Stone Drainage Aggregates

To provide consistent and reliable long-term drainage, readily available and commonly used drainage materials such as crushed stone, pea gravel, or small-diameter river rock must be double-washed and cleaned of fine particles so that they drain freely. The type of stone and particle size distribution may be similar to that used in conventional terra firma subdrainage systems. It should not be from a parent material that can be easily pulverized during quarrying, installation, or long-term use. Fines can clog the air spaces of the drainage aggregate or materials such as geotextile fabrics that are adjacent to the drainage layer.

The weight of a stone aggregate and its maximum water-holding capacity must be considered because the system can become quite heavy and the structural slab must be sized to accommodate it. The additional thickness of the slab can add cost and reduce the allowable planting depth.