Ask the Builder: Trench will help keep basement dry

DEAR TIM: I’ve got a chronic problem with water entering the 4-foot tall crawl space under the duplex I live in. This year has been bad with all the rain we’re having. We installed a swale, but that has not helped. I know the groundwater is causing the problem, but I’m at my wits’ end about how to solve it. Is there a silver-bullet solution that will keep the water out of the crawl space? – Mike H., Maryville, Missouri

DEAR MIKE: Hundreds of thousands of homeowners suffer from chronic water seepage into crawl spaces and basements. Areas that have clay-rich soil seem to have the greatest problems.

It helps to understand what’s going on below the surface of the ground if you want to solve water problems in crawl spaces and basements.

A basement or crawl space that’s below grade is not much different from a shallow pond. It’s a hole in the ground that will naturally fill with water because the bottom is lower than the edges.

Ponds stay filled with water as long as it rains because the soil at the bottom contains lots of clay that doesn’t allow water to pass through.

As you dig into ground, the soil profile changes. Topsoil near the surface contains lots of organic debris and air. The deeper you dig, the denser the soil gets and there’s very little air.

When broadcast on dry topsoil, water starts to fill the air voids, then starts to move downslope below the surface. Most people think that the water in soil moves straight down, but when it hits the denser clay subsoils, it typically moves parallel to the surface of the ground.

You can waterproof your crawl space and/or basement if you capture and divert this moving underground water before it gets to your house.

The best way to capture this underground water is to dig a trench 6 inches wide by 2 feet deep. This trench should be about 4 to 6 feet from your foundation wall and should surround the house on at least three sides like the letter U. You can make the trench completely surround the house like a moat if you desire.

If you want guaranteed results, you want the bottom of this trench to be below the level of your crawl space or basement floor. This can be problematic for deeper basements. Nevertheless, I’ve stopped water from entering many basements with a trench drain that’s just 2 feet deep in clay-rich soils.

For the system to work, there must be a low spot on your land that’s lower than the bottom of the trench. Most lots have this kind of fall. What you think is a level lot often has quite a bit of fall to it in 50 or 100 feet of horizontal run. One leg of the trench extends from the house to this low spot on your land.

Install a perforated 4-inch diameter drain pipe in the trench on top of about 2 inches of rounded gravel the size of walnuts. Once the pipe is in place, fill the entire trench with the same gravel (it shouldn’t have any sand or smaller pieces of gravel in it). Water readily flows through gravel like this. Water in the soil wants to find the path of least resistance, and this trench is that place.

When it rains and water is flowing through the soil toward your crawl space, the water pushes its way through the topsoil and comes to the gravel. It’s easier for the water to flow through the gravel than through the soil, so it goes down to the bottom of the trench and starts to flow in the gravel and into the pipe if there’s a heavy volume of water.

If you make the bottom of the trench level while the ground in your yard is sloping downhill, eventually the bottom of the trench will emerge from the ground. That’s where the water in the drain systems exits and flows further down toward the next house. Your crawl space stays dry because the trench acts like the hole in the bottom of a kitchen sink.

The gravel-filled trench drain is simply a huge drain system that is always working. It relies on gravity and it’s very effective, if you have enough fall on your land.