The different methods of waterproofing a basement and benefits of basement waterproof tanking systems

At the time of writing, it is getting colder and the rain can be incessant. Spending time in a cellar
might not, therefore, hold much appeal. One might venture down there to rummage for the
Christmas decorations, notice the damp and the wet, shrug, then head back upstairs thinking of
festivities warmed by log fires and the odd tipple.

That’s Christmas. But what about the New Year? Resolve, resolutions and renewal? Maybe; maybe not.
Yet it’s as good a time as any to consider how converting your cold, damp cellar might offer more
living space and, for the investment, a proportionally greater increase in property value.
In a climate such as ours, a damp cellar is a fact of life: without protection – in older buildings, say,
with floors built from rough slabs, stone or brick – hydrostatic pressure will result in water ingress.
Soil-retaining walls will be vulnerable – particularly where the cellar floor joins the wall – and, at best,
the cellar will only be suitable for basic storage (so remember not to park those decorations too
close to the walls).

It’s self-evident why tanking is named so: in effect, a waterproof tank is created. However, its job is
to keep the water out not in.

There are three methods of waterproofing a basement, they can be used separately or sometimes together:

A) Barrier protection. This process relies on an unbroken ‘monolithic’ membrane to keep
moisture out. It is achieved by applying different impervious coatings – cement renders,
slurry, resins, bitumen-based products, spray on systems and bonded sheets – to the existing
floor and walls.

B) Structural integral protection. In this method, the structure itself (waterproof reinforced
concrete) is the protection. This method might be used when constructing a new basement,
usually in combination with A or C, to meet new building warranty requirements.

C) A cavity drain membrane (CDM). This works by placing the drainage on the inside of the
structure, between it and a plastic membrane. Water then drips down the outside of the
membrane, gathering in a drip tray-style drain before it flows to a sump pump and then an
external drain.

We will discuss cavity drain membrane (CDM) some more, since not only is it our most popular solution, it is also the cleverest and, arguably, the most effective.

For starters, a CDM prevents a gradual pressure build-up against the construction. With options A)
and B), it would be necessary to dig drainage on the outside of the tank. With option C), however,
the drainage is on the inside; the moisture’s progress is halted by the plastic membrane. Moreover,
the air gap allows the structure to ‘breathe’ and dry out.

The membrane is usually a high-quality, high-density plastic such as polyethylene. It will be studded
in appearance – allowing water to drip down to the drain – and comes in various thicknesses e.g.
3mm, 5mm, or 8mm, and up to 20mm for the floor membrane. It will also be gas and vapour-proof
and will have a long-lasting guarantee e.g. 10 years. It is applied using sealed, fixing plugs.
The drainage channels are created around the basement perimeter, leading to the sump chamber,
while floor insulation will first be applied to offer maximum resistance to ground moisture
penetration, before a floor membrane is laid for added waterproofing.

Where the wall membrane meets the floor membrane, vapour tape is used. In this way, a secure
moisture barrier is placed over the seams and it also strengthens the joins, keeping the membrane in
place.

The sump pump, meanwhile, is installed at the beginning of the process i.e. when only the basement
shell exists and before the membrane is applied. It is fitted in the sump chamber – the lowest point
of the basement – and connected to the drainage channels underneath the membrane walls. The
groundwater is then pumped out to an external drain.

With a lid on the chamber, the screed will be laid over the floor membrane and, depending on the type
of membrane used, plaster might be skimmed on to it directly.
Another solution is ‘drylining’, in which metal or wooden framing is fixed to the membrane, allowing
for a drywall finish. After that, the basement may be ‘fitted out’, with electrical and plumbing
fixtures added.

The work is always done in accordance with BS8102- 2009 – the code of practice for protection of
below ground structures against water from the ground.
At this point, the waterproofing would be completed. However, if the work were part of a larger
project, Danford Brewer & Ives might hand over to a partner e.g. a builder, architect or property
developer.

If, for example, we were assisting a property developer, we might advise them on making the best
possible use of available space and land. It might also be necessary, say, to provide underpinning if
the conversion of a basement compromises the structural integrity of the property by weakening the
foundations or removing walls.

The potential offered by tanking is enormous. And by making use of our expertise (Danford Brewer
& Ives are qualified surveyors with both CSRT (Certificated Surveyor in Remedial Treatment) and
CSSW (Certificated Surveyor in Structural Waterproofing) credentials) both householders and
building partners can gain significant living space and add value.

Moreover, we are skilful, efficient and cost-effective. If you would like more information on tanking,
structural waterproofing or any of the other building services Danford Brewer & Ives offer, such as
timber treatment, extensions or building maintenance – then please contact us. As always, we’re
more than happy to help.