Ground gas characteristic situations CS1 to CS6 explained

The characteristic situation is the single number that decides what gas protection a building needs. It runs from CS1 to CS6, it is derived from a gas screening value calculated from monitored concentration and flow, and it feeds directly into the protection score that BS 8485 requires for the floor construction.

Reviewers who only ever see the conclusion should know how fragile the number is. A characteristic situation derived from four monitoring visits in settled summer weather is not the same evidence as one derived from twelve visits including three during falling atmospheric pressure, even where both reports print CS2 on the front page.

Why ground gas gets its own classification

Ground gas behaves differently from soil and groundwater contamination. The exposure pathway is accumulation inside a confined space rather than ingestion or dermal contact, the hazard is asphyxiation and explosion rather than chronic toxicity, and the risk is controlled by construction detail rather than by removing material.

The gases that matter in most assessments are methane and carbon dioxide. Methane is flammable between roughly 5 and 15 per cent by volume in air, and carbon dioxide is an asphyxiant that displaces oxygen and is heavier than air, so it collects in voids, service ducts and basements.

Sources divide into three broad groups. Landfill and made ground with a degradable organic fraction. Natural sources including peat, alluvium, organic clays and some mudstones. And mine gas in coalfield areas, which behaves differently again and needs the coal mining risk to be considered alongside it.

Volatile organic compounds and vapour intrusion sit outside the CS framework and are assessed separately.

How gas screening values are calculated

The gas screening value is the product of the maximum gas concentration recorded, expressed as a percentage by volume, and the maximum borehole flow rate, expressed in litres per hour. The result is a notional emission rate in litres per hour.

Three points where reports go wrong:

• flow rate must be a measured borehole flow, taken with an appropriate low-flow instrument, rather than a default or an assumed value
• the concentration and flow used should be the values that represent the worst realistic case for the site, and the report should say how they were selected
• a negative flow reading is not zero, and a period of borehole inflow tells you something about the pressure regime that a single peak reading does not

Where flow rates are at or below the detection limit of the instrument across every round, the screening value collapses towards zero, and the assessment then rests on concentration and on the conceptual model rather than on arithmetic.

The bands and what each implies

The classification derives from the CIRIA guidance on hazardous ground gases and is used alongside BS 8485.

Treat the bands as a starting point rather than an answer. The guidance is explicit that the screening value alone does not determine the classification, and that concentration, the conceptual site model, the source term and the building type all inform it. A site with a low screening value but a persistent 20 per cent carbon dioxide concentration is not a CS1 site in any meaningful sense.

At CS5 and above, generic protection tables stop being appropriate and the design moves to a bespoke engineered solution.

Monitoring rounds and weather conditions

The monitoring programme is where most reviews find their objections.

1. Number of rounds. Lower risk sites are commonly monitored across at least six visits over three months, and landfill-affected or higher risk sites across twelve or more visits spanning six months or longer. A programme of three visits over three weeks does not support a low classification.
2. Atmospheric pressure. Gas emission rises as barometric pressure falls, so the programme has to include visits during falling pressure and at low absolute pressure, conventionally below 1,000 millibars. Record the pressure and the trend at each visit.
3. Seasonality. Winter monitoring on a site with a shallow water table gives different results from summer monitoring, because rising groundwater displaces gas from the unsaturated zone.
4. Installation quality. Response zone depth, screen position relative to the source and the water level, bentonite seal integrity and headworks all determine whether the borehole is measuring the source at all.
5. Instrument and method. Record steady-state and peak concentrations, flow, differential pressure, water level and time since opening.

A reviewer's fastest test is to plot the readings against atmospheric pressure. If every peak coincides with the lowest pressure recorded, the programme has probably not yet captured the site's worst case.

Protection measures under BS 8485

BS 8485 works on a points system. Each characteristic situation and building type has a required protection score, and the score is made up from three contributing elements: the structural barrier provided by the floor slab, a gas-resistant membrane, and a ventilation or dispersal layer beneath the slab.

The design freedom is in how you assemble the points. A cast in-situ reinforced concrete slab designed and detailed as a gas-resistant barrier earns more than a beam and block floor. A well-designed passive subfloor void with cross-ventilation earns more than a granular venting layer. Membranes vary by material, and vary more by the quality of the jointing and the detailing around penetrations.

Two practical rules hold across sites. Verification matters more than specification, because most membrane failures are installation failures at services, laps and upstands. And passive ventilation is more reliable than active systems over a building's life, because nobody maintains a fan for thirty years.

Illustrative example: a small residential scheme on made ground over alluvium returns a maximum methane of 1.2 per cent, maximum carbon dioxide of 6 per cent and a peak flow of 0.3 litres per hour. The resulting screening value falls within the CS2 band. The design uses a reinforced concrete ground bearing slab, a proprietary gas membrane with third-party verification of the installation, and a ventilated subfloor void. The figures are illustrative.

How this feeds a contamination condition

Councils condition contaminated land in four familiar parts: site characterisation, a remediation or mitigation scheme, verification on completion, and reporting of unexpected contamination found during works. The gas assessment sits inside the first two, and the membrane verification report discharges part of the third.

Get the sequencing right. A gas monitoring programme of six months cannot be squeezed into an eight-week determination period, so start it before submission wherever the desk study flags a source. The contaminated land assessment covering Phase 1 and Phase 2 work is a report type where a suitably qualified and experienced person is required, and an AI-assisted draft is a starting point for that professional rather than a substitute for their judgement. Where ground conditions also drive foundation design, run the geotechnical assessment from the same investigation.

On coalfield sites the Coal Authority is consulted, and its response sits alongside the environmental health officer's. Knowing which statutory consultees will see the report tells you who the audience is.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which ground condition documents your site is likely to need.