Structural Waterproofing
What is Structural Waterproofing?
Quite simply it is the installation of a correctly designed system to help keep the internal environment fit for the purpose it is intended. In order to do this the chosen system will either stop water from entering the structure or it will manage any water that penetrates the structure. There are 3 types of system as defined by B.S.8102:2009 Type ‘A’ Barrier Protection (internal or external) Type ‘B’ Structurally Integral Protection (incorporated within the structure) Type ‘C’ Drained Protection. (Internal)
It is now considered best practice to choose a dual system for New Build Basements using any 2 of the above types. For upgrading existing basements it is only possible to use either type ‘A’ or type ‘C’
Once the Waterproofing system has been chosen it is then important to consider the intended use of the basement in order to achieve the required internal environment. It is usually necessary to provide ventilation – either natural or mechanical. Habitable spaces also need to be heated.
Type ‘A’
This is often called barrier protection as the waterproofing is applied directly to the surface so as to prevent water passing to the inside of the building. When it is applied externally it will protect the structure from the effects of water ingress. When water pressure comes to bear against the waterproofing it will push it onto the structure.
When it is applied internally it cannot protect the structure from water ingress. When water pressure comes to bear against the waterproofing it will try to push it off. It is imperative that a good bond is achieved by correct preparation and application. When applied to walls internally it should have multi coat cement renders applied on top of it known as ‘loading coats’ to strengthen it.
When applied to floors internally it should have a cement screed overlaid. When it is applied externally a protection/drainage layer is placed against it to prevent damage from back fill and also to allow water to drain freely to the installed land drain. This type of system can be achieved with cementitious products, liquid applied membranes and adhesive sheet membranes.
Type ‘B’
This is an integral method of waterproofing. What this means is that the waterproofing is within the structure. A common method of achieving this today is to use additives in the concrete, which classify the concrete as ‘watertight’. In situations where the water table is high it is considered best practice to use a watertight concrete.
In the past waterproofing has been ‘sandwiched’ within the structure. This type of system is impossible to repair in the event of failure, and therefore is not considered good practice anymore.
Type ‘C’
This is an internal cavity drainage membrane system. It involves fixing a membrane to the walls and floor, and incorporates an internal drainage system. The drainage system delivers water to a sump, or externally to a drain, or natural outfall if site conditions allow.
The sump – which may be internal or external –houses a submersible pump(s) to remove the water. These pumping stations should have a back up system incase of electrical power failure or pump failure. Remember, if you only have pumps powered by mains electricity, in the event of a power cut you don’t have a pump.
Power cuts are often caused during adverse weather, which is when you really need your pumps to work. Back up systems can be battery powered or by connection to a generator. The drainage system should have inspection/flushing points to allow maintenance of the system. If the drainage system cannot be flushed it may well block up over time. This is a water management system as opposed to a water barrier system.
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