When building below ground the requirement to protect structures against water ingress is key. However, the potential for the penetration of ground gases must also be assessed. The ideal scenario is to use a solution that will protect against both water and ground gases.
For waterproofing the standard to adhere to is BS 8102:2009 – Code of Practice for protection of below ground structures against water from the ground. The Basement Information Centre provide guidance to aid designers in meeting the required standard. For ground gases the British Standard is BS 8485:2015 – Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings.
With the need to assess the site conditions at the early stage of a project, architects and developers are seeking advice on structural waterproofing and now also ground gases. However, the complex number of scenarios and a lack of synergy between BS 8102 and BS 8485 means that the advice isn’t simple.
Radon, Methane, CO2, VOC’s and even Mercury, are some of the gases that have been highlighted as potential threats to human health on basement sites. Methane and carbon dioxide is dealt with in BS 8485, with NHBC guidance also available. CIRIA 735:2014 also refers to VOC’s and radon, with more detailed information on VOC’s in CIRIA 748: 2014. There is also guidance for ground investigations in BS 8576: 2013, as well as BRE, CL:AIRE and more.
It soon becomes apparent that far from simply specifying a “gas membrane” to protect structures against water and ground gas ingress, there is a complex matrix of variables which must be taken into account to arrive at a gas protection proposal which in turn may be subject to scrutiny by a client-appointed third party environmental consultancy which may well have been involved at the GSV stage (gas screening value site survey – boreholes and monitoring) and/or the desk study to arrive at a theoretical “CS value”/threat level. Likewise, if a verification (on site testing) process is required to prove the integrity of the installed ground gas measures then this will need to be undertaken by a third party independent company with no commercial interest.
With all the above in mind when making gas mitigation proposals seek the advice of ground gas specialists, and for structural waterproofing consult with structural waterproofing experts. The client will need to establish a network of third party experts who can explains the various layers of protection clearly and devolve design liability into the realm of the established gas protection industry experts.
In summary, the various layers of gas protection, as defined in BS 8485:2015 are:
- Gas Protection membrane.
To arrive at a logical design solution you need to balance these three mitigation tools against the Building Type (there are A,B,C,D,E etc. categories for residential, commercial, industrial buildings etc.) and then balance these against the type of gases that have been found to be present or potentially present in the site, and then to balance these against the CS Value of 1-6 (the GSV screening determines the CS Value) that has been determined for the site.
Finally, in order to achieve effective protection, the Standard attributes different gas mitigation methods with certain values. For example a sealed and pressurised membrane system that combines both waterproofing and ground gas protection would be attract the following points:
- Basement construction to Grade 3 standard – 2.5 points
- Integrity tested waterproofing membranes – 2 points
- Active ventilation element – 2.5 points
- Pressure relief element – up to 0.5 points
With a maximum of 7 points, such a solution could provide effective gas protection as required by BS 8585.
Edited by Claire Ackerman, the Basement Information Centre based on a discussion paper by Michael Vernon, Delta Membrane Systems and a manuscript by Richard Crossley, Newton Waterproofing.