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Neil I would consider the void below a timber suspended floor to be accessible - it is easy for people to lift floor boards to lay cable in the void for example (I've done it myself!). Therefore put a venting layer below the membrane and make sure the concrete over the membrane is sufficiently thick to protect it. The sand blinding and well compacted hardcore will not be suitable as a venting layer as they will not be permeable enough. You need as an absolute minimum 20mm single size material for a small house with perforated pipes or geocomposite strips in it . The pipes should have with sufficient perforations in them to give the required gas/air flow and the composite should have sufficient permeability to give the required air flow. If you do not put a venting layer below you should make sure the verification and testing of the membrane meet the highest level in the latest CIRIA guidance as you will be placing a lot of reliance on it. Steve Wilson, UK Registered Ground Engineering Adviser Technical Director on behalf of EPG Tel 07971 277869 <http://www.epg-ltd.co.uk> www.epg-ltd.co.uk -----( Disclaimer )----- > > Information contained in this e-mail is intended for the use of the addressee only, and is confidential and may contain commercially sensitive material. Any dissemination, distribution, copying or other use of this communication, other than for which it is explicitly intended, without the permission of the sender is strictly prohibited. If you have received this e-mail in error, please advise the sender immediately and delete it from your system. Whilst all e-mails are screened for known viruses, the company cannot accept responsibility for any which have been transmitted. From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Neil Stirling Sent: 25 August 2014 18:22 To: [log in to unmask] Subject: Gas Membrane for Suspended Timber Floor Hi All Has anyone come across a gas protection system for a suspended timber floor where the ventilation layer (i.e. solum (sub-floor) vented by air bricks) is above the gas membrane? How confident would you be in the effectiveness of such a solution whereby the membrane is providing the primary protection and the ventilation secondary (as opposed to the standard solution where ventilation is the primary defence mechanism)? The above would seem to represent a standard detail for protection from Radon in buildings with a suspended timber floor. The membrane lies below a concrete screed forming the base of the sub-floor and is laid over a sand blinding layer which in turn overlies a layer of well compacted hardcore. Therefore, would it not be possible to vent the layers below the membrane and indeed would this not represent a better solution? Any comments gratefully received. Many Thanks Neil Neil Stirling Scientific Officer (Contaminated Land) Environmental Health Environment Services Aberdeen City Council Business Hub 15 3rd Floor (South) Marischal College Broad Street, Aberdeen AB10 1AB Tel: 01224 523211 Fax: 01224 523887 IMPORTANT NOTICE: This e-mail (including any attachment to it) is confidential, protected by copyright and may be privileged. The information contained in it should be used for its intended purposes only. If you receive this email in error, notify the sender by reply email, delete the received email and do not make use of, disclose or copy it. Whilst we take reasonable precautions to ensure that our emails are free from viruses, we cannot be responsible for any viruses transmitted with this email and recommend that you subject any incoming email to your own virus checking procedures. Unless related to Council business, the opinions expressed in this email are those of the sender and they do not necessarily constitute those of Aberdeen City Council. Unless we expressly say otherwise in this email or its attachments, neither this email nor its attachments create, form part of or vary any contractual or unilateral obligation. Aberdeen City Council's incoming and outgoing email is subject to regular monitoring.