The same thing is going on with the rainwater. It soaks right through the dirt, but can’t go past the clay. It flows along the contours of the clay substrate. So when people with wet basements slope their dirt away from their foundation it does little good. The water flows along the clay. When thinking about a wet basement you can ignore the topsoil and just think about the clay.

Often when homes are built, builders dig a trench, put in the foundation, then back-fill with anything around (stone, clay, dirt, whatever). This backfill is water-permeable. So, thinking only about the clay, you have a trench around your house. If the eaves drip down inside this trench the water will go right through the dirt, hit the clay, and flow towards your foundation. This is especially important when thinking about downspouts. You have to emit the water far enough away from the house so they are outside of the foundation trench.

What should be done to prevent this when building a new house?

1. Contour the clay substrate to slope away from the house.

For an excellent start, position the house on a rise. Take samples of the ground. If the topsoil is only a few inches deep and you don’t find any surprises (pits, old foundations, etc.), you can tell what the clay is doing.

2. Try to keep your trenching minimal.

Remember, for the most part, the contour of the undisturbed clay is the contour the rainwater is going to follow. If you can use trenchers instead of bulldozers, or only bulldoze from inside; and you can leave a nice sharp wall only a foot or so outside where the foundation will be you’ll be in good shape.

3. Design your eaves and downspouts to extend beyond your foundation trench.

You want the rainwater to fall far enough away, so when it hits the clay substrate, it flows AWAY from the house.

4. Lay in a perimeter drain.

Since you’ve designed a good clay substrate you should have very little water flowing towards your house and into your basement. But for added insurance, a perimeter drain is nice redundancy. I’ve seen several disagreements about what should cover your perimeter drain, and it depends upon how your clay substrate is layed out. I personally suspect that gravel, or ‘rip rap’ covered with gravel, covered with fine mesh is probably the best.

What can you do to fix it after-the-fact?

1. Check your downspouts.

Are your downspouts emitting the water far away from the foundation? Can you pipe the water away into a drain? This is usually the major contributor to wet basements.

2. Check your gutters.

Any chronically overflowing gutters might be contributing a lot of water to your basement troubles. Fix your gutters. Keep the leaves out so they don’t overflow. Fix the slope so they drain into the downspouts. If they are sagging in the middle, fix the supports. If they have holes, fix the holes or replace them.

3. Install perimeter drains.

This can be ridiculously expensive, or not too bad. It depends upon your conditions. But often just gutter and downspout repair can help dramatically.

4. Make your basement sea-worthy.

If you think of your basement as a boat, and you make it water tight, you can simply keep the water out that way. It’s a better idea to alleviate the source, but if the problem is not from gutter and downspout water, this is often the only affordable solution. Plug up the holes with cement, and paint the foundation with a product like Dri-Lock which stops the water from coming through. Seeping or leaking water does more damage than still water, and if the water is not coming through you won’t have the humidity and mold problems.

It’s a common problem.

Wet basements are very common. Building materials and practices have gotten better and better, but time and cost is still a factor in construction. If you consider how the clay is laid out, you can design a drier basement from the start, or have a good chance at fixing a wet basement.

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