Myth: Cavity drainage systems are the best solution – fit and forget

Cavity drainage (Type C) systems are often chosen for site convenience but come with costly ongoing maintenance that is sometimes overlooked. They rely on pumps, discharge licenses, and regular servicing, and can be impractical for complex civil-engineered foundations. The most appropriate solution should be based on structural risk and long-term durability, not just short-term buildability. Type C systems are best suited to retrofit situations where options are limited and where minimising the impact on the structure is important.

At EPG, we believe in cutting through the noise with facts grounded in experience and guidance. Let’s raise the standard in geoenvironmental and construction practices together.

Myth: There is no point paying for independent specialists to design my gas protection system when the product supplier can give me a design for free!

Free designs from suppliers can lack independence and site-specific context. Using an impartial, qualified specialist ensures the design is appropriate, risk-based, and fully accountable.

If you ask a supplier to specify the products – at best you are handing them a blank cheque to over-spec the job, and at worst you’ll end up with a design that doesn’t work, and no-one to hold to account.

An independent design will include a full review of the site investigation data and gas regime on the site. We will design-out the gas protection if we can, and provide the most cost-effective design for the risk-profile of the site. Our designs include a set of site-specific drawings and construction details to allow site certainty and confidence in what is required to be installed and verified. The design report will specify the products to be used in the design and ensure these are compatible with the construction methods, structural waterproofing requirements, and gas regime for the site. All of this is backed by our contaminated land risk assessment and gas and water mitigation design expertise, and professional indemnity insurance. Advice you can trust – than we will stand by.

Would you trust the car salesman to tell you which car to buy? Or ask an estate agent how to build a house?

Myth: Pressure relief is needed (as a minimum) for all gas protection designs

BS8485 indicates that as a minimum all gas protection systems should include at least pressure relief for gases which might otherwise build up under the building, in reality for low gas risk situations (CS2) the flow of gas from the ground is sufficiently low so that pressurised gas will not accumulate below the building.

A pressure relief pathway should be considered for CS3 and above for sites where underfloor ventilation is not included.  Pressure relief systems are often overdesigned. Pressure relief pathways should consider the flow of gas from the ground which could potentially accumulate below a building and provide sufficient flow capacity so pressurisation will not occur. They do not need to keep gas below a design concentration.

High concentrations can accumulate in pressure relief systems – but the gas shouldn’t be under pressure. The sub-base under the slab if often sufficient on it’s own.

Do you see pressure relief systems in every gas protection design?

Myth: VOC membranes stop ingress of VOCs

Membranes do not stop vapour intrusion completely. All membranes have transmission rates for different contaminants, VOC membranes (if properly installed and verified) will slow down ingress of certain vapours. Depending on the levels of contamination on the site, the contaminant(s) of concern, and the form of construction it may not be enough to mitigate the risk. In particular where free product is close to the building or in direct contact with VOCs in groundwater.

The performance of VOC membranes can vary significantly between products. Inclusion of any VOC membrane in a design MUST be backed by a suitably detailed vapour intrusion risk assessment.

To inform the vapour intrusion assessment you need VOC permeation data for your membrane that you can trust. Terminating permeation tests early doesn’t allow for “break-through” of contaminants and can give a false impression of the diffusion rate. That’s why at EPG we want to see the full test results to verify the shape of the curve and confirm the published permeation data is representative. We might even send samples for testing as part of the verification, if it’s crucial to the integrity of our design.

Thickness will also affect membrane performance.  Checks should be undertaken during installation to ensure the thickness is consistent with the assumptions used in the risk assessment as it can be less than shown on product datasheets increasing vapour permeation.

As designers we have a responsibility to ensure that the products being specified are suitable and not just accept the information provided.

How often are we calling for a VOC membrane without fully quantifying the risk? Or relying on them to stop ingress entirely? Is this ever an acceptable approach? 

Myth: 0.4mm thick gas resistant membranes are suitable under a warehouse

In our designs we require gas membranes under cast in-situ concrete to be at least 0.5mm thick (between any reinforcement scrim) and free from aluminum foil.

Not all gas protection membranes are the same! They vary in terms of the materials they are made of, thickness, levels of reinforcement – and if they are bonded (integral link to cast in-situ concrete) or self-adhesive.

