Perimeter drain
A perimeter drain is a slotted ag pipe in aggregate around a slab or retaining wall footing, intercepting groundwater before it reaches the footing zone.
Ask Chalkline about this →A perimeter drain is a slotted subsoil pipe laid in a gravel bed around the perimeter of a slab or retaining structure, intercepting groundwater before it reaches the footing zone. It is the primary defence against moisture accumulation beneath reactive-soil slabs and against hydrostatic pressure behind retaining walls.
What it consists of. A standard installation uses a 100 mm diameter slotted or perforated pipe (rigid PVC or corrugated HDPE) seated in a free-draining gravel surround. A geotextile fabric sock wraps the pipe and gravel to prevent soil fines from migrating into the aggregate and silting the pipe. The whole assembly sits at or just above footing level so water is intercepted at the lowest point before it enters the footing zone.
Where it is required. AS 2870:2011 expects perimeter drainage as part of the site drainage specification for residential slabs on reactive soil sites (verified 2026-06-11, Standards Australia). The engineer’s drawings must show drain location, fall, and discharge detail. AS 4678:2002 requires the same drainage aggregate plus subsoil drain arrangement at retaining structure bases to relieve hydrostatic pressure.
Falls and discharge. NCC 2022 Housing Provisions Part 3.3.4 requires subsoil drains to fall at a uniform minimum 1:300 (verified 2026-06-11, ABCB). Discharge goes to a legal point of discharge (stormwater pit, kerb, or council main) per AS/NZS 3500.3. Discharging near the footing is not accepted on reactive clay sites: it reintroduces moisture into the zone the drain protects.
Common defects.
- No fall or reverse fall: pipe ponds, fills with silt, drain ceases to function.
- Missing geotextile sock: soil fines block the gravel surround within a few wet seasons.
- No outlet or outlet sealed: water fills the pipe but has nowhere to go.
- Crushed or kinked pipe: common where backfill is over-compacted before concrete is placed.
- Discharge near the footing: undermines the reactive-soil protection the drain provides.
Also known as: perimeter ag drain, footing drain, edge drain, perimeter subsoil drain.
Category: Drainage / foundations / civil.
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Last updated: 2026-06-11. Verified: 2026-06-11.