside actually creates a “cap” of polyurethane on the outside of the wall that helps seal it against water.

When the polyurethane has cured it remains flexible in order to prevent minor foundation movement from causing the crack to re-open.  Polyurethane crack repairs generally carry a lifetime warranty.

OK, so that’s how to fix a wall crack from inside but what cracks in the floor and the cove joint?  There’s one solution that takes care of both of them.

Interior Drain Tile – The best way to deal with seepage caused by hydrostatic pressure is to create a means of alleviating the pressure and giving the water that causes it somewhere else to go.  This is accomplished by installing interior drain tile, a system of perforated pipe under the basement floor.

Installing interior drain tile begins with removing a strip of concrete floor approximately 12 inches wide along the affected wall or walls, exposing the footing.  Then, a shallow trench is dug to the bottom of the footing and a layer of washed gravel is poured in and leveled.

Lengths of perforated corrugated plastic pipe are then encased in a “sock” of filtration fabric to keep out dirt and debris and are laid on top of the gravel bed.  The pipes are connected and routed to a sump basin at one or both ends.  More gravel is then added to cover the pipe and promote water drainage and the concrete floor is replaced.

When it has been installed properly interior drain tile requires no maintenance.

Of course, interior drain tile won’t do much without a proper sump pump so let’s have a word about this workhorse of any basement waterproofing system.

Sump pumps come in many sizes, with many features, and are available from big box hardware stores and plumbing suppliers as well as from basement waterproofing contractors.  The waterproofing professional will know the best brand of sump pump as well as the necessary pumping capacity and discharge requirements to make sure the basement stays dry and the pump has a long, productive life.  Also, a sump pump that is professionally installed is guaranteed to do the job and is never subject to the “Does that look right?” analysis that is the downfall of so many DIY attempts.

When a homeowner discovers water in his or her basement the first thing needed is a professional diagnosis of the problem and a recommendation as to whether it can be repaired from the inside or on the exterior.  To ensure this is done the right way, the homeowner will need a professional basement waterproofing contractor to make that diagnosis and to perform the necessary repairs.

At U.S. Waterproofing, we’ve been waterproofing basements on the inside  (and the outside as well) for more than 57 years and we have brought that experience and an expertise built on the latest technologies to work for each of our more than 300,000 satisfied customers.  Why not ask for our free advice?

Tags: waterproof basement inside, inside basement waterproofing

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