How-To Guide
How to Trench for Drainage
Drainage only works if the trench holds a consistent fall. Here's a practical walkthrough of getting depth, fall and backfill right the first time.
1. Design the Fall Before You Cut
Drainage relies entirely on gravity, so the fall from the highest point of the run to the outlet has to be established before trenching starts — not adjusted on the fly. Confirm the outlet depth and the fall the drainage design requires over the full run length.
2. Mark the Line and Set Depth Control
Mark the trench line accounting for outlet position and any obstacles. A trencher with continuously variable, laser-ready depth control makes it possible to hold a precise fall over a long run, rather than cutting to a fixed depth and correcting sections afterwards. See our Trenching Depth Guide for how depth control varies across the AFT range.
3. Match Trench Width to the Pipe or Coil
Cut only as wide as the pipe or drainage coil diameter plus the bedding clearance the design calls for. A wider trench doesn't drain any better — it just costs more in spoil handling and backfill material.
4. Cut, Lay, Backfill in Sequence
A trencher with a conveyor system places spoil to one side, keeping the trench clear for the crew laying pipe or coil directly behind the cutting head — then backfilling can follow immediately rather than waiting for a second pass to clear spoil out of the way.
5. Check the Fall Once Backfilled
Confirm the finished fall holds after backfilling and any compaction — settlement can affect a marginal fall, and it's far cheaper to correct before the site is fully reinstated than after.
- Establish outlet depth and required fall before trenching starts
- Use laser-ready depth control where the fall tolerance is tight
- Match trench width to the pipe/coil diameter plus bedding clearance, not wider
- Sequence cutting, pipe-laying and backfill to avoid a second pass
- Re-check fall after backfill and compaction, before final reinstatement
Common Questions
How to Trench for Drainage: FAQ
How do you keep a consistent fall over a long drainage run?
With continuously variable, laser-ready depth control, which lets the operator hold a precise fall over a long run rather than cutting to a fixed depth and correcting sections afterwards.
What trench width does a drainage pipe or coil need?
Only as wide as the pipe or drainage coil diameter plus the bedding clearance the design calls for. A wider trench doesn't drain any better — it just costs more in spoil handling and backfill material.
Why does fall matter more than depth for drainage?
Drainage relies entirely on gravity, so a consistent fall from the highest point of the run to the outlet is what makes the system work — depth on its own doesn't guarantee the water actually moves.
Is agricultural drainage trenching different from general drainage trenching?
The principle is the same, but agricultural drainage is usually run across paddocks at scale with a tractor-mounted trencher, while general drainage trenching is often shorter runs with tighter depth/fall tolerances near structures. See our Agricultural Trenching page for the farm-specific view.
Ready to Put a Trencher to Work?
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