Q: I read your column regularly even if the problem you discuss does not directly affect me. Your comments are always interesting.
I’m writing you to ask if you know if it is possible to waterproof a concrete slab?
My garage sits at the bottom of a long driveway. I am planning to tear it down (before it falls down) and build a combined garage, workshop and studio in its place. I’d like to be able to use the existing slab if at all possible.
Winter rains accumulate at the entrance to the carport that lies in front of the garage, and moisture seeps up through the slab in the garage, soaking anything that sits directly on it. Unless items are stored in plastic bins or placed on plastic, they become soaked.
A: There are concrete sealers on the market, but they won’t solve your problem.
Concrete sealers are used to inhibit surface staining of concrete. They do not prevent moisture underneath a slab from leaching to the surface. To prevent moisture from rising to the top, a vapor barrier of plastic is installed under the slab prior to pouring the concrete.
We’d be surprised if your slab has a vapor barrier. We would also be willing to bet that the soil on your property doesn’t drain well. The soil most likely contains a high volume of clay, which doesn’t allow the water to percolate and disperse.
The fact that your garage is located at the bottom of a sloped lot provides a catch basin for winter runoff. As the ground under the slab gets saturated with runoff, moisture leaches up through the concrete and makes a soggy mess of everything stored on the floor.
One solution is to remove the slab and start anew. Once the slab is broken up and removed, grade the site and install a vapor barrier of 6 millimeter plastic. Then install a 4-inch layer of gravel and pour the new slab over the gravel.
The vapor barrier will ensure that moisture does not leach up through the slab and the gravel will provide a porous yet solid base for the concrete.
If you don’t want to break up the existing slab, another solution is to install a French drain around the perimeter of the existing slab.
The purpose of a French drain is to channel water that would normally seep under the slab into drainage pipes and away from the garage. Although a true French drain contains no pipe (only gravel), we like the idea of using drainage pipe to channel the water.
To install a French drain the first step is to dig a 6-inch-wide by 18- to 24-inch-deep trench around the perimeter of the slab. Line the trench with heavy plastic. Then, fill the bottom of the trench with 2 to 3 inches of 3/4-inch gravel. Next, install 4-inch drainage pipe in the trench. Drainage pipe has holes on one side to accept water. Place the pipe with the holes pointing down.
Start at the highest point and make sure the pipe slopes at least a quarter-inch per foot so the water will drain. We also suggest that you install a riser with a plug, called a “cleanout,” at this point so that if the pipe becomes clogged you’ll be able to clear the blockage. Use “sweep” or “long bend” fittings at all turns to lessen the chance of restrictions.
For the cleanout, use a “sweep tee” to transition to both sections of the drain. This allows you to approach a blockage from either direction. At the top of the riser install a threaded plug for access to the line.
At the low point of the system connect a solid 4-inch pipe and extend it to a point where it can discharge onto the surface. This may be the gutter, or perhaps on the ground.
Once the drainage line is in place, cover it with filter fabric. This material, which is available at nurseries and home centers, allows water to enter the pipe while restricting entry of dirt from the gravel. Finally, fill the trench with 3/4-inch gravel. This sounds like a lot of work and it is, but not as much as demolishing the slab. It’s cheaper, too.
