aterproofing offers structural integrity, the ability of the structure to retain its strength, function and shape, without failure when subjected to the loads imposed throughout the structure’s service life.
Structural waterproofing is a fundamental aspect of the build process.
Structural waterproofing is a relatively new terminology; it covers areas such as tunnelling, basements, reservoirs, top decks and balconies, podium decks, green roofs and car parks; to name a few. The basic principle for structural waterproofing is waterproofing the structure.
Historically basement waterproofing was divided into two key areas:
A terminology often used for waterproofing methods used where the below-ground structure sits within the water table continuously or periodically. Hydrostatic pressure is the pressure that is exerted by a fluid at equilibrium at a given point within the ground due to the force of gravity.
Hydrostatic pressure increases in proportion to the depth measured from the surface because of the increasing weight of fluid exerting downward force from above. Hydrostatic pressure likely to be imposed on the structure requires consideration to the full encapsulation of the basement structure in a tanking membrane, which is installed under the slab and to the structural walls. The positive or negative side may be considered.
Tanking (in current-day terminology)
A terminology for waterproofing methods applied using typically liquid-applied, sheet-bonded or substrate reactive components such as migratory systems.
A terminology for waterproofing methods used where the water table is lower than the structure, and there is good free-draining fill. The damp-proof membrane deals with the shedding of water and the ingress of water vapour only – with no hydrostatic pressure – in above-ground scenarios.
From the late 1990s, the construction industry saw technological advances in waterproofing systems, solutions and materials available and the inclusion of a new set of British Standards.
BS 8102:2009 (Code of practice for protection of below-ground structures against water from the ground) is the standard that has been developed to give recommendations and provides guidance on methods of dealing with and preventing the entry of water from surrounding ground into a structure below ground-level.
It covers the use of:
- a) Waterproofing barrier materials applied to the structure
- b) Structurally integral watertight construction
- c) Drained cavity construction.
BS 8102:2009 sets out various considerations to include the three ‘Types’ or ‘Forms’ of waterproofing construction. These systems are categorised as Types A, B and C.
Type A is defined as ‘barrier protection’. A material is used to offer a barrier to the passage of water. A barrier protection which physically holds water back on either the positive side of a below-ground level structure (externally), the negative sides (internally) or sandwiched into the construction. Cementitious coatings, bituminous coatings or flexible sheet membranes are categorised within Type A ‘barrier protection’.
Type B is defined as ‘structurally integral protection’. Type B protection applies that the structure itself is constructed as an integral water-resistant shell. A concrete structure built inclusive of watertight concrete admixtures, hydrophilic strips/waterbars (passive or active). When considering and/or specifying a Type B integral system, a superior knowledge and understanding of concrete construction is required to meet BS 8102:2009 specifications. A further design consideration and robust strategy is to design the concrete in accordance with BS EN:1992 Euro Code 3.
There are two main types of integral waterproofing systems: the hydrophilic and the hydrophobic. A hydrophilic system typically uses a crystallisation technology that replaces the water in the concrete with insoluble crystals. Hydrophobic systems use fatty acids to block pores within the concrete, preventing water passage.
Type C is defined as ‘drained protection’ and functions by diverting a passage of water away from the structure (which is usually collected into a submersible pump). A drained cavity system collects and manages any moisture which breaches the integrity of the structure by channelling, collecting and discharging such free water via a suitable evacuation point.
A Type C waterproofing system will not increase or exert water pressure on a structure nor alter the dynamics of a structure.
Waterproofing systems have become sophisticated and are a very specialised area.
One of the leading causes of water damage to building structures and of personal injury is the use of improper products, used for the wrong application.
Waterproof is a loose term, used for numerous products. Each product has a very specific area of application. Failure of following manufacturer specifications and installation procedures can cause severe consequences when the systems fail.
How to avoid failed waterproofing systems
Waterproofing failures can be expensive and disruptive. Waterproofing makes up between 1 and 2% of the total cost of construction but accounts for huge costs in remediation. While prevention is the obvious first choice for waterproofing success, there are occasions of error in design, during construction and throughout operation. Water infiltration will not fix itself. Inexperienced attempts at treating systems of waterproofing failure will only serve to trap or redirect moisture.
British Standard 8102:2009 (Code of practice for protection of below ground structures against water from the Ground [BS 8102:2009]) recommends that every design team/waterproofing project incorporates a waterproofing design specialist.
As set out in Section 4.2 of BS 8102:2009 under the heading ‘Design Team’, the code states: “A waterproofing specialist should be included as part of the design team so that an integrated waterproofing solution is created.
The waterproofing specialist should:
- i) Be suitably experienced
- ii) Be capable of devising solutions that accommodate the various project constraints and needs
- iii) Provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.”
A waterproofing design specialist provides expertise in structural waterproofing. A waterproofing design specialist should have:
- CSSW as a minimum standard of qualification
- Be able to list the principal considerations for a robust waterproofing design
- Offer knowledge on waterproofing systems available
- An in-depth understanding of BS 8102:2009 and its requirements
- Desktop study and risk assessment knowledge – these should form part of any designer’s report and waterproofing design
- An understanding of sources of water (such as how it flows through the soil and interacts with the structures)
- Structural knowledge
- Knowledge of ground gases
- Geotechnical knowledge (to be able to understand the implications of a soil report)
- The ability to produce a design report, method statements and waterproof design drawings.
Delta Membrane Systems offers CSSW training and examinations at its fully functional training facilities in Epping, Essex, hosted by the PCA. All of Delta’s technical team are CSSW-qualified surveyors and are members of the Waterproofing Design Register.