A French drain is an underground trench filled with loose stones or rubble surrounding a perforated PVC pipe. The trench, which runs along the interior side of the footing beneath the floor slab, intercepts water and channels it to a sump pump system for removal.
Dig a channel along the perimeter of the basement about 14 inches wide and 12 inches below the floor-slab level. This trench will intercept water at the lowest point of the foundation and direct it into a sump pump chamber. When digging is complete, lay 4-inch-diameter perforated PVC drainage pipe with the holes facing up. To help protect the drain, fill the trench with clean, crushed stone 0.75 to 1.5 inches in diameter.
To discharge the water collected by the drainpipe, it's necessary to install one or two sump pumps. Using two sump pumps - a small submersible pump and a larger backup pump - usually provides the most effective water removal. The final step is to install discharge pipes, which route water removed by the sump pumps outside through the foundation wall.