Intermediate

Ventilated Sub-Base Under Slab: Guide and Installation

11/04/2026 12 min read

Under your concrete slab, air and moisture normally stop circulating once the pour is done: everything rising from the ground — water vapour, radon, decomposition gases — hits the DPM and can eventually weaken the slab or work its way up through any weak point. A ventilated sub-base solves this by turning the coarse gravel layer into a true ventilation network: air circulates permanently through the sub-base, sweeps away moisture and evacuates radon to the outside. Mandatory in radon zones 2 and 3 (French classification), recommended everywhere else on damp ground — here is how to build one.

Ventilated sub-base: principle and role

A standard sub-base (20 to 30 cm of 20/40 crushed aggregate or hardcore) plays three roles: bearing platform, vertical drainage and capillary break. The problem is that it remains hermetically sealed by the slab and DPM above: the trapped air no longer circulates, and water vapour migrates slowly through the slab if it finds any gap in the waterproofing.

The ventilated sub-base adds a network of perforated drains embedded in the gravel, connected to:

Air inlets on one facade (rodent-proof grilles)
An air outlet at high level, typically through the roof, to create natural stack-effect draught

What ventilation achieves in practice

Problem solved	How
Radon (radioactive ground gas)	Drawn away by the airflow before it can migrate through the slab
Water vapour rising from the ground	Captured and evacuated continuously
Condensation under the DPM	Eliminated by air renewal
Termites (risk zones)	Dry, aerated environment hostile to their development
Decomposition gases (organic fill)	Evacuated before they accumulate

💡 Tip — A ventilated sub-base is not just a radon precaution: on clay or damp ground, or over former fill, it extends the life of the slab and prevents moisture rising into interior wall linings. For £150–350 of extra PVC drain, the cost-benefit ratio is unbeatable.

Standard or ventilated sub-base: how to choose

Question

The choice depends on three criteria: the radon zone, the nature of the ground and whether you have a ventilated crawl space. If you have already opted for a ventilated crawl space, the ventilated sub-base loses its point — ventilation is already provided by the void. On a ground-bearing slab, however, the question arises systematically.

flowchart TD A{Type of ground floor?} -->|Ventilated crawl space| B[Standard sub-base
ventilation = crawl space] A -->|Ground-bearing slab| C{Radon zone?} C -->|Zone 2 or 3| D[VENTILATED SUB-BASE
mandatory or strongly advised] C -->|Zone 1| E{Damp or clay
ground?} E -->|Yes| D E -->|No| F{Former fill or
made ground?} F -->|Yes| D F -->|No| G[Standard sub-base
20-30 cm 20/40 aggregate] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style C fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style E fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style F fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style D fill:#56C6A9,stroke:#56C6A9,color:#fff style B fill:#F58220,stroke:#F58220,color:#fff style G fill:#F58220,stroke:#F58220,color:#fff

Radon zones

In France, the IRSN publishes a radon potential map by commune, classified into three levels (note: this is the French radon authority’s map):

Zone 1 — Low potential. No obligation, but vigilance on granite ground.
Zone 2 — Low potential with aggravating factors (permeability, fractures). Ventilation advised.
Zone 3 — Significant potential. Mandatory measures for new buildings and public buildings: ventilated sub-base, ventilated crawl space or enhanced ventilation.

In the UK, the British Geological Survey (BGS) and the UK Health Security Agency (UKHSA) publish equivalent radon maps by postcode. UK Building Regulations Part C and the BRE radon atlas define protective measures for new builds. High-risk areas include Cornwall, Devon, Northamptonshire, Derbyshire and parts of Scotland.

⚠️ Warning — Radon is the second leading cause of lung cancer after smoking. In a well-insulated, airtight house, natural dilution is insufficient: without sub-floor ventilation, radon accumulates in the living space. Installing a ventilated sub-base on a high-performance airtight build in a radon zone is non-negotiable.

Composition of the ventilated sub-base

The principle is simple: a bed of coarse, even-graded aggregate in which perforated drains are embedded, linking an air inlet to an air outlet.

1. Formation level

As with a standard sub-base, start from stripped and compacted ground:

Strip topsoil to 20–30 cm below the finished sub-base level
Compact with a plate compactor (2–3 crossed passes)
Level to avoid low spots that would retain water

2. Aggregate — grading and depth

Unlike a standard sub-base where crushed run is acceptable, the ventilated sub-base requires coarse, single-sized aggregate to preserve the voids between stones through which air will circulate.

Characteristic	Recommended value
Grading	20/40 mm or 40/60 mm (never 0/X all-in)
Type	Washed rounded or crushed aggregate
Depth	200 mm minimum, 300 mm recommended
Compaction	Light only (preserve voids)
Quantity	~0.25 m³/m² of floor area

⚠️ Warning — Never use all-in hardcore or crushed run for a ventilated sub-base. Fines block the voids between stones and completely suppress air circulation. The aggregate must be washed and open-graded — this is non-negotiable.

3. Perforated drains

These are the lungs of the system: perforated PVC pipes or land drains laid within the gravel, spaced and connected to form a network.

Parameter	Typical value
Diameter	DN 100 mm (perforated PVC drainage pipe)
Spacing	2 to 3 m between parallel drains
Orientation	Perpendicular to the longest dimension
Depth in sub-base	Mid-height (100–150 mm below slab)
Number of drains	1 drain per 2–3 m of width

For a 10 × 8 m house, allow 3 longitudinal drains of 10 m, connected to a transverse manifold that feeds the air outlet.

4. Air inlets and outlets

This is what makes the difference between a properly ventilated sub-base and a pile of gravel with useless pipes.

