Sub-slab Ventilation of Methane CO2 & Hydrocarbons

Current guidance (CIRIA Reports & Partners in Technology), has emphasised the importance of Sub-Slab Ventilation, whether it be an active system that is required, or a completely passive system to ensure the controlled and safe removal of ground gases beneath the building.

Venting Layer Vent Types Soil Gases Ventilation Systems
  • Underfloor void
  • Polystyrene void former
  • Geocomposite void former
  • Granular blanket (with or without pipe or geocomposite drain networks)
  • Periscopic air bricks
  • Ground level vent boxes
  • Static vent cowls
  • Rotating vent cowls
  • Gravel trench

Ventilation (Dilute & Disperse)

Purpose: to allow removal and dilution of gases from beneath buildings by the provision of an underslab venting layer. Passive Venting: relies on natural venting due to climatic conditions (predominantly wind). Active Venting: mechanical extraction to safely extract the gases beneath the building and vent to the atmosphere.

Principle of Venting Beneath Buildings

Passive Venting

Passive venting relies on natural venting due to climatic conditions, (predominantly wind). Below is an example of Passive Venting.

Gas Ventilation Beneath Buildings

Active Venting

Active Venting: mechanical extraction to safely extract the gases beneath the building and vent to the atmosphere. Below is an example of Active Venting.

Gas Ventilation Beneath Buildings


The ACS S-S D S system is a hybrid of the industry standard method of venting sub-slab gases to atmosphere, as recommended in CIRIA 149 gases should be extracted and vented to atmosphere.

ACS S-S D S has been developed to protect developments constructed on gas contaminated land.  The system comprises of a number of depressurisation diffusers located beneath the building floor slab and linked into a depressurisation unit comprising a flameproof fan mounted within the extract pipe-work. 
The fan is designed to be permanently running and ensures that a slight vacuum or negative pressure is created beneath the slab.  This removes any ground gases moving upwards towards the slab and allows them to be safely vented to atmosphere.


  • Ease of installation
  • Installation as building work progresses or laid with mass poured floor slabs
  • Remote Audible and Visual alarms at your choice of location
  • Telemetric alarms installed via BT landline
  • Works well with conventional barriers and void formers
  • Full Professional Indemnity Insurance
  • Low Cost Fixed Price maintenance
  • No Perimeter installation work

The system is designed to vent all gases at sub-slab level, once installed it is connected to low power permanently running depressurisation units requiring low cost fixed price annual maintenance.  The system is checked using our unique portable gas monitoring equipment.  Remote audible alarms are installed at your choice of location.
A telemetric alarm can also be installed if required utilising a BT landline.

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Sub-slab Venting Soil Gas Ventilation - ACS Lining Building Gas Protection. Gas membranes, Building gas protection, Soil gas barriers, Methane gas protection barriers, gas separation membranes for industrial buildings, houses, factory, gas membrane installation.