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There is no hard and fast rule in respect of any design or size of a French drain; therefore, the following suggested data is given as a guideline only, and is applicable when read in conjunction with our copyright sketch plans.

The depths and lengths of a herringbone configuration may vary according to site conditions, such as the quantity of water being discharged and soil permeability. Further, the advantages to be considered in the design/location of a French drain are (a) a high percolation rate of the soil in the area, and (b) prolific vegetation in the area, such as evergreen trees or any other form of growth with a year-round water demand. Alternatively, in the planning stages, it might well be advantageous to locate the herringbone drain beneath a garden or lawn.

Until recent years, it was common practice to dig as large a hole as possible and fill it with every size of boulder, brick rubble and stone in sight. Modern day methods have proved that the most efficient type of French drain / soakaway is one that creates the maximum area of wall surface, which permits the fluid to exit horizontally in all directions.

For example, in the past a hole of say 3 x 3 x 1 metres would necessitate an excavation of 9 cubic metres, producing a wall percolation area of 12 square metres. Our design, as per sketch plans B-W 73/1 and B-W 73/2 (applicable to the “Poly-Rib” septic tank), produces approximately 35 square metres of wall percolation area for an equivalent of 9 cubic metres excavation. The advantages are thus incalculable over designs of yesteryears, and far more simple to construct.

The specifications and guidelines contained in the full French Drain brochure are copyright to ourselves, although they are offered freely to intended users of our “Poly-Rib” septic tanks.


Click here for the full brochure on our French Drain / Soakaway System