Basement Waterproofing: Slab on Grade Foundations


A customer at a home show once told me, “I’m never going to need your company’s services.”  When I asked why she was so sure, she replied “Because my house was built on a slab.  I don’t have a basement.”

My response?  “Here, take my card.  You’ll need it someday.”

What?  Well, it is true that your basement can’t leak if you don’t have one.  But, your foundation can leak anywhere it sits on or against soil, whether on the surface or eight feet below it.  And, a leak in a slab on grade foundation is more likely to impact your living space than one in an unfinished basement.

Slab foundations are very common in commercial buildings. Construction methods are similar, just on a larger scale, as are the possible problems.

Let’s look at how a slab foundation is built.  An excavation is made around the perimeter of the new house and concrete footings are poured a few feet below grade.  Short foundation walls (called "frost walls" or "stem walls") are poured on these footings, up to grade level.  The soil in the middle is graded, a layer of gravel is poured in and a plastic vapor barrier is laid down.  The concrete slab, which forms the first floor of the house, is then poured and finished.

Of course, before the concrete is poured, pipes and ducts for the house’s mechanical systems are installed so plumbing supply and drain pipes, electrical wiring and heating and cooling ducts will be buried under the concrete floor.

What Happens when Water Comes In?

Water is most likely to enter a slab foundation because of openings in the frost walls for sewer pipes and other mechanicals.  Water can also come through the foundation walls the same ways it can seep into a basement – through cracks and the cove joint.  In fact, because these walls are often surrounded by gravel instead of compacted soil, it's easier for water to penetrate.  It’s true that there's usually a vapor barrier under the floor, but vapor barriers aren't intended to stop ground water, just water vapor.

Water will seep into the living space through hairline cracks in the floor, around HVAC vents and any other place the floor has been breached for mechanical systems.

Even if it stays under the slab, ground water can create a host of problems with your mechanical systems.  Although they should be sealed before installation, pipes, conduits and ducts are subject to water infiltration:

  • Electrical circuits can be shorted or destroyed by water;
  • Drains can back up due to ground water infiltration; and
  • HVAC ducts can fill with water, blocking air flow, damaging furnaces and air conditioners and contributing to high levels of moisture in the living space above.

How can this Seepage be Prevented?

Many of the same methods used in basement waterproofing can also be used to repair seepage problems in a slab foundation:

  • Interior drain tile can alleviate hydrostatic pressure and prevent water coming up from below and causing damage to mechanical systems.  Placement of heating duct and other mechanicals under the slab will complicate installation, but the drain tile will be effective as long as it is installed below the level of any HVAC ductwork;
  • An exterior drain tile system, with exterior waterproofing membrane applied to the frost walls, will prevent further penetration of water through them; and,
  • Of course, proper yard drainage and water management, such as keeping gutters clean and flowing and extending downspouts away from the house will help to keep the slab dry.

So, just because your house was built on a slab foundation don’t think you’ve drawn a “Get out of Jail Free” card when it comes to seepage problems.  If you’re seeing signs of water in your home, contact a basement waterproofing professional.  At U.S. Waterproofing, we’re familiar with the problems of slab foundations because we’ve waterproofed many of them for some of our 300,000 satisfied customers.  Why not ask for our free advice?

Want to talk about your basement – or that space under your home that isn’t a basement?  Drop a line in the Comments box below.



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