6 Drainage and water
Clean, washed stone without fines. Six jobs about water: stopping it, passing it, storing it or filtering it.
- 6.1Capillary breakStops ground moisture wicking up into a floor slab. At least 100 mm, better 150 mm, of washed 16–32 mm stone under a membrane.
- 6.2Permeable layerThe base under permeable paving. 2–8 mm grit bedding on 150–250 mm of washed 4–20 mm stone.
- 6.3Drainage against wallsFree-draining stone behind retaining walls and next to basement walls, with a drain pipe at the bottom.
- 6.4Drainage trenchA French drain: a perforated pipe in washed stone, wrapped in geotextile, laid to a fall.
- 6.5SoakawayA stone-filled pit that lets roof water seep into the ground. Soil test, pit size calculator and siting rules.
- 6.6Filter layerSand, grit or geotextile that keeps fine soil out of drainage stone. The 4–5 × rule and which combinations hold.
The stone in this chapter all washed, no fines
| Size | Used for | Voids | Loose weight |
|---|---|---|---|
| 2–8 mm | Bedding and joints in permeable paving (6.2), filter grit (6.6) | – | ≈ 1.4 t/m³ |
| 4–20 mm | Permeable base (6.2) | ≈ 40 % | ≈ 1.5 t/m³ |
| 16–32 mm | Capillary break (6.1), wall drainage (6.3), drainage trench (6.4) | ≈ 40 % | ≈ 1.5 t/m³ |
| 32–63 mm | Soakaway (6.5), permeable sub-base (6.2) | ≈ 40 % | ≈ 1.5 t/m³ |
Void ratio about 0.4 for clean single-size stone, from Virginia DEQ and the Minnesota Stormwater Manual. Loose weight from FHWA tests on open-graded stone (No. 8, No. 5 and No. 57; 32–63 mm uses the No. 5 value). Metric sizes are the nearest match to the US sizes in the sources.
Why no fines
Every job here depends on open gaps between the stones. Large gaps stop water climbing (6.1) and let it flow and be stored (6.2–6.5). A size starting with 0– includes the fines, which fill those gaps, so it belongs in the compacted bases of chapter 3, not here. Where fine soil meets clean stone, a filter (6.6) keeps the gaps open.
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