How Much Slope to Put on a Trench When Installing a Drain Tile?

French Drain Slope

• A French drain consists of perforated pipe known as drain tile that is embedded in clean gravel. Ground water, following the path of least resistance, penetrates the gravel and enters the drain tile through the perforations. It flows down to a pumped sump, dry well, storm drain or surface outlet for disposal. Recommendations vary on how steeply to slope drain tile. Some local codes require drain tile to have the same 2 percent slope as sanitary sewers. That means the tile must drop by 2 vertical feet for every 100 linear feet of tile. Other experts recommend approximately a 1 percent slope. A 1 percent slope means the drain tile will drop by about 1 vertical foot for every 100 linear feet.

Drain Trenching

• The first step in installing a French drain is digging a trench. The trench should be about a foot wide and as deep as needed to achieve the desired slope. To check slope, set a reference stake near the structure and drive other stakes along the route of the trench. Run a string from the reference stake to the trench stakes and tie the string exactly level. Measure the depth from each string to the trench bottom and later to the top of the drain tile. Start trenching from the low spot where the water will drain into your sump, dry well, storm drain or surface drainpipe. When using a French drain to relieve wet basements or crawl spaces, the trench depth should be below the top of the foundation footing.

Lay Tile

• Once your trench is done, line it with permeable landscaping fabric and fill it with washed 1-inch gravel to a depth of about 4 to 6 inches. Drain tile comes with rows of holes at the 4 o’clock and 8 o’clock positions. You should use drain tile that’s at least 4 inches in diameter. Lay the drain tile with the holes down. The collected water will enter the pipe through the holes and flow down the “invert,” the gutter-like space between the rows of holes.

Never Upward

• Never lay drain tile with the holes upward. This defeats the goal of collecting the maximum amount of ground water and invites clogging. Fill the trench with more gravel up to a point about a foot below the surface. Cover the gravel with more permeable landscaping fabric and backfill with soil, working from the high end to the low end. Connect the drain tile to your sump, dry well, storm drain or surface outlet and cover as appropriate.