Calculate trench materials
The gravel estimate subtracts the approximate volume occupied by the round pipe. Fabric wraps the trench bottom and sides with the selected overlap.
Drainage trench formulas
Gravel = trench volume minus pipe volume, plus allowance
Fabric area = trench length × the combined bottom, two sides and overlap. Pipe length includes 5 percent for connections and trimming.
Resolve the drainage path first
- Identify where water enters the system and where it can legally discharge.
- Confirm the required slope using measured elevations.
- Keep runoff away from foundations and neighbouring properties.
- Check underground utilities before excavation.
- Use cleanouts and access points appropriate to the selected system.
Adding a trench does not correct a site with no viable outlet. Professional design may be required for foundation drainage, retaining structures, high groundwater or large catchment areas.
Last updated August 28, 2026. Confirm local requirements and the complete drainage design before construction.