The Importance of Engaging a Waterproofing Specialist at Design Stage
We recently received an urgent enquiry from an engineer requesting a waterproofing specification both compliant with NHBC Chapter 5.4 and BS8102:2009. The specification was required yesterday as building work had already began.
The below sketch was received:
The structure consisted of a reinforced concrete retaining wall set away from a blockwork cavity wall. A strategy that is becoming more popular allowing designers to achieve the same edge of slab detailing throughout.
We provided a design rationale for the project which was both cost effective, compliant with NHBC 5.4 and BS8102:2009 and in line with onsite requirements.
It is our understanding that the waterproofing design on this project should comply with NHBC Chapter 5.4, hence all materials should carry third party accreditation relevant to their proposed application. NHBC Chapter 5.4 contains illustrations showing the required waterproofing in various scenarios to guide waterproofing design. The closest illustration to this project is the “Retaining walls forming light wells” scenario shown below.
Figure 1: NHBC Chapter 5.4: Retaining walls forming lightwells
As Grade 3 protection is required to the structure, as defined in BS8102:2009 and NHBC Chapter 5.4 two lines of waterproofing defence are required to the retaining wall. No waterproofing is required to the slab as shown in Figure 1.
For primary waterproofing we would recommend Type A protection in the form of atal Bentofix 5000, protected/drained using atal Flo Board 4e with a land drain installed at the edge of the foundation to ensure hydrostatic pressure does not come to bear on the structure. Waterproofing continuity should be provided using atal Premsil 100 into DPC. This material is BBA certified for this use.
For secondary waterproofing a Type C drained cavity without membrane would be utilised. This system would need to be maintainable to comply with BS8102:2009 and NHBC Chapter 5.4. To achieve a Grade 3 environment the living space would be adequately ventilated.
The current land drain has been positioned on top of foundation of the retaining wall which is not compliant. To conform with NHBC guidelines and BS8102:2009 a land drain should be placed at edge of foundation or lower, the current land drain placement should be amended.