How to Create a Soakaway — Step-by-Step UK Guide

What You'll Need

Before You Start

• Conduct a percolation test (see Step 1 below) to confirm the soil drains adequately.
• Locate the soakaway at least 5m from any building and 2.5m from boundaries.
• Check for underground services — call your utilities or use a CAT scanner.
• Size your soakaway using our Soakaway Sizing Calculator and calculate the pipe fall using our Drainage Fall Calculator.
• If in doubt about soil conditions, get a formal percolation test from a drainage engineer.
• Wear steel-toe boots when digging deep excavations. Shore or batter the sides of any excavation over 1.2m deep.

Step-by-Step Instructions

Step 1: Perform a Percolation Test

Dig a test hole 300mm x 300mm x 300mm deep at the proposed soakaway location. Fill it with water three times over 24 hours to saturate the soil. On the final fill, time how long it takes for the water level to drop from 75% full to 25% full. Divide this time by 150 (the mm of water drop). This gives you the Vp value (seconds per mm). If Vp is between 12 and 100, the soil is suitable for a soakaway.

Step 2: Calculate the Soakaway Size

Use BRE Digest 365 or our Soakaway Sizing Calculator. You need the roof area draining to it, the local rainfall intensity (typically 50mm/hr for most of England, higher for western regions) and the Vp percolation value. For a typical UK house extension (30m² roof), expect a soakaway of approximately 0.8-1.5 cubic metres.

Step 3: Excavate the Pit

Dig the soakaway pit to the calculated dimensions. The base of the pit should be at least 1m above the water table. Keep the sides as vertical as possible. If the pit is deeper than 1.2m, use trench sheets or batter the sides to 45 degrees for safety.

Step 4: Line with Geotextile

Line the pit with geotextile membrane, leaving plenty of overhang at the top. This prevents fine soil particles from clogging the soakaway crates over time while allowing water to pass through.

Step 5: Install Soakaway Crates

Stack the polypropylene crates in the pit. Modern crates click together and can be stacked in any orientation. Cut a hole in the side of one crate for the incoming pipe. Crates provide approximately 95% void space compared to about 30% for rubble — meaning a much smaller pit.

Step 6: Connect the Drain Pipe

Run 110mm underground drainage pipe from the downpipe or gutter outlet to the soakaway. The pipe must fall at a minimum gradient of 1:100 (1% or 10mm per metre). Bed the pipe on pea gravel and backfill with the same. Install a rodding eye or inspection chamber at every change of direction.

Step 7: Wrap and Backfill

Fold the geotextile membrane over the top of the crates, overlapping by at least 300mm. Backfill around and above the crates with clean gravel, then top with excavated soil. The top of the soakaway should be at least 200mm below finished ground level.

Step 8: Test the System

Run a hose into the gutter or drain connected to the soakaway. Check for leaks at all pipe joints. Watch the soakaway area for any water surfacing — this indicates a problem with the percolation or sizing. The water should disappear within a few hours.

Common Mistakes to Avoid

• Skipping the percolation test: Installing a soakaway in clay soil without testing means it will fail in the first heavy rain.
• Too close to the house: A soakaway within 5m can undermine foundations, especially on clay soil that swells and shrinks with moisture.
• No geotextile: Without the membrane wrap, soil silts up the soakaway within a few years, reducing its effective volume.
• Using rubble instead of crates: Rubble provides only 30% void space — you need a pit three times the size compared to crates. Crates are worth the extra cost.
• Insufficient pipe fall: A pipe that is too flat will silt up and block. Maintain 1:100 minimum fall at all times.

Cost Estimate (2026 UK Prices)

Use our Soakaway Sizing Calculator and Drainage Fall Calculator for precise sizing and pipe fall calculations.