How to Make a Basement French Drain

Co-authored by David Balkan

Last Updated: October 8, 2024 References

A French drain removes water from a basement by collecting moisture in a shallow trench, pulling it into a perforated pipe, and expelling it to a basin and sump pump. Because of the need to dig into the foundation of the building, installing a French drain is a major project and should only be tackled by those with significant building or maintenance experience. Before putting a French drain in your basement, you should already have a basin, sump pump, and exterior drain to which the water collected in the basement can flow.^{[1]
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Steps

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1
Plot out and mark the path of the French drain.^{[2]
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- The drain should run about 1 foot (or 30 cm) away from the basement's exterior wall. Place it in the area with the greatest moisture. It should run to a collection basin in the basement (ideally placed in a corner), from which a sump pump will expel the water to the outside.
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Dig out the floor of your basement along the path of the drain.^{[3]
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- The trench in which you place the drain should be about 8 inches (or 20 cm) wide and 18 inches (or 45 cm) deep. Use a pickaxe or a jackhammer to break through the floor of the basement. Remove the soil beneath with a shovel.
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3
Grade the bottom of the trench.^{[4]
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- Your drain needs to slope downwards in order to effectively carry water to the basin. Tamp down the soil at the bottom of the trench with your shovel, and make the trench 1 inch (or 2.5 cm) deeper for every 8 feet (or 2.4 meters) of length. For example, if you have a trench that is 24 feet (or 7.2 meters) long, the end of the trench should be 3 inches (or 7.5 cm) deeper than the beginning.
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6
Fill the trench around the piping with drainage gravel.^{[9]
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- The drainage gravel allows water to seep down to the bottom of the trench, where it goes into the perforated holes into the piping and flows into the basin. Make sure the trench is filled with gravel all around the piping, including on top, but do not pack the gravel down tight. Cover the gravel with tyvek and tuck under the outside edges (this is to try and keep the cement from seeping into the gravel)
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Seal the trench with cement.^{[8]
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- Quick-setting or quick-dry cement is the easiest option for sealing the trench. If water is coming in from below the floor (high water level) then seal over the entire trench. If water is coming in from leaks in the wall then leave a 2 inch (or 5 cm) gap so as to allow water to run down the wall and into the drain. Easiest to take the edge of the tyvek and pull it back, put a 2x6or 8 tall ways in the trench between the tyvek and the floor edge toward the wall (make sure its above the floor level so it can be removed after the cement dries), then let the tyvek drape over the wood. Mix the cement mix with water and pour it over the gravel that lies on top of and to the side of the piping. Tamp down with a trowel. Let dry for 24 hours. Remove the wood to leave a 1 1/2" weep trench.

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Question

Do I need to put any drainage gravel under the pipe?

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Absolutely, both above and below in order to support any heavy loads instead of having the pipe support anything (and to keep dirt from migrating into and clogging the system). It also allows for some minimal overflow until the pump can catch up. Make sure your piping (black corrugated drain tile) is set at or slightly below the bottom of the foundation's footer.

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Question

We have a french drainage system in our basement but I want to close off the gap between the wall and floor. We don't have water dripping down the walls so the gap isn't necessary. Can we do this with quick drying cement without hurting the drain?

Andy Orr

Community Answer

I could not possibly recommend doing that unless you wanted to render your drain useless. Where will the water run to if it's blocked?

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Question

Can I frame a wall leaving the space between the wall and the floor open without blocking it at all?

Community Answer

Are you refering to a "floating wall"? If a basement, (any) wall is not going to be load bearing, it can "float" top or bottom of wall (usually bottom) with large spikes into floor material. These floats are used for expansion and contraction of the structure so the walls wont bind and crack concrete, sheetrock, etc.

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Tips

All elements of your French drain system--including the collection basin and sump pump at the end--are readily available at any home supply or hardware store. Be sure to buy perforated pipe (which has holes that allows moisture to seep into the drain) and drainage gravel (which allows water to fall through) rather than limestone gravel.

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By placing the piping face-down and surrounding it with gravel, your French drain should remain unclogged. However, if you are particularly worried about silt or other solid matter getting into the drain, consider wrapping the piping in a drain sleeve, which allows water in but filters out larger particles.

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Warnings

Do not use limestone gravel in your pipe trench. Limestone can degrade with moisture and produce a viscous, cement-like substance that will clog your pipes. Always use drainage gravel.

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