Tanking Vs Cavity Drainage Waterproofing for New Construction
This article digs in to the main differences between cavity drainage and external tanking systems for use in new construction waterproofing.
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).
Internal Cavity Drainage systems (Type C Drained Protection)
Cavity drainage protects by collecting, controlling and removing penetrating groundwater, using a planned drainage system, while cavity membrane vapour barriers isolate the interior from any moisture behind. These systems work on a fundamentally different basis from tanking systems, they don't block water out, they manage it.
Advantages of cavity drainage waterproofing:
- Inherently considers and manages defects, by preventing pressure from bearing upon membranes (any water ingress is drained before it can stand and pressure).
- System is internal and accessible and repairable.
- Can be water tested during the construction to ensure function.
Disadvantages of cavity drainage waterproofing:
- Does nothing to reduce the permeability of the structure.
- Reliant on managing the volume of water that penetrates the structure.
New construction outcome
What's the better system in new construction? In reality it's not the question we should be asking ourselves. As above, both 'types' of waterproofing have their advantages and disadvantages, but they are completely different systems that work in very different ways.
What we advise you, is to consider what degree of liability any advisor (on waterproofing) carries. For example we design and install your waterproofing, and carry responsibility for both. If we fail to deliver a waterproof structure, there isn't anyone we can blame but ourselves. Our priority isn't to sell you materials, it's to sell you a dry basement we'll put our name to.
Further to this, in new construction for habitable space, it's pretty typical that we'll specify a 'combined protection' waterproofing system, comprising external tanking AND internal cavity drainage. The tanking minimises scope for ingress; the cavity drainage provides belt and braces because any minimal ingress has been designed for. This is explained in greater detail in another article you can read on our site
Who are Trace Basements?
Established 1974 we have completed over 50,000 surveys and 18,000 projects, for a broad client base from national institutions like the NHS, to private clients, working in homes and £100m+ new builds. We believe that good design is fundamental to success as is installation by skilled, experienced and trained technicians. Find out more about our structural waterproofing services here.
Trace are based in North and we cover the North and parts of the Midlands - We get a lot of enquiries from the South and if you're looking for reputable Designer/Contractors that will take design responsibility and also install, so that you avoid split liabilities, we can suggest:
Guardian Waterproofing - https://www.guardianwaterproofing.co.uk/ Based in Cheltenham.
Stonehouse Basements - https://stonehousewaterproofing.co.uk/ Based in Henley on Thames.
Advanced Basements - https://advancedbasements.co.uk/ Based in Hampshire
Maclennan Waterproofing - https://maclennanwaterproofing.co.uk/- Based in Salisbury