Ground Gas Venting

Ground gas venting systems disperse hazardous ground gases from beneath and around a building, working alongside a gas barrier membrane to form a complete gas protection strategy. 

What is a ground gas venting system?

A ground gas venting system is a network of components installed beneath and around a building to collect hazardous ground gases and disperse them to atmosphere before they can accumulate. On contaminated or gassing ground, a gas barrier membrane alone prevents gas entering the structure, but on higher-risk sites the gas beneath the slab must also be actively managed. The venting system provides that route, creating a void beneath the floor slab through which gas is collected and channelled away, either passively or, where required, with mechanical assistance.

A complete system has three stages: a collection layer beneath the slab, a network of pipework that carries the collected gas, and external outlets that release it safely to atmosphere at the building perimeter or within the surrounding landscaping.

When is a gas venting system required?

The level of gas protection required on a site is determined by the ground investigation and the risk characterisation carried out in accordance with BS 8485:2015+A1:2019. On lower-risk sites a gas barrier membrane may be sufficient on its own. As the characteristic gas situation increases, a venting layer is added beneath the slab to relieve pressure and dilute gas concentrations, reducing the load on the barrier and providing a second line of defence.

Methane and carbon dioxide are generated by decomposing organic material and are a particular risk on brownfield land, former landfill and made ground. Radon is a naturally occurring radioactive gas found across parts of the UK, and volatile organic compounds are associated with former industrial sites. Where the ground investigation identifies elevated gas concentrations, a venting system designed to the specific site conditions forms part of the protective measures required before construction of the ground floor slab.

Ground gas venting system components

The venting range covers each stage of the system, from collection beneath the slab through to the external outlet.

Gas collection

The gas vent mat is a cuspated geocomposite laid beneath the reinforced concrete floor slab to create a shallow venting void. Gases collected across the mat are drawn towards the pipework for dispersal. On sites requiring a defined collection route, slotted carrier pipe is laid within the venting layer to gather gas along its length and feed it into the solid pipework. Mat connectors provide the transition between the collection layer and the pipe runs.

Vent pipework and fittings

Solid vent pipe carries the collected gas from the collection layer to the external outlets. The pipework is built and routed using preformed HDPE fittings: couplings to extend straight runs, bends to change direction, T fittings to form branch junctions, end caps to close runs, flexible connectors to take up alignment and movement, and adaptors to connect to outlet components. The smooth internal bore of the fittings maintains consistent gas flow through the network.

External outlets

At the building perimeter, the collected gas is released to atmosphere through an external outlet. Gas vent boxes are set into paths, patios and trafficked ground, with a heel-safe locking lid suitable for pedestrian areas. Gas vent bollards provide an above-ground outlet where the dispersal route cannot be taken through a perimeter masonry wall, such as at curtain walling or glazed elevations. Airbricks, fitted with an adjustable vent and extension sleeve, release gas through the external wall face while blocking wind driven rain and insects.

Passive and pressurised venting

Most low-rise and residential applications use passive venting, where gas disperses through the system under natural pressure differences and airflow, without mechanical assistance. This is the simplest and most maintenance-free approach and is suitable for the majority of sites where a venting layer is required.

On higher-risk sites, or where the building footprint and layout limit natural airflow, a mechanically assisted or pressurised system may be specified, using fans to actively draw gas through the venting layer and out through the outlets. The choice between passive and pressurised venting is a design decision based on the site gas regime, the building type and the guidance in BS 8485:2015+A1:2019.

Choosing the right venting components

The venting system is specified as part of the overall gas protection design for the site, and the components required follow from that design. The gas vent mat provides the collection layer; the number and type of outlets follow from the free air flow capacity required and the building perimeter; and the pipework and fittings are selected to build the runs that connect them. Because the system works in conjunction with the gas barrier membrane, the two should be specified together against the site gas assessment. Where you need help matching components to a gas protection design, our team is available to assist.