Module 7 · Proportionate Remediation
7.2 External and Drainage Works First
8 min read
A downpipe discharging against a wall puts hundreds of litres into it in a single storm. Ground raised above the internal floor level bridges every barrier the building has. A blocked gulley concentrates a roof's entire catchment at one point. These are the largest water inputs a building receives, they are the cheapest to fix, and they are routinely left out of specifications in favour of interventions costing twenty times as much and addressing a fraction of the volume.
Learning objectives
By the end of this lesson you will be able to:
- Prioritise interventions by the volume of water each addresses.
- Carry out an external inspection that finds the actual inputs.
- Specify drainage and ground-level works with the detail needed to execute them.
- Explain to a client why the cheap works come first.
Key terms
- Catchment concentration
- A roof's whole rainfall arriving through a small number of outlets. A single blocked or leaking one delivers a very large volume to one place.
- Ground level differential
- The relationship between external ground and the internal floor and any DPC. Where external is higher, every barrier is bypassed.
- French drain
- A gravel-filled drainage trench alongside a wall, intercepting water and lowering the effective ground level against it.
- Splashback zone
- The band of wall wetted by rain rebounding off a hard surface below — typically the lowest 300–450mm above a path or paving.
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