14 Aug 2014

Deeper underground

28

Whether building a new basement or upgrading an existing one, it’s crucial to get the waterproofing right. Michael Earle, Technical Director at Safeguard Europe, explains what systems are available and where they should be used.

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As well as knowing a thing or two about railways and sewers, Victorian designers were pretty savvy about housing. They understood that by adding a basement or cellar to a property, they could make the most of the land they were building on. A century later, basements are enjoying a resurgence in popularity. Damp Victorian ones are being re-invented as bedrooms, playrooms and offices; new ones are being added to houses in high-value property areas such as Kensington and Chelsea; and some builders are even including them in new developments.

However, basements are an area of risk for the builder, as figures released last year by NHBC demonstrate. Claims on basements built since 2005 have cost the industry £21m, with 66% of them being due to tanking defects.

If a basement is to be a habitable room – a living room or bedroom – no water penetration is acceptable, according to the British Standard concerning waterproofing, protection of below ground structures against water from the ground.

The British Standard defines three types of waterproofing system: Type A which provides a physical barrier to the water either internally or externally; Type B which sees the waterproofing integral to the structure; and Type C, drained protection, which involves installing a membrane which channels water to a low point from which it is usually pumped out.

Types A or C can be applied to refurbishment and extensions, with the choice depending on the fabric and condition of the building and the level of water present. All three types can potentially be used in new-build situations. As the British Standard points out, sometimes a combination of systems will be the best solution, either to provide redundancy, or – in the case of a renovation project – to maximise the space available.

When a barrier is best

Perhaps the most familiar type of waterproofing system is Type A, barrier protection, which involves physically holding back the water pressure and possibly vapour too if the property is in an area where harmful gases such as radon or methane are present. There are many types including cementitious, bentonite, liquid-applied polymers, polymer sheet membranes and combinations of those.

For a new build, site investigation can establish the type of soil present and the ground drainage characteristics which can then inform the choice and extent of waterproofing. For an existing basement, this is not usually feasible, so the standard says that we must assume water will come to bear to the full height of the basement.

For existing basements, internally applied barriers are the norm for obvious reasons. Whatever the tanking system, it is crucial that it can be applied to damp substrate, forming a strong bond, and it must be able to resist the water pressure.

A typical Type A system for a refurbishment would see a render-backing coat applied first if the wall is uneven, followed by a tanking slurry. The junction between wall and floor is a potential weak spot and here it is important to include a waterproof fillet using a specialist mortar.

Getting a Type A system application right requires attention to detail, including preparing the wall meticulously, making sure the system is compatible with the substrate and following the manufacturer’s instructions on curing. If anything goes wrong, a barrier system can be difficult and expensive to fix.

Barrier for protection

In new applications, the structure of the basement can be designed to be waterproof, a Type B system. In practice this means reinforced concrete. Another standard, BS EN 1992, sets out how the concrete structure should be designed.

It is worth noting that the additional steel reinforcement required by the standard can make this a prohibitively expensive solution. And it cannot protect against harmful gases – an additional vapour barrier would be required.

Again, joints are the potential weak spot, with the junction between wall and floor needing particular attention as well as movement and construction joints. Possible solutions include ‘active’ slurries which penetrate concrete to make it water-resistant; expanding internal waterstops which swell once the concrete has cured; fillet seals and repair mortars; and construction joint tapes.

Construction joint tapes, such as those by Vandex, are best applied on the external wall of a basement. A design without a toe, extending the base slab beyond the wall externally, works best with this type of protection.

Drained and dry

Drained protection, Type C in the British Standard, is a popular solution for refurbishment projects. It is a good option if the substrate is weak or contaminated, it can cope better with ground movement and there are versions available which can also protect against radon and methane.

Water is directed down the walls into a channel running around the perimeter of the room, using a profiled cavity drain membrane. Detailing should be such that water cannot migrate from the cavity across the slab. A sump and pump at the low point of the room discharges the water.

These types of system do require regular maintenance of both the pumps and the drainage channels, so access must be built in. It may be worth considering a pump with a battery back-up or dual pumps, depending on the pumping capacity required and the potential impact of a mains failure or pump failure.

This system probably isn’t a good option if complicated geometries are involved. And it doesn’t work if the soffit of the basement is flat. Here a combination system could be used, for example, a tanking slurry on the ceiling lapping onto a cavity drain membrane on the walls.

Specialists must specify

Though an understanding of the types of waterproofing systems and their potential applications will help at the early stages of a project, it is important that specialist advice be brought in to help finalise the waterproofing design. NHBC’s research into the claims made due to basement failures shows that problems are usually due to detailing at joints and around service penetrations.

It is worth taking the time up front to choose the right system, get the detailing right and making sure that the manufacturer’s instructions are followed to the letter. This is always time well spent if it avoids costly remedial work. Contact specialist companies such as Safeguard to discuss the most appropriate solution for the particular basement waterproofing project in hand.

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