Why should you use Cavity Drain for basement waterproofing?
Fair warning, below we've got a lengthy article on Cavity Drain systems by Ian MacLennan, who as some of you will know (we've barely mentioned it) was on the team that rewrote BS8102* in 2022.
There's been some debate recently and we thought it best to weigh in with a comprehensive assessment of what it is, and why it works.
(*BS8102, 'Protection of Below Ground Structures Against Water Ingress' is a code of practice which takes the form of recommendations and guidance.)
You might ask, What is Cavity drain?
In basic terms, a cavity drain system is normally a plastic membrane applied to a wall and/or floor with a channel at the wall/floor junction to remove water.
It is not strictly a structural waterproofing system,
It's a water management system.
It has always been understood that basements must be built watertight or waterproofed after construction.
This is achieved by good concrete design and crack control and if necessary, the application of membranes, renders or slurries to further waterproof the structure.
Why should you use cavity drain?
1/4 - Why do we need cavity drain?
The reason is that the industry and by that, I mean the construction, engineering and insurance industry have realised you can manage the risk of water coming in, but it is not possible to eliminate every possibility for water coming in on every project.
Large projects are impossible to deliver without any defects. Smaller projects are often carried out by less experienced site staff, leading to defects.
Why should you use cavity drain?
2/4 - Court cases* have established in law that applied products cannot be installed defect free.
(*Outwing Construction v. Thomas Weatherald, 13 Sep 1999)
The institute of Civil engineer’s state that 50% of basements leak during construction and a significant number continue to leak after construction. A system is often not tested until de watering is taken away by which time the basement is complete and often the building above to provide loading before removing de watering. Basements are often started in Spring, finished by Autumn, and not tested by water until Winter.
Waterproofing guarantees for structural waterproofing have proven to be offering no protection to clients, insurers or design teams so claims invariably end up with huge costs for contractors who accept the loss to be able to finish the project and get paid or they can end up in expensive court cases.
Why should you use cavity drain?
3/4 - The remediation of waterproofing issues
The issue is when type A or B systems fail or there even just minor leaks. They are inaccessible for repair. They are either buried or they have walls, lining, insulation, and expensive permanent finishes in front of them.
If you cannot access them, you cannot repair them. Many years ago, the industry realised that form and feasibility of repair had to be in the initial design.
If a structure can be accessed for grouting, resin injection or localised repairs then it is practical and reasonably inexpensive to remedy.
However, if it is inaccessible due to finishes, obstructions, structure then it needed a system that could in most circumstances be repaired without the expense and disruption of stripping the whole basement out to find the source of a leak and to carry out repairs. Hence, The cavity drain.
To put this into some context, Maclennan have In the last 12 months had to install a full cavity drain system into at least 5 multi million pound basements after they were completed and waterproofed with type A and Type B systems by main contractors.
All the basements were completed but could not be handed over to fit out contractors due to leaks and noncompliance with insurer requirements.
The delays were hugely expensive, insurance would not accept the basements, clients would not pay for the basements..
All of them had guarantees for additives and Type A waterproofing. None of them could claim off manufacturers or additives companies. In most cases the main contractor had to pay for hundreds of thousands of pounds worth of remedial work.
Have you got a project that needs waterproofing?
Do you need technical advice or a design only package?
We design, supply and install on our Pi.
Why should you use cavity drain?
4/4 - If Basements are like boats, Don't get caught without a life raft.
Manufacturers and additive companies still claim that it's safe to install basement waterproofing and proceed to fit out the whole basement with hundreds of thousands in finishes without any fail-safe protection.
This would be like setting out to sea in a ship without a reinforced hull, bilge pumps or lifeboats... the ship builder says it's fine, they built it watertight.
The Titanic is a household name for a reason.
Movement and defects occur after construction. Water tables change, services leak in the ground, buildings get built nearby changing the environment around the building... there are inevitable undetected defects.
These are our icebergs, the cavity drain is the hull, bilge pumps and lifeboats.
We must have a cavity drain to deal with the consequences of leaks with minimal disruption and cost to the main contractor, design team and property owner.