Air inlets (low level):

Stainless steel rodent-proof grilles, minimum 100 mm diameter
Positioned on the windward facade
At least 150 mm above external ground level to avoid surface water
1 inlet every 2–3 m of facade

Air outlet (high level):

Vertical stack rising through the structure to the roof
At least 2 m above the ridge to ensure draught
Rain cap at the top
Cross-section at least equal to the sum of all inlets

💡 Tip — To boost airflow on a project with high radon exposure, you can add a low-consumption extractor fan (10–15 W) at the top of the stack. The system moves from natural ventilation to mechanical ventilation, and the flow rate becomes stable regardless of weather conditions. Budget £130–220 for a controllable helical extractor.

Step-by-step installation

Conseil

The order of work is critical: each layer determines the next. Do not improvise on drain layout or the route of the air outlet stack — these two points are virtually impossible to fix once the slab has been poured.

Step 1 — Formation level preparation

Strip topsoil to 20–30 cm below the base of the future sub-base
Compact with a plate compactor in two crossed passes
Check flatness with a 3 m straight-edge: tolerance ± 20 mm
If the ground is clay, lay a geotextile membrane to prevent fines migrating into the aggregate

Step 2 — Setting out the air outlets

Before laying any aggregate, mark out the drain routes with a string line:

Mark each drain route on the ground
Mark the air inlet positions on the perimeter walls
Mark the position of the vertical outlet stack — it will need to penetrate the slab, ground floor and every storey up to the roof

Step 3 — First layer of aggregate (100–150 mm)

Spread a first layer of 100–150 mm of 20/40 aggregate across the whole area. Level with a straight-edge without compacting: the aim is a flat surface on which to bed the drains at the correct height.

Step 4 — Laying the perforated drains

Unroll the perforated PVC pipes according to the layout plan
Orient the perforations downward (to draw up ground moisture)
Assemble tees and bends toward the manifold
Connect the manifold to the vertical outlet stack
Connect each drain end to an inlet grille on the facade via an elbow
Temporarily cap the free ends to prevent aggregate entering during backfill

Step 5 — Second layer of aggregate

Top up with 100–150 mm of aggregate to the final sub-base level. Spread by shovel — never roll or compact heavily over the drains as they do not resist crushing.

Step 6 — DPM, insulation, slab

The remainder is identical to a standard slab: 250-micron DPM with 300 mm laps, XPS or EPS insulation, reinforcing mesh, concrete pour. See the detailed guide on pouring a concrete slab.

⚠️ Warning — During the concrete pour, protect the vertical outlet stack with a temporary PVC cap and a rag. A single trickle of cement slurry into the drain can drastically reduce airflow once hardened.

Dimensions and typical bill of quantities

For a 100 m² ground-bearing slab (10 × 10 m) in a high radon zone:

Item	Quantity	Indicative cost
Washed 20/40 aggregate (250 mm depth)	25 m³	£600–900
Geotextile 150 g/m²	110 m²	£90–160
Perforated PVC pipe DN 100	40 m	£160–260
Manifold + fittings + bends	1 set	£65–130
Stainless steel rodent-proof grilles	4–6 units	£50–100
Outlet stack (PVC pipe 100)	8–10 m	£65–100
Low-consumption extractor fan (option)	1	£130–220
Total materials		~£1 160–1 870

The extra cost over a standard sub-base is around £330–530 — mainly for pipes, grilles and stack. The aggregate difference (washed 20/40 vs all-in hardcore) adds £160–330 depending on the region.

✅ Best practice — If you are in a moderate radon zone (not mandatory) but are unsure, have a radon measurement carried out after construction (2-month dosimeter, £25–40). If the concentration exceeds 300 Bq/m³, you can still add an extractor fan — but you cannot redo the sub-base. Better to play it safe from the start for a few hundred pounds.

Mistakes to avoid

Aggregate with fines — Using all-in hardcore instead of washed 20/40: zero air circulation, system inoperative.
Perforations facing upward — Fines and water enter the pipe rather than being drawn away.
Outlet stack too short — Below ridge level, the draught reverses in strong winds. Always 2 m above the ridge.
Forgetting the low-level air inlet — Without an inlet, there is no airflow: the outlet only draws vapour through slab defects.
Rolling compactor over the drains — Crushed pipes, system destroyed.
Blocking the outlet stack at handover — It must pass cleanly through each floor and remain accessible for maintenance.
Neglecting the rodent-proof grilles — Mice love drain inlets and may nest in the sub-base.

Maintenance and inspection

The ventilated sub-base requires virtually no maintenance once in place. Two annual checks are sufficient:

Inlet grilles: clear of leaves, snow and cobwebs. Allow 5 minutes.
Roof outlet: cap in place, no bird nests. Do this during the annual roof inspection.

If in doubt about operation, a smoke test (smoke bomb in an inlet grille) gives a visual check of airflow through to the outlet. Budget £8–16 for a smoke bomb.

To validate actual performance, a 2-month radon measurement (Kodalpha or Radonova dosimeter) gives an official reading in Bq/m³. The target under UK Building Regulations and WHO guidance is < 300 Bq/m³, ideally < 100.

Links with other works

The ventilated sub-base is part of a chain of groundworks and structural works that must be coordinated from the planning stage:

Choosing your foundation type — to set the base level of the sub-base
Pouring a concrete slab — the logical next step above the sub-base
Insulating the ground floor (crawl space or slab) — insulation to lay between DPM and slab
Drainage pipes EU/EP: falls, inspection chambers and installation — must be done before the sub-base to avoid clashes