How to Build an Internal French Drain

Break up and remove concrete from the area from the floor slap around the perimeter where the drain will be placed and the location between the perimeter drain and the sump pump. A jackhammer sill be most ideal for this but a concrete saw or another tool may be utilized. Remove concrete only from the slab. Avoid damaging the footing, which must remain intact to support the wall.

Dig a trench along the perimeter where the drain pipe will go. A mattocks or spade and hand trowels may be most ideal for this. The trench should be located next to the footing and be about 4 to 6 inches deep and at least as wide. The existing location and depth of the sump pump basket or intake must be taken into account, as the pipe must slope to this outlet.

Drill a series of 3/8-inch weepholes along the bottom edge of the wall if the wall is made from concrete masonry units or has a similarly-open interior. Use a rotary hammer to drill these holes and be prepared for water to pour out of these weepholes immediately.

Line the trench with landscaping cloth or filter fabric, if desired. In especially silt soils, landscaping cloth may be necessary to prevent the pipe from clogging.

Place an inch or two of clean, coarse gravel in the bottom of the trench.

Lay the drain pipe atop this gravel. Different types of piping may be used, ranging from perforated PVC or corrugated plastic to specialized interior drains. The drain pipe should have a slight slope, generally between about 1 and 2 percent, toward the sump pump intake or basket.

Fill in the space around the pipe with gravel. Cover the pipe and gravel with landscaping cloth, if desired.

Place a plastic or metal weep flange on the footing or trench so that it extends up the foundation wall. A flange is designed to leave a small gap along the wall to force water flowing down the wall into the sub-slab drainage pipe.

Make any adjustments necessary to attach the drain pipe or otherwise direct water to the sump basket. A connector pipe may be required. If the sump basket is perforated to allow water inflow, the sump basket should be surrounded by gravel.

Fill in most of the remainder of the trench with gravel, leaving a space for the necessary depth of concrete patch.

Apply concrete patch to the top of the drain and any area around the sump where the slab was removed to accommodate the drain. Prepare and apply concrete patch according to manufacturer instructions. Apply the patch so that the only visible portion of the system is a portion of the vertical section of the flange.