6.1 Capillary break
A layer of clean, coarse stone under a floor slab. Water climbs up through soil and fine sand, but it cannot climb through the large gaps between stones.
At a glance
- Difficulty
- 1 of 4
- Used under
- Concrete floors on the ground (4.1), basement floors, slabs for sheds and outbuildings
- Tools
- Spade, wheelbarrow, rake, straight board, spirit level
- Parts
-
- 1 Capillary break
- 2 Membrane
- 3 Slab (4.1)
What you need per m², 150 mm layer
| Material | Size | Type | Amount |
|---|---|---|---|
| Capillary stone | 16–32 mm | Washed, clean, crushed or rounded | 230 kg |
| or | 8–16 mm | Only if 16–32 is not available | |
| Membrane | – | Polyethylene sheet, overlapped at joins | 1 m² + laps |
Why no fines?
Small grains leave small gaps, and water climbs up small gaps on its own. In fine sand it climbs about 0.5 m, in silt a metre or more.
Washed stone leaves gaps too large for water to climb. Already in fine gravel the rise is only about 15 mm, so 150 mm of clean stone stops it with plenty to spare.
Step by step
-
1
Dig out to firm soil and level the bottom.
Remove topsoil and roots. Dig deep enough for 150 mm of stone plus the slab and anything that goes between them.
The dug-out, levelled bottom -
2
Spread 150 mm of washed 16–32 mm stone and rake it level.
Every source asks for at least 100 mm. 150 mm leaves margin for an uneven dig. Check the depth with a straight board and a spirit level.
Raking the stone level -
3
Keep the stone clean.
Do not walk mud onto it or let soil slide in from the sides. Every bit of fine material in the gaps lets water climb again.
Soil edge kept back from the stone -
4
Lay the membrane, then cast the slab on top.
The stone stops liquid water. A polyethylene sheet stops the vapour. Overlap the joins. Cast the slab directly on the sheet. If sharp stones could puncture it, cover the stone first with a thin layer of fine, compactable material.
Membrane laid with overlaps
Specifications
| Part | Sieve size | Thickness | How |
|---|---|---|---|
| 1 Capillary break | 16–32 mm or 8–16 mm | 150 mm (at least 100 mm) | Clean, at least 12.5 mm. Placed and levelled. |
| 2 Membrane | – | – | Polyethylene sheet directly under the slab, joins overlapped. |
| 3 Slab | See 4.1 Casting a concrete floor. | ||
How high water climbs
| Material | Capillary rise |
|---|---|
| Fine gravel | 15 mm |
| Very coarse sand | 40 mm |
| Coarse sand | 150 mm |
| Medium sand | 250 mm |
| Fine sand | 500 mm |
| Very fine sand | 1,000 mm |
| Clay | over 800 mm, up to several m |
Gravel and sand from Fetter, clay from FAO, both as reproduced in one review (source 4). Silt values in the same table are uncertain, so they are left out.
Stone size
The sources ask for clean stone of at least ½ in (12.5 mm). 16–32 mm meets that with margin. 8–16 mm is partly finer, so use it only if 16–32 is not available. Ask for washed stone.
Compaction
Place and level the stone (sources 1 and 5). Clean single-size stone can only get about 14 % denser between its loosest and densest state (source 6), so it settles little. Firm it so it does not move underfoot.
Membrane
A capillary break and a vapour barrier both belong under a floor slab on the ground. Lay the polyethylene sheet on the stone and cast the slab directly on the sheet (sources 1 and 2). The sheet also keeps wet concrete out of the gaps between the stones. If sharp stones could puncture the sheet, cover the stone first with a thin layer of fine, compactable fill.
Sources
- Oak Ridge National Laboratory (US DOE), Building Foundation Design Handbook, section 4.1 Slab-on-grade foundations: gravel layer under the slab “at least 4 inches thick, and of clean aggregate no less than 1/2 inch in diameter”; 6-mil polyethylene on top of the gravel, directly under the slab, lapped at least 12 in. Section 2.1 adds that the sheet keeps concrete out of the aggregate as it is cast.
- US DOE Building America Solution Center (PNNL): capillary break beneath slab: “The large spaces between the individual stones prevent capillary action from occurring”; ½ in gravel, 4 in deep; slab poured directly over the polyethylene
- US EPA, ENERGY STAR v3 Water Management Guidebook: “Install a 4 in. bed of at least 0.5 in. clean aggregate” beneath slabs
- Extent of capillary rise in sands and silts (Western Michigan University thesis): capillary rise table after Fetter, Applied Hydrogeology (1994), and FAO (1985) values for clay
- NREL case study, slab capillary break: “Place and level ½-in. aggregate at least 4 in. deep”; uniform-size stone, no fines
- FHWA-HRT-15-034, open-graded aggregates: No. 57 stone, loosest 95.4 lb/ft³ (1.53 t/m³), densest 108.7 lb/ft³ (1.74 t/m³)
Amount: 0.15 m × 1.53 t/m³ ≈ 230 kg per m², loose. Clean stone can settle by up to about 14 % (loosest to densest, source 6), so order about 15 % extra to cover settling and waste.
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