Focus On: Waterproofing a Basement

by Build It
1st November 2019

Waterproofing a basement is a critical point in the initial stages of house design. John Spearman, director at Surrey Basements, gives the lowdown on why good design and specialist installation are key to ensuring a properly waterproofed basement.

What types of waterproofing are there for basements?

When you’re creating a habitable basement, getting it properly waterproofed is an absolute necessity to avoid issues with ingress and damp. Moisture penetrating the walls or floors of your basement could cause damage and create an unhealthy environment, so finding the right solution is vital.

For retrofits – adding a basement beneath an existing home – there are two main options to choose from. Cavity drain membrane systems involve using an eggcrate-style polyethylene membrane internally, along with a sump pump (which removes water that has accumulated via the drainage channels), to create a fully sealed zone. The alternative is a cementitious-type installation, such as the application of a waterproof render or slurry.

If you’re building a new house from scratch, most warranty providers require the use of two waterproofing systems.

Typically, you would use a cavity drain membrane internally, along with another solution either externally or integrally. On the exterior face, you could go for a geotextile membrane with a perforated land drain incorporated.

A variety of other options exist. We’ve fitted Volclay systems, for example, which are basically mats of absorbent material that are installed under your basement’s floor slab and up the external wall surfaces.

When water comes into contact with the clay in the mat, it swells and seals the breach. You can also get waterproof paints, so there are plenty of products out there. An integral system involves using concrete that incorporates a waterproof admixture.

How can you decide on the best way to waterproof?

Your basement designer should recommend something tailored to the project. I’d always suggest getting a certified surveyor in structural waterproofing (CSSW) to come and look at the basement, regardless of whether it’s a new build or an existing one. They will then go and design a system that will be most appropriate for the situation.

Always use a specialist who has a qualification behind them and can demonstrate their experience. If you put your faith in your standard local builder, they may not have the expert knowledge to get this right.

Stepoc basement construction

For this project, Surrey Basements installed a geotextile membrane to the floors and a Volclay system below slab level. A cavity drain membrane and sump pumps were then fitted internally to provide a second form of waterproofing

For example, you might need geotechnical surveys and soil reports, which could change the required solution. If there is clay in the ground, for instance, you have to take into consideration that this is impermeable soil and it will take a long time for rain or any water to discharge through that clay layer (known as a perched water table).

In that situation, the design can’t include a perforated land drain, because in clay soils this would be overwhelmed and under constant water the whole time – so it wouldn’t work. A general contractor might not be aware of such implications.

When should you start planning waterproofing?

At the design stage. That’s when you’re dealing with engineers and architects, so key information your waterproofing specialist needs to identify the correct system could come to light. Plus, if they’re involved now, they might be able to make it a bit more efficient price-wise.

For instance, if the engineer decides to use a waterproof admixture in the concrete for the structure of a new build basement, your team may only need to fit an internal membrane to deliver the two forms of protection (an external solution might be superfluous).

Or the water table may be very high on a plot, which might demand three systems to provide robust protection. It may also be possible to isolate some internal walls and eliminate them from having to be treated before they’re built. The message is that communicating with a waterproofing specialist at the design stage can identify the right course of action.

What’s the average cost of waterproofing a basement?

We charge between £35 and £40 per m2 for a cavity drain membrane solution. That doesn’t include the price of the sump, which would be around £1,500. For a cementitious system, eg a slurry, we’d charge a similar sort of price.

So that’s a good ballpark for treating a structurally sound basement zone. As for how long the work takes, it depends on the size and nature of the project. We always install to the British standard BS 8102, but a local builder may not adhere to this.

Working to that code of practice involves being diligent and methodical – but you get confidence that the work is being done right. As a rule, on a straightforward job with no convoluted shapes or other difficulties, we can install around 150m2-200m2 of waterproofing membrane in a day.

What are the legal requirements for waterproofing a basement?

Not as such, but you’ll want to make sure the installation is carried out correctly and the company is stacked up with accreditation. Use a registered fitter, a CSSW-certified designer, and ensure they can provide an insurance backed guarantee.

Look for membership of the Property Care Association, a trade body in the waterproofing and building preservation sector that fully regulates and audits its members.

Follow those guidelines and you shouldn’t have any trouble. Pretty much all the failures in waterproofing we’ve come across have been due to work carried out by companies that aren’t properly accredited. So they’re not specialists and the basement has suffered from poor workmanship and possibly even the wrong design.

Try not to cut costs on this element, because at the end of the day, your basement living space will only be as good as the waterproofing.

For more information visit the Surrey Basements website.

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