When we replace the drainage system on a building it is important that the lowest point of the French drain is the evacuation. A good drainage system works only by gravity. Despite the height of the foundation footings that may vary due to garages or extensions that have been added over the years, all the water must go towards the evacuation. The pipe that directs the water to the inner pit must be unperforated to prevent outside water from splashing under the concrete slab.