Once the perforated pipe is past the house a few feet, it can transition to a solid pipe. The bottom of the trench can now be dug level so that it gets shallower and shallower until the pipe exits the ground at, or near, the lowest part of your lot. (Tim Carter)

Spring rains are almost here and your basement may soon transform into an indoor swimming pool. In almost all cases, you can stem the tide and create a living or storage space that would be the pride of the Atacama Desert, one of the driest places on Earth.

It’s important to realize that waterproofing a basement in new construction is easy. There are best practices with respect to foundation height above grade, footing drain tile, and there are plenty of magnificent products and systems a builder can use to keep water from entering a basement space.

However, there are many more existing homes with chronic basement leaks than new houses being built in the Washington area. You may wonder what your options are when it comes to keeping water from entering your basement or crawl space. Fortunately, my college degree in geology with a focus on groundwater has allowed me to stop water from entering tens of thousands of existing homes just like yours.

The water that’s trying to enter your basement comes from three primary places: your roof, overland from land higher than your lot and water flowing through the soil toward your home.

All too often, I see roof downspouts dump water onto splash blocks next to a home’s foundation. This is poor practice. Roof water should be piped to local storm water drains if allowed. If this is not an option, then pipe it to the lowest part of your lot away from your home.

In periods of heavy rain, parts of your lot may resemble a small stream or a pond. Your lot should have been graded so that the soil tilts away from your foundation. The minimum slope should be six inches of fall in the first 10 feet of horizontal distance away from your foundation. More slope is better.

If you don’t have the proper slope around your foundation, you may want to take some measurements and see what can be done to correct it. Patios need to slope away from foundations and shed water just like bare ground.

There are quite a few retrofit basement waterproofing systems that contractors sell. Based on 40 years of solving leaking basements, I can tell you that stopping the water outside your basement is a far better remedy. Many other systems try to collect the water after it’s entered your basement.

When it rains, water soaks into the topsoil in all the land that’s above your home. There could be acres and acres of land that slopes toward your home. This water tries to go deeper into the soil, but in almost all cases it hits a dense layer of subsoil and starts to move sideways, or downslope, through the soil toward your basement.

You can capture this underground water before it gets to your basement. In almost all cases, a simple six-inch-wide trench about two feet deep is all that’s required. I’ve had success with trenches as shallow as 16 inches should you have a relatively flat lot.

This trench should be placed in your yard about six feet away from your foundation walls for the best results. The trench needs to be placed on the high side of your lot where the upland ground is sloping toward your home. The trench then wraps around your home on one or both sides.

The bottom of the trench can be level, but it’s best if you can incorporate a minimum amount of fall. One-eighth of an inch per foot is plenty of slope. A two-inch layer of washed gravel the size of common grapes is placed in the bottom of the trench and then a four-inch diameter perforated drain pipe is placed on this gravel. Fill the trench to within 1 1/2 inches of the surface with more of the washed gravel. Be sure there’s no sand in this gravel. Put pieces of sod on top of the gravel.

Once the perforated pipe is past the house a few feet, it can transition to a solid pipe. The bottom of the trench can now be dug level so that it gets shallower and shallower until the pipe exits the ground at, or near, the lowest part of your lot.

Thousands of gallons of water flowing through the soil will encounter the trench and fall through the gravel. The water finds it easier to flow into and through the pipe rather than push its way through the soil toward your basement. The amount of water flowing out the end of the trench drain in wet weather will amaze you.

Don’t let contractors talk you into installing filter fabric in the trench or around the pipe as a sock. It’s not required and it slows the movement of the water. Water flowing through soil is silt-free. This is why well water is clear.

You can view a video of a working trench drain at my AsktheBuilder.com website. Just type: linear french drain video into my search engine and you’ll be on your way to a dry basement in no time!

Tim Carter, author of the Ask the Builder column, wrote this story for The Washington Post. He can be reached at AsktheBuilder.com.