Innovating safely: Protecting against ground gas while using modern methods of construction

It might seem contradictory, but modern methods of construction – or MMC for short – are really nothing new.

True, the technologies and materials involved have changed with the times, with ever-new methods being deployed by builders eager to improve speed, quality, safety and cost-efficiency.

But the desire to reap the rewards of non-traditional off-site construction has been around for decades, even in the wake of the First World War, when socioeconomic challenges required different approaches.

What was considered ‘modern’ then might seem entirely old hat today, of course.

Yet in the home building world, some things don’t change – such as the need to mitigate against the potential risks of ground gas contamination.

That’s true of any build; both to safeguard homeowners from any detrimental health effects and avoid expensive unforeseen delays.

On occasion, these important factors have led to developers being reluctant or uncertain about using MMC on sites affected by ground gases.

So what do you need to know – and how can your project take advantage of the latest innovations safely and effectively?

Same considerations. Same standards.

First, let’s recap the basics.

Ground gases can originate from natural ground conditions or historical land use, with methane, carbon dioxide, radon and volatile organic compounds (VOCs) among those that pose the main concern.

These gases only represent a risk when they migrate into buildings and accumulate to harmful levels.

Ground gas risk is assessed using a well-established source-pathway-receptor methodology that is widely accepted by regulators, building control bodies and warranty providers.

It’s an approach that ensures protection is only provided where a credible risk exists, that measures are proportionate to site conditions and that avoids over-engineering and unnecessary construction impacts.

For conventional buildings, measures and approaches for ground gas protection designs are well established, supported by numerous authoritative technical standards and guidance.

These include recognised UK guidance like the BS 8485 and CIRIA publications, relevant building regulations and warranty provider requirements.

Applying the guidance to MMC

So what about projects that involve non-traditional methods?

While many in the industry remain less familiar with how to apply existing guidance correctly when using MMC, a recent Off-Site Alliance publication now offers a practical guide to help provide confidence and clarity.

It’s a welcome addition to the collection of existing resources available – which includes NHBC Publication NF94, our essential guide for housebuilders on managing hazardous ground gas.

Don’t let complacency creep in

Off-site and modular construction methods may include factory-produced floor systems and controlled construction environments that give inherent resistance to gases.

That might invite the temptation to rely on such features – but it’s never wise to proceed without the full picture.

Applying standard solutions without thorough evaluation can result in unnecessary costs, construction delays or buildability issues.

The good news is that where additional protection is required, it can often be achieved by:

• integrating measures directly into the modular building system, such as gas-resistant floors or ventilated spaces within or beneath the building module
• installing systems that use gas-resistant membranes, vent layers or concrete slabs within the foundations or below the units to act as a barrier
• using a combination of these approaches and options.

What do you need to do?

The Off-Site Alliance’s new publication makes one thing explicitly clear: any chosen approach must be justified by site investigation findings and specialist assessment.

The guide sets out that designs should be prepared by competent, appropriate specialists and that gas and VOC membranes must be durable, protected and compatible with MMC installation.

It explains that vent layers (gravel, geo-composite or voids) require calculation and careful detailing and notes how concrete and insulation may provide sufficient resistance, but only if tested.

Meanwhile, the guide also points to poor detailing and damage during installation as common causes of failure.

And as with any health and safety design measure, the protection on offer is only as good as the installation quality – so verifying the system's performance is crucial.

Verification, training and competence

While factory-installed systems can rely on manufacturing quality assurance, the Off-Site Alliance’s new guidance calls for on-site works to receive specialist verification based on the good-practice processes laid out in the Construction Industry Research and Information Association’s C735 book.

Installers should also be NVQ Level 2 qualified or fully verified to ensure their competence and knowledge.

The benefits of expertise

There’s obviously no one-size-fits-all approach to ensuring every MMC project protects against ground gases effectively.

The solutions you’ll need will depend on the specific sites and systems involved on a case-by-case basis.

It’s clear, however, that getting early input from specialists and adopting risk-based designs and buildable solutions is the surest way to manage smartly in modular and off-site construction.

Although overly conservative approaches may increase costs, well-justified MMC-appropriate designs can improve safety, programme certainty and the odds of warranty acceptance significantly.

That’s why our Land Quality Services team is here to help.

With experienced specialists who can help you understand what lies beneath your land at the earliest stages, we can support you to make the most of MMC in the knowledge that your systems will meet compliance requirements.