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Protecting against the big C

technical & practice

With the imminent increase in scope of Approved Document Part C, which comes into force this year, we outline the likely changes and provide guidance to the risks from radon One health and safety issue that relates to the occupation of completed buildings rather than their construction is the prevalence and control of radon gas. Radon is a colourless, odourless, radioactive gas that moves through fissures in ground rock or sub-soil. In open spaces, it is released safely into the atmosphere. However, in buildings it can find its way into occupied areas where, if it is allowed to accumulate, its concentration can be raised to a point where it can present a cancer-significant health risk.

This point is called the 'action level' and equates to 200Bq/m 3(bequerels - the SI unit of radioactivity - per cubic metre).

Radon is believed to be responsible for half the total radiation exposure of a typical person in the UK, with current estimates suggesting that it is responsible for 2,500 lung cancer deaths a year. Mindful of this risk, the government has reviewed Part C of the Building Regulations for England and Wales, and plans to publish a revised edition of the Approved Document next month, with the requirements coming into force in October . It is likely that many of the proposals made in the public consultation document will appear in the new edition. For example, the guidance on radon protection is likely to be expanded to include building types other than dwellings (eg workplaces) and the requirements extended to include developments that involve a material change of use to residential purposes - for example barn and warehouse conversions.

Determining actual risk Because the level of risk differs across the country according to local geology, the risk depends on where your site is. The existing BRE Report BR211, Radon: Guidance on protective measures for new dwellings (1999 edition), is the main source of guidance on risk and solutions. It includes two sets of maps for determining the need for radon protection, as well as detailed information on construction solutions.

However, this BRE report is also likely to be updated to coincide with the publication of the revised Approved Document and is expected to include:

lnew consolidated maps based on the most up-to-date statistical data;

lguidance on timber and lightweight floor construction;

lguidance on extensions and conversions;

lguidance for non-domestic buildings.

Although this revised guidance document is unlikely to be available until late 2004, the principle of using a map to determine the required degree of radon protection (whether 'basic' or 'full') and, from this, appropriate construction solutions, will remain.

Your local authority building control officer or a local approved inspector should also be able to help you determine the level of risk in your area but, like the maps, they can only give a broad indication.Ultimately, if you are in an area that is potentially at risk, there is no substitute for an assessment close to your actual site - you may be in a pocket where the geology is different from the region as a whole, for example. That could mean that the required protection can be downgraded, or even make radon protection unnecessary. The specifics of each site will determine the correct response:

lif you are in an area where the maps indicate that no protection is required, there is no need to take further action;

lif the maps indicate the need for 'basic' or 'full' protection, the only way to see if the level of protection can be downgraded is to commission a geological assessment from the British Geological Survey (BGS);

lif the standard BGS geological assessment suggests that 'full' protection is required, it is worth confirming this by requesting a detailed assessment, which focuses more accurately on your location.

Basic protection For 'basic' protection, you need to provide an airtight barrier covering the entire ground floor of the building, linked to the dampproof course (if necessary using cavity trays). The cavity tray prevents radon moving through the wall cavity and into the building through cracks in the inner skin.

All junctions between the floor membrane and cavity trays should be sealed.

By using a single manufacturer's radon protection system, you will have the peace of mind that:

lthe radon-proof membrane will be delivered complete with all necessary components;

lthere will be chemical compatibility between the various membrane and sealing materials used;

lthe system will have proven radonresisting performance.

Some manufacturers offer a complete package of materials in kit form, specially tailor-made for a range of house types, with the gas membrane factory-welded to the exact footprint of the dwelling.

Full protection For 'full' protection, it is necessary, not only to provide a radon-proof floor membrane, but also to have an underfloor depressurisation system.

Depressurisation can be achieved by natural or mechanical underfloor ventilation or, if there is no underfloor space to ventilate, by a radon sump with an associated powered extraction system.

In fact, natural underfloor crossventilation (in conjunction with a radon membrane) may be sufficient to provide 'full' protection, making it unnecessary to install a fan unless it appears to be necessary through postoccupation testing.

Construction solutions 'Basic' protection can be achieved with both ground-supported and suspended (ventilated) concrete ground-floor constructions by installing a radon-proof membrane system. However, a suspended concrete slab has the advantage that the space beneath the floor construction is available to ventilate radon safely away should 'full' protection be required.

If 'full' protection is required and you prefer to use an in-situ slab in contact with the ground, the slab should be supported on the inner leaf and a radon sump incorporated beneath it. The sump should be fitted with an extract pipe that can be capped until it is shown through testing that mechanical exhaust is required.

In the case of timber suspendedfloor construction, full protection can only be provided by installing a radon barrier (in conjunction with a radon sump) across the whole footprint of the building, below the sub-floor space.

Mike Finbow is an architect with NBAT, specialising in technical standards. Tel 0207 2842272.

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