An article by FS Verification — independent ground gas design and verification to BS 8485, CIRIA C735 and BRE BR 211.
A drawing pack is not a design
On many sites, the "gas protection design" sent to the local authority is a set of manufacturer standard details: a typical slab-edge detail, a typical service penetration, a typical cavity tray, and a product data sheet. Sometimes a company logo and a site address are added to the title block.
Those details are useful. But they are not a gas protection design, and treating them as one is one of the most common reasons planning conditions go unanswered, verification fails and remedial work is needed after the slab is poured.
This article explains the difference, what BS 8485 actually expects a design to contain, and what to ask for before you rely on a pack of standard details.
What a standard detail is for
A manufacturer's standard detail shows how their product should be installed at a typical junction. It is written for a generic building on a generic site. Its job is to protect the product's performance and the manufacturer's warranty.
It does not know:
- how much gas is on your site, or what kind (methane, carbon dioxide, radon, VOCs)
- the Characteristic Situation (CS) your gas monitoring supports
- what type of building it is, or who will occupy it
- how your foundations, slab and services are actually arranged
- whether the site needs ventilation beneath the building as well as a membrane
A standard detail answers "how do I fit this product?" A gas design answers "what protection does this building need, and how will it be achieved here?" Only the second question satisfies a planning condition or building control.
What BS 8485 actually asks for
BS 8485:2015+A1:2019 sets out a process, not a product. A compliant gas protection design works through it step by step, and shows its reasoning at each stage:
- Site characterisation. Use the ground investigation and gas monitoring data to work out the Gas Screening Value and the Characteristic Situation, from CS1 to CS6.
- Building type. Classify the development as Type A to D, according to how it is owned, managed and used.
- Required protection score. Combine the CS and building type to find how many gas protection points the building needs.
- Choice of measures. Decide how those points will be earned from the structural barrier, ventilation or dilution beneath the building, and a gas-resistant membrane.
- Detailed design. Show how each measure works on this building: slab type, venting layout and outlets, membrane specification, and every junction and penetration.
- Ventilation design. Where venting earns points, justify its performance for this footprint and layout, rather than assuming a product's generic rating.
- Verification plan. Set out what will be inspected and tested during installation, and to what standard, in line with CIRIA C735.
A pack of standard details covers, at best, part of step 5. The other six steps are where the design decisions are made, and they are the steps regulators look for.
Where standard details fall short on site
No link to the gas risk. A standard detail looks the same on a CS2 site as on a CS4 site. Nothing in it shows that the protection has been matched to the measured gas regime, or that the required points have been achieved.
The awkward junctions are missing. Standard packs cover straight slab edges and single pipe penetrations. Real buildings have stepped foundations, lift pits, piled caps, drainage clusters, service ducts, steel column bases and junctions between different floor types. Those are exactly where gas gets in, and where installers are left to improvise.
The structure is ignored. A standard detail rarely credits the slab or substructure. That leaves protection points unclaimed, and it can mean the membrane is asked to do work the structure could have done.
Venting is assumed, not designed. A detail may show a venting layer and a periscope vent, but not how many outlets are needed, where they go, or whether the layout works for the footprint.
Mixed products, no compatibility check. Sites often end up with a membrane from one supplier, tapes and top-hats from another, and a venting product from a third. A single manufacturer's details will not confirm that the combination works together.
No verification route. A detail shows the finished product, not what the verifier needs to inspect, test and record before it is covered up.
What it costs when details stand in for design
Planning conditions left open. Contaminated land officers are increasingly returning submissions that contain only product literature and typical details. Each round of queries costs time, and occupation or sign-off can be held until the condition is discharged.
Problems found at verification. When the verifier arrives, junctions that no drawing covered are the first things to fail. The fix is usually done on the spot, under programme pressure, and not always well.
Remedial work after the slab is poured. Once the floor is cast, a missing detail or a failed junction is expensive to put right. In the worst cases it means breaking out and relaying, or retrofitting active ventilation.
Unclear responsibility. If a standard detail was used without a site-specific design, it can be hard to say who designed the protection. The manufacturer produced a typical drawing, the installer followed it, and the gap between them belongs to no one — until something goes wrong. A typical detail comes with no professional indemnity cover for the design, so if the protection fails, there may be no insurance behind it.
Wasted money on over-specification. Without a design that works out the points required, the easy answer is the heaviest membrane in the range. That costs more and still does not prove compliance.
What a proper gas design pack contains
Before you submit, or before you accept a design from anyone else, check that the pack includes:
- A design statement that summarises the gas monitoring data, the Gas Screening Value and the Characteristic Situation
- The building type and the BS 8485 protection score required
- A points calculation showing how the score is met from the structure, ventilation and membrane
- A site-specific layout plan of the membrane, venting layer and vent outlets
- Details for this building's junctions: slab edges, steps, pits, pile caps, service entries and floor-type changes
- A membrane and ancillary specification, with compatibility confirmed across all products used
- Justification of the ventilation layout where venting earns points
- A verification plan to CIRIA C735, stating what is inspected, what is tested and what is recorded
- A named designer who takes responsibility for the design, backed by professional indemnity (PI) insurance
Manufacturer details still have a place in the pack. They are supporting information, used where they fit the design, not a substitute for it.
Conclusion
Standard details tell an installer how to fit a product. A gas protection design tells everyone — the regulator, the warranty provider, the groundworker and the verifier — what this building needs, why, and how it will be achieved on this site.
If your "design" is a set of typical drawings with a site name on them, the thinking has not been done yet. It is far cheaper to do it at design stage than to discover the gaps when the verifier arrives, or after the slab is poured.
About FS Verification
FS Verification is an independent ground gas specialist based in Lancashire, working on developments nationwide. With over 20 years' experience in ground gas membranes and verification, we offer:
- Ground gas design — site-specific, product-neutral protection designs to BS 8485:2015+A1:2019
- Gas membrane verification — independent inspection and testing of installed membranes to CIRIA C735, with same-day reporting
- Radon protection — design and verification in line with BRE BR 211
Every design we produce is backed by professional indemnity insurance, so you have real cover behind the design, not just a product warranty. We take no commission from any manufacturer or supplier, so our only interest is a safe, compliant building at the right cost.
Talk to us about your next site: call 07813 788537, email info@fsverification.com, or book a visit at www.fsverification.com.