Membrane suitability depends on the site-specific risk, construction sequence and durability. Under a large cast in-situ warehouse slab, thin membranes and those with an aluminum foil core are likely to be damaged by the construction process. Taped seams are also unlikely to be durable enough.

A thin composite membrane might have a low gas transmission rate and be easy to work with on site for detailing – but all that’s no good if it gets ripped, punctured, or suffers chemical attack during construction of the floor.

How often do you see membranes >0.4mm being specified for gas protection?

Have you ever seen evidence of a membrane being damaged after it’s been laid and verified?

Myth: Gas membranes need to comply with BS8485 

BS8485 is a Code of Practice for designers to use when specifying appropriate gas protection products, it includes guidance for competent professionals to use to inform their designs. It is not a specification and a product cannot comply with a Code of Practice, so a membrane cannot comply with BS8485. 

Specification of the right gas resistant membrane and ancillary products requires a full understanding of the ground gas regime and other design considerations. A competent designer asks all these questions (amongst others): 

• What is the bulk gas regime? (CS classification) And is there also a radon risk and/or vapour intrusion to consider?  

• Where is the membrane placed, above or below the floor? 

• Is there the potential for chemical attack of the membrane from curing concrete or contact with soil contamination? 

• How does the gas protection integrate with structural waterproofing and damp proofing requirements? 

• How will the membrane be detailed at edges, joins, penetrations, level changes, etc., and is it sufficiently flexible for the detailing required? 

• Are there compatible products available to achieve a continuous gas resistant layer across the full footprint of the building (self-adhesive, sealants, liquid applied)?  

• What stresses and strains will it come under during construction, is it robust enough and do I need to specify protection layers? 

• Is there the potential for ground settlement or heave? 

• Does the design life of the structure require joins to be welded rather than taped, are there other reasons welded joins could be required, e.g. VOCs? 

• Could a more robust membrane with a higher gas transmission rate be suitable for the development where mitigation is also provided by the structure?  

At EPG, we provide a comprehensive independent gas protection design service, in accordance with the BS8485 Code of Practice, but also backed up by a wealth of experience in this area. We understand that clients and developers want things to be simple, unfortunately gas protection design isn’t as simple as some manufacturers or installers would like you to believe – but with EPG on board, we can ask all the questions for you – and provide you with a site-specific design that can you can rely on.  

Have you seen a claim that a membrane complies with BS8485? 

Myth: Ground gas from shallow coal workings can be easily fixed with just a gas membrane, and if the ground investigation doesn’t find any gas I won’t even need that.

Risk assessment for emissions from coal mine workings requires a full understanding of all the potential pathways for gas and possible future changes. It’s generally more complex than gas flux from biogenic sources (landfill/Made Ground) because there is more potential for preferential pathways and accumulation of large volumes of gas in voids in the ground. Keep in mind these key points: 

• It’s easy to miss mine workings when boreholes are widely spaced.  

• Information about groundwater levels and how they change over time is key to determining if there is a risk. Flooded workings cannot generate or store gas! 

• Spot monitoring can easily miss short duration significant gas fluxes – consider using continuous methods instead. And even then, the response zones and monitoring period needs to be designed right to be robust.  

• Gas flux modelling calculations are required to design protection measures for coal mine gas emissions – you cannot use BS8485 unless you have a very low risk conceptually.  

• Some high risk mine gas sites won’t be suitable for residential development.  

If you have shallow unflooded coal workings or a shaft linked to workings on or very close to your site WATCH OUT: absence of gas during a short monitoring period does not mean a site is risk-free. 

Check out the CL:AIRE guidance for more detail HERE or the SoBRA webinar Introduction to Coal Mine Gas Risk Assessment and Protection Design by Head of Geoenvironmental, Amy Juden HERE

Do you find site investigation and monitoring for coal mine gas challenging?  

Myth: Barometric pressure is always a key driver for bulk ground gas risk

Barometric pressure can influence bulk gas movement, but is not always the dominant factor. Site geology, source strength, and construction type are also critical in understanding gas behaviour.

Soil and Type 1 sub-base have two key properties that limit the influence of pressure drops to a very small depth below a floor slab – friction to flow (head loss) and entry pressure. Testing by EPG has shown that the zone of influence from pressure drops will only extend at most 500mm below the slab and probably less than that. That is not enough to draw sufficient gas out of soils to pose a risk. This means that unless there is a large open source and an open pathway into the building then barometric pressure has minimal if any influence on gas movement into the building. Gas emissions at sites over Made Ground with a low organic content or over Alluvium will not be affected by barometric pressure changes.

