In 2014 CIRIA C735 Good practice on the testing and verification of protection systems for buildings against hazardous ground gases was published. This offered guidance to levels of independent verification required on site along with methodology for the risk-based approach to determine this and levels of integrity testing. It also outlines the pro’s and con’s of different types of integrity testing and offers points to consider when selecting these. The following recommendations were set out by the authors of this document as key points.

Ciria 735

"Good practice on the testing and verification of protection systems for buildings against hazardous ground gases"


  • The role and responsibilities of the LA regulators (in particular the contaminated land officers or environmental health officers as advisors to the local planning authority) should be more fully recognised and understood. In particular, with respect to the assessment and approval of the developers’ planning proposals, the imposition of planning conditions and their discharge by reference to satisfactory verification reports. Early and appropriate consultation with the regulator throughout the process is recommended
  • Planning conditions should continue to be used to require appropriate investigation, assessment, remediation and verification in a continuation of the good regulatory practice built up over the last several years.
  • Regulators, clients, consultants and contractors should aim to discourage behaviour whereby insufficient verification activities are carried out. It is recommended that unsuitable and incomplete verification reports should be systematically rejected by the LA regulators, to encourage good practice in future.
  • There is a general need for up-skilling across the sector to better disseminate good practice. It is recommended that a programme of training and dissemination (relevant to all sectors) is devised and implemented. It is proposed that Construction Skills should develop a competent persons scheme for the installation of gas protection systems. Once such a scheme is in place it is recommended that it is supported by all relevant parts of the industry (manufacturers, installers, contractors, consultants, and regulators).
  • It is recommended that installers obtain the relevant NVQ diploma (or equivalent) such that over the next few years this becomes an accepted minimum standard for these specialists. Anyone carrying out the installation of gas protection systems (especially involving gas membrane installation) will have, or should be working towards, the relevant NVQ (or equivalent) qualification.
  • The verification plan (which includes consideration of the need for any integrity testing) should become an integral part of the design of the gas protection system.
  • Independent verification should be recognised as an important element in the process and its use will strengthen and raise the quality of the installation.
  • The degree and intensity of independent verification should reflect the assessed level of risk, the nature of the gas protection system, the quality of the product(s) and the competence of the installer.
  • Those involved in such independent verification should be appropriately independent of the installer and should be appropriately competent and experienced.
  • In most circumstances an unreinforced 1200g gas membrane will not survive the construction process intact. So it is recommended that a proprietary gas membrane should be specified having full regard to the gas/vapour concerned, the material it is manufactured from (which should be virgin polymer), its physical properties (tensile strength, puncture resistance) and permeability to the gas in question. Thickness also becomes a key parameter if joints are welded. Adequate protection of the gas membrane (appropriate to the build sequences of follow-on trades and the robustness of the gas membrane used) will always be required to ensure that installed gas membranes have retained their integrity.
  • It is recommended that a programme of research into integrity test methods is carried out to provide independent evidence of efficacy, optimum test method etc. This should include research and development of injection methods that do not rely upon the deliberate creation of defects in the gas membrane.
  • The standard pro forma presented in Appendix A of C735 is recommended for all site inspections carried out as part of the verification of gas protection systems to buildings.