An article by FS Verification — independent ground gas design and verification to BS 8485, CIRIA C735 and BRE BR 211.
The "free design" that isn't free
On most UK sites affected by methane, carbon dioxide, radon or VOCs, a gas protection design is needed before building control or the warranty provider will sign off. Many developers and contractors get that design for nothing: a membrane manufacturer, or a designer closely tied to one, produces the drawings as part of the product sale.
It looks like a saving. In practice, the cost of the design is simply moved into the materials, and the design is shaped around one company's product range rather than around your site. An independent designer, paid for the design and nothing else, has only one job: to meet BS 8485 and the regulator's requirements at the lowest whole-life cost and risk to you.
At FS Verification we design and verify ground gas protection with no ties to any manufacturer. This article explains where that independence makes a difference — in the bill of materials, on site, at verification and years after handover.
How manufacturer-tied design works
A manufacturer-tied designer is paid, directly or indirectly, by the sale of product. That might be an in-house technical department, a "preferred" design consultancy that receives referrals, or an approved installer network that must specify one brand to keep its status.
None of this means the people involved are dishonest. It means the brief they work to is different from yours:
- Their question: which of our products meets the requirement?
- Your question: what is the least costly, most buildable way to meet the requirement on this site?
Those two questions often have different answers. A tied designer can only draw from one catalogue, even when a competitor's membrane, a different venting product or a structural solution would do the job better or for less. And because the design costs are recovered through product margin, there is a quiet incentive to specify more product, not less.
Where the savings come from
BS 8485:2015+A1:2019 works on a points system. The site's Characteristic Situation (CS1 to CS6) and the building type (A to D) set a required score. That score can be met from three sources: the structural barrier (the floor slab and substructure), ventilation or dilution beneath the building, and a gas-resistant membrane. How those points are shared out is where most of the money is won or lost.
1. Crediting the structure you are already paying for. A well-detailed cast in-situ or suspended reinforced slab can earn protection points in its own right. A product-led design tends to lean on the membrane and treat the slab as a given. An independent designer works with the structural engineer to claim every point the structure legitimately offers, which can reduce the membrane or venting specification.
2. Choosing the right venting approach. Passive sub-floor ventilation, a void former, a geocomposite blanket, or simply the void under a beam-and-block floor can all contribute points. The cheapest effective option depends on the foundation type and the site. A tied designer will favour the venting products in their own range.
3. Matching the membrane to the risk. On a low-risk site, a basic compliant membrane may be all that is needed. Over-specifying a premium reinforced or composite membrane across a large footprint adds cost with no extra points.
4. Not paying twice for waterproofing and gas protection. On basements, the gas and waterproofing systems can often be designed as one envelope. An independent designer can coordinate both, rather than layering two separate product systems.
5. Competitive tendering. An independent specification is written to a performance standard, so several suppliers and installers can price it. A single-brand specification removes that competition, and the product price is whatever the one supplier sets.
6. Challenging the gas risk assessment itself. A good independent designer will review the ground investigation and monitoring data. Where the data is thin or has been interpreted cautiously, further monitoring can sometimes lower the CS, and with it the whole protection requirement. A manufacturer has little reason to argue for less protection.
Why the outcome is better, not just cheaper
Designed for your building, not a catalogue. Most gas protection failures happen at the details: service penetrations, steps in the slab, lift pits, pile caps and junctions with walls. An independent designer can pick the best detail for each junction, even if that means mixing compatible products from different suppliers, as long as compatibility is confirmed.
Buildability. An independent designer has no reason to prefer a product that is hard to install in wet weather or awkward around dense services. They can choose systems your groundworkers can install well, which cuts rework and delay.
Clear, impartial verification. BS 8485 and CIRIA C735 expect the installed membrane to be inspected and verified, with the level of scrutiny rising with the risk. Verification is most credible when the person signing it off has no stake in the product. Regulators and warranty providers increasingly look for that separation.
Straight answers to the regulator. Local authority contaminated land officers and warranty surveyors want a design that shows its reasoning against the standard. A performance-based, product-neutral design is easier to defend than one that reads like a sales specification.
Advice that follows the project. If the programme changes, the foundation design is revised or a product becomes unavailable, an independent designer can adapt the specification without starting again with a new supplier.
"But the manufacturer's design is free"
"We don't pay for the design." You do, through the materials price, and usually with no competing quote to compare against. On anything beyond a single plot, a modest independent design fee is commonly recovered several times over through a leaner specification and competitive tendering.
"The manufacturer offers a system warranty." Check what it covers. Many product warranties cover the material, not the design or the installation, and may depend on using approved installers. An independent designer carries professional indemnity insurance for the design itself, and a product-neutral specification can still require manufacturers' warranties for the materials chosen.
"One company taking responsibility is simpler." Simplicity is worth having, but not at the expense of an impartial check. The independent designer becomes your single point of accountability for the design and verification, while suppliers remain accountable for their products.
"Our site is too small to need it." Small sites often suffer most from over-specification, because a standard product package is applied without looking at the actual gas data or the slab design.
Questions to ask before you appoint a designer
- Do you receive any payment, commission or referral from a product manufacturer or supplier?
- Will your specification name alternative products, or be written to a performance standard so it can be competitively tendered?
- How will you use the structural slab and sub-floor ventilation to reduce the membrane requirement?
- Will you review the gas monitoring data and tell us if more monitoring could lower the Characteristic Situation?
- Who will carry out verification, and are they independent of the product supplier and the installer?
- What professional indemnity insurance do you hold for the design?
- Can you show a BS 8485 points calculation for a recent project, with the reasoning set out?
If the answers keep coming back to one brand, you are buying a product, not a design.
Conclusion
Ground gas protection is a life-safety system, and it has to be right first time: once the slab is poured, the membrane is out of reach. The best designs start from the site data and the structure, then choose products to fit — not the other way round.
An independent designer gives you that order of thinking. They claim every protection point your building already earns, open the specification to competition, and provide verification that regulators and warranty providers can trust. The result is usually a lower build cost, fewer problems on site and a stronger compliance record — which is exactly what the "free" design promised, but rarely delivers.
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 — product-neutral protection designs to BS 8485:2015+A1:2019, built around your site data and structure
- 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
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.