Tanking Vs Cavity Drainage Waterproofing for New Construction

What's involved with tanking & cavity drainage waterproofing for new construction?

External barrier systems (Type A Tanked Protection)

External barrier tanking systems seek to protect against groundwater penetration, by including a material which acts as a barrier to block water out. There are many different types of membrane system, from pre-applied sheets to post-applied liquid membranes, but all of them work on the premise of blocking water out.

Advantages of external barrier tanking systems:

• External waterproofing is referred to as waterproofing of the 'positive' side of the structure. This is because water is pressing the tanking on to the structure, not away from it, which is the case where barrier tanking is installed on the inside (the 'negative' side). Therefore, to some extent there is less reliance on the bond achieved with the structure, and more flexibility around the types of material that can be employed. For example, flexible bitumen materials can be employed externally, but not internally.
• Materials are available which can treat a wide variety of structures, from concrete to masonry to insulated concrete formwork (ICF). They function to reduce the extent of any ingress through what might otherwise be a permeable structure.

Disadvantages of external barrier tanking systems:

• Once an excavation (the hole formed to build the structure) is back-filled, you cannot access the waterproofing, to repair if, there is any issue.
• BS8102 advises that you/Designers must assume risk of defects in any system, and if you do not have a strategy for repair, you must change your design, and so external tanking should generally not be used in isolation, particularly in habitable space where there is no viable strategy for repair (locating and repairing the defect).