PRS: basement waterproofing, part 2

Causes of Basement Wall and Floor Cracks

Cracks show up in the home's basement floor for three main reasons.

The first cause is simply poor craftsmanship during the house's initial construction. Builders may have cut corners, and the cracks and subsequent leaks don't show up until long after the home warranty expires.

The second cause is that homes tend to settle naturally over the course of time. This will naturally show up in the form of foundation problems, basement cracks, and holes.

Finally, cracks in the floor and walls are caused by water pressure from the outside. This factor will definitely exacerbate the first two causes.

Hydrostatic and lateral pressure attack!

The fancy term for the water pressure that causes cracks in the basement floor is hydrostatic pressure. What this means is that the ground water pressure is moving up. When it finds the path of least resistance -- any tiny fault in the basement floor -- it takes advantage of it, and the result is a weeping crack.

When the cracks manifest on the basement walls, it's called lateral pressure. There are a couple of things that can cause it. The first is the water pressure that builds up around your basement walls. This is sometimes called the "false water table."

Secondly, are the downspouts from your roof gutters dumping too close to your foundation? If so, they're just adding water pressure to the walls. If this is the case, see Bill Volk's excellent article, "Rain barrels made easy." It just makes sense to recycle that rain water -- and take the pressure off your basement at the same time.

Do a visual and seal the small dry cracks

The first thing to do is to conduct a walk-through and locate the cracks that are less than 1/8" wide. You can seal these using a commercial waterproofing material. Generally, these products are made up of a latex cement and epoxy compound; the latex ingredient allows for a bit of flexibility.

And now for the medium-sized dry cracks...

For any cracks that are wider than 1/8" but under 3/4", plan on preparing your own mortar mix to seal the crack with. Not a problem! Here's your recipe: mix two parts of fine sand to one part portland cement. Next, add enough water to make a stiff mortar.

Now just trowel your mortar into the crack, working your way carefully from one end to the other. Mash it in with your trowel and squeegee it first in a downward motion, and then, with more mortar on your trowel, squeegee in and towards the far end of the crack.

This way, you won't leave any air pockets -- choice areas for the crack to reopen, especially if there is any significant water pressure from the outside of the basement walls.