First what is a French drain and what does it do to keep water off the floor of your Basement?
A French Drain is designed by cutting around the floor of your basement at the TOP of the Footer. A Box channel plastic drain is placed around the edge of the basement wall. As water leaks down the walls and underneath the footer the French drain captures the water, sends it to a sump pump and pumps it out of the home to keep the water off the floor.
I recently inspected a foundation were a French Drain was used to correct a water problem. First it is true that there was no water on the floor. Two humidifiers ran to dry the air in the basement and pumped that water directly into the sump pump and out of the home. The Cinder Block Walls were leaking white lime. The walls were wet and seeping water. The walls were cracked and bowing and there was mold and mildew growing everywhere.
The problem is that even though the French Drain was running correctly, a French Drain was never designed to Address the issues with the walls.
Here are the 3 reasons a French Drain will not solve your Foundation issues:
1) Pressure on the walls continues from the Ground water. In fact because the flooring was cut it weakens the support of the wall and may cause the wall to move, crack and buckle faster.
2) Mold and Mildew continues to grow inside the walls, as water is still leaking into the wall from ground and surface water.
3) The wall will continue to crack and crumble as the water comes down the wall around the footer and under the floor. The footer will continue to sink into the ground and will create step cracks in the wall.
The only solution to solve your foundation issues is to correct the issues with the wall, footer and the floor. You must drain the wall and relieve the pressure on the wall. You must Solidify the Footer by creating alternative routes for the water. You must dig down to the Bottom of the Footer to drain all the water out beneath the floor. It is important to provide spacer with the floor as not to release the support between the floor and the wall. As simple French drain at the top of the footer solves none of these issues.