Perforated Pipe Calculator — Drainage Field Length & Trench Layout

A drainage field (also called a soakaway field or leach field) disperses treated effluent from a septic tank or sewage treatment plant into the ground via perforated pipes laid in gravel-filled trenches. The soil provides final treatment by filtering bacteria. Design follows Approved Document H 2015 (Building Regulations Part H), with BS 6297:2007 as the underlying standard.

Per Approved Document H 2015 §1.44, the drainage field floor area At (m²) = p × Vp × 0.25, where p is the number of persons served by the tank and Vp is the percolation value in seconds per mm (from a percolation test). Pipe length = area / trench width. A 4-person household with Vp of 20 sec/mm needs At = 4 × 20 × 0.25 = 20 m², so roughly 33m of pipe in 600mm-wide trenches (20 ÷ 0.6 = 33.3m). Use our Percolation Test Calculator to work out your site's Vp value first.

Approved Document H 2015 does not set a numerical maximum trench length, but common UK installer practice limits individual distribution trenches to about 18 metres to keep effluent evenly distributed along the pipe. If more length is needed, use multiple parallel trenches fed as a continuous loop from the inspection chamber (AD H §1.44). AD H §1.42 requires 2 metres of undisturbed ground between parallel trenches (edge to edge) — with 600mm-wide trenches that is 2.6m centre to centre.

Venting is optional under Approved Document H 2015. Where provided, the diagram note (AD H 2015, Diagram 1/2 note 1) states the UPSTREAM ends of the distribution pipes may be extended vertically above ground and capped with a cowl or grille. This allows air circulation that supports the aerobic treatment process in the soil. It is good practice on larger fields; check any specific requirement from your sewage treatment plant manufacturer.

Per Approved Document H 2015 §1.41, the perforated pipe is laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm; §1.42 then fills the trench to 50mm above the pipe. With a ~110mm distribution pipe that is roughly 0.46m of granular fill in total. A geotextile membrane is laid over the gravel (§1.42) before backfilling with soil to prevent silt entering the gravel. The pipe must be at least 500mm below the surface.