Section 5.2.5

DPM and Gas Membranes

Damp proof membranes and damp-proof course are designed to preventing damp rising from the ground and into the property. It also plays a vital part in preventing ground gases from entering the property. Attention to detail is critical to its success.

  • Preparation for the installation of the DPM must be complete before commencement.

  • Block and beam fully grouted with no gaps around the penetrations.

  • Free from debris and other materials which could puncture the membrane.

  • Damp proof membrane laid across the block and beam.

  • Pulled tort with limited folds and joints.

  • Ballast carefully placed on top of the membrane to prevent movement.

  • Damp proof membranes require sealed laps of 300mm or as per the manufacturer’s recommendations.

  • Gas membranes require the laps to be sealed.

  • At party walls the membrane must form within the cavity to a minimum depth of 225mm.

  • This is to comply with Robust details.

  • Internal and external corners must be fixed and sealed as per the design.

  • Prefabricated corners can be used.

  • Penetrations require an appropriate seal.

  • Seals need to be complete around both base and pipe.

  • Approved tape should be applied to the membrane the pipe work or ducting penetration.

  • Mechanical fixing used around the pipe/duct to secure the “top hat”.

  • Unless stated in the design, DPM must have a minimum lap of 90mm on to the internal leaf of the external wall.

  • DPC over laps the DPM on the internal leaf.

  • DPC on the external leaf must be placed flush with the external face of the brick or over hang slightly.

  • Gas membranes must be cut flush with the external face of the brick of the external leaf.

  • A cavity tray must be provided, and weep holes installed to allow moisture that enters the cavity to egress.