Can You Tunnel Under a Staircase When Installing an Interior French Drain?

Two-Directional Staircase

• If the staircase runs down along one basement wall to a landing and then turns and runs along a second basement wall, you’ll have to remove the portion of the staircase below the landing to access the corner of the basement floor.

Single-Wall Staircase

• If the staircase sits only against one basement wall and you have access to the basement floor on both sides of the staircase, you can try to tunnel under instead of removing the bottom stair treads. A boring contractor has equipment that tunnels in tight spots like this, but hiring one is expensive.

Removing the Concrete

• In a standard French drain installation, the contractor makes a line, parallel to the basement wall and 8 inches in from the wall. He makes another line 8 to 10 inches on the inside of that line. He cuts along these lines with a concrete saw and then uses a jackhammer to break out the concrete between the lines. He then removes the soil beneath the basement floor to a depth of about 10 inches. This creates a trench all the way around the basement floor.

Drain Tube

• The French drain pipe, which is a perforated flexible tube, covered with a mesh “sock,” fits into the trench like a long snake, and the two ends of the tube fit into a sump basin that the contractor digs beneath the floor. The contractor fills beneath, around and above the drain tube with 1-inch gravel that acts as a filter to keep sediment from entering the tube and clogging it.

Tunneling Process

• Before you can tunnel under the staircase, you’ll have to break out the concrete trench as close to the staircase as possible on both sides. By placing an iron pipe in the trench beside the staircase, you can drive the pipe with a sledgehammer through the soil beneath the concrete to the other side of the staircase. If you start with 1-inch iron pipe, once you get it through, you can pull it out and drive a larger-diameter pipe through. Eventually, you’ll need to drive a large enough pipe through the hole to insert the drain tubing and have a couple of inches of extra room to work gravel around the tubing.

Considerations

• Unless you have only a foot or two to tunnel beneath the floor, it’s probably simpler in the long run to remove the bottom treads of the staircase and replace them once the French drain is in place. If you don’t tunnel a hole large enough to fill around the drain tube with gravel, the risk of sediment clogging the tube increases.