Absorption disposal systems must be designed with sufficient capacity to store the inflow of a one in three months average recurrence interval design storm, with an emptying time of less than 24 hours.

As a guide in satisfying the above criteria for a roof catchment area of 150 m², an absorption disposal system being an absorption trench would require a volume of 6 m³ (depending on site soil conditions, see above). For each additional 25 m² of roof area or part thereof, 1 m³ of additional absorption trench must be provided.