Have you assessed whether changes in gas concentration in a monitoring well are due to oxygen flowing into the ground and diluting static gas rather than an increase in gas flow from the ground? Are any correlations with barometric pressure simply due to the presence of the monitoring well and do not reflect what is happening in the surrounding ground? Barometric pressure drops often coincide with rainfall which can be the actual driver.

A common myth is that falling atmospheric pressure sucked gas out of the landfill site at Loscoe and caused the methane ingress into a house that exploded. This is not correct. The gas slowly diffused into a pit in the garden that was connected to the house via a pipe and the pressure drop sucked the gas that had accumulated in the pit via an open pathway into the house. The same mechanism occurred with carbon dioxide at a site in Gorebridge in Scotland where gas accumulated below the floor slab in stone columns and pressure drops sucked it from there.

Do we focus too much on barometric pressure conditions? Should we spend a bit longer on the CSM and ingress pathways instead? Do you consider when barometric pressure drops are a risk driver for ground gas, and when they are not?

Myth: Gas risk assessment in accordance with BS8485 is as easy as multiplying two numbers together and adding up points! Anyone can do it!

Gas risk assessment requires professional judgement. BS8485 includes a scoring system, but using it effectively demands understanding of site-specific factors, geology, and gas behaviour.  

The gas screening value (GSV) in BS8485 is not calculated – it is derived based on professional judgement and a full understanding of the site conceptual model, gas sources, pathways and data quality assessment. Hazardous gas flow rates are calculated per monitoring well, per monitoring event – and are not the same as the GSV.  

We also often see misapplication of BS8485, when the data quality assessment is missing, data from deep or flooded wells is used, where response zones cross-strata or where there is no consideration of the source of the gas in the ground.  

While CIRIA C665 may provide some useful background guidance on how to do a risk assessment, watch out – the definition of a GSV has changed! Be aware of the difference – BS8485 is the current Code of Practice that should be used.

A GSV should not be used for existing buildings or on its own to consider coal mine gas risks. 

The scoring system for gas protection design in BS8485 provides a generic framework for determining the scope of protection measures, based on the bulk gas regime or characteristic situation (CS2, CS3, CS4, etc.). But producing a full design involves a greater understanding of the proposed development, risk, and an ability to check and verify the findings and recommendations of site investigation reports.

Myth: Gas monitoring wells do not need designing

Gas monitoring wells should be designed on completion of the hole, based on the strata and groundwater encountered. Response zones should be designed so that they are above the groundwater table and isolated into a single stratum that is either a pathway or source of gas that could pose a hazard.

Designing a site investigation – ensure that there is supervision and flexibility to allow design of individual response zones on completion of each borehole. Make sure that the response zones are suitable for the risk assessment method you are adopting.

Not all gas monitoring data can be used to determine the worst case gas screening values (GSVs) – flow rate and gas concentration has to be from the same stratum as defined in BS8485.

Myth: Every ground investigation on brownfield land has to include gas monitoring

Not all brownfield sites require gas monitoring. The decision should be based on a robust preliminary risk assessment and Conceptual Site Model. Whilst on some sites it’s absolutely required – unnecessary monitoring can waste time, resources, and carbon.

Gas monitoring in natural superficial deposits often leads to elevated CO2, which doesn’t pose a risk of hazardous emissions but is often misinterpreted.  If wells are installed in Alluvium, the likelihood is that they become flooded and any data is not representative – i.e. useless!

If the only viable source is urban Made Ground, are you better off doing organic carbon testing on the soil? (see CL:AIRE RB17)

Or installing a groundwater monitoring well and collecting gas monitoring data from it, just because it’s present? Perhaps gas monitoring for a road or rail schemes, where there is no receptor? We see this kind of thing all the time, and it’s frustrating.

Follow this golden rule: Gas monitoring should only ever be in the gas source (in the unsaturated zone) or in a permeable (unsaturated) pathway between the source and the receptor. If you are doing anything else, STOP. (More on monitoring well response zones coming up on the MythBusters!)

What do you think? How often do you design out gas monitoring from your site investigation?