A soakaway manages excess surface or storm water by allowing it to percolate slowly back into the surrounding soil. Combined with a French drain — a perforated pipe in a gravel trench — it forms one of the most effective stormwater management systems for South African properties, especially in winter-rainfall regions like the Western Cape. This guide compares soakaways and French drains, explains how soakaway crates work, and sets out what installation and maintenance looks like for a typical Cape Town property.
Table of Contents
What is a Soakaway?
A soakaway is an underground void that holds stormwater temporarily and allows it to seep gradually into the surrounding ground. Traditional soakaways were simply rubble-filled pits; modern designs use lightweight plastic soakaway crates wrapped in geotextile fabric. The crates create a high-void-ratio chamber (typically 95% void) that stores far more water per cubic metre than rubble fill, and they are designed in modular blocks so you can size the soakaway to the catchment area and the soil’s infiltration rate.
Soakaways are commonly used to dispose of roof runoff, paved area drainage and stormwater from yards where connection to a municipal stormwater system is impractical. The underlying principle is the same regardless of construction method: store, then infiltrate.
What is a French Drain?
A French drain is a trench filled with gravel that contains a perforated pipe. Surface water and shallow groundwater enter through the gravel, drain into the pipe, and the pipe carries the water away to a designated discharge point — which is often a soakaway. Where a soakaway disposes of water in place, a French drain moves water from where it collects to somewhere it can soak in or be discharged. Read our companion guide on how French drains work for installation detail.
Soakaway vs French Drain: Which Do You Need?
For most Cape Town properties the answer is “both, working together.” A French drain collects water along a wall, driveway edge or yard low spot, and the pipe discharges into a soakaway sited a few metres away. The two systems combined are sometimes referred to as a “French drain soakaway.”
- Roof and paved runoff with no municipal stormwater connection: soakaway only.
- Wet patch in a garden or against a wall: French drain feeding a soakaway.
- Sub-surface seepage on sloping ground: French drain only, discharging to a lower-ground watercourse.
- Heavy clay soil: soakaway alone may not work — percolation testing is essential.
Installing a Soakaway with a French Drain
Step 1: Percolation Test
Before designing the soakaway, the soil’s infiltration rate must be measured. A simple percolation test (BRE Digest 365 method, also referenced in SANS drainage standards) involves digging a test pit, saturating the soil and measuring how fast a known volume of water drains away. Sandy soils typically pass; heavy clay often fails.
Step 2: Sizing
The soakaway volume is sized against the catchment area and a design rainfall event. Undersized soakaways flood; oversized soakaways waste money. A 100 m² roof in Cape Town typically requires 1.5–3 m³ of soakaway crate volume, depending on soil infiltration.
Step 3: Excavation and Trench Preparation
Excavate the French drain trench (typically 300–450 mm wide, 600 mm deep) and the soakaway pit (sized to take the crates plus 150 mm of gravel surround). Line both with geotextile fabric to prevent fines migrating in.
Step 4: Pipe and Crate Installation
Lay perforated HDPE pipe in the French drain trench on a gravel bed, with perforations down. Connect the outlet to the soakaway crate array. Backfill the trench with 20 mm clean stone up to within 100 mm of the surface.
Step 5: Backfill and Reinstate
Wrap the crate array fully in geotextile fabric, backfill with site-won soil, and reinstate the surface. The whole system should be invisible once the grass grows back.
South African Considerations
- Winter rainfall regions (Western Cape): design for the wet-season storm intensity, not annual average.
- Summer rainfall regions (Highveld, KZN): short-duration high-intensity storms drive the design — a smaller, faster-draining system is usually appropriate.
- Building regulations: SANS 10400 and SANS 10252 cover stormwater drainage from buildings; your local municipality (e.g. City of Cape Town) sets the discharge rules.
- Recycled materials: crushed concrete and recycled HDPE perforated pipe are both eco-friendly and cost-effective.
Maintenance Tips
- Inspection chamber: include one at the inlet to the soakaway for periodic checks.
- Annual clear-out: remove leaves and silt from any inlet gullies or inspection chambers.
- Watch for ponding: water standing on the surface above the soakaway suggests the system has clogged or saturated — a professional drain clearing team can jet-clean the perforated pipe.
- Avoid heavy loading: don’t drive vehicles over crate-style soakaways unless they’re rated for traffic loading.
Conclusion
A well-designed soakaway — usually fed by a French drain — is the most reliable answer to surface water problems on properties without a municipal stormwater connection. Get the percolation test, the sizing and the geotextile detailing right and the system will run for 20 years with minimal attention. Get them wrong and the result is a soggy lawn at best, structural damp at worst. If you’d like Drain Blasters to assess a wet area on your property and recommend a soakaway specification, get in touch via our contact page.
