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Which? Trusted traders tips on waterproofing your basement

By Rebecca Milligan

When space is at a premium, extending your home downwards can be a good option. We spoke to our Which? Trusted traders to find out more about waterproofing your basement.
basement room once waterproofed and converted into a bedroom

Most of us would welcome some extra space in our homes. If you’ve already got a cellar, converting it into a habitable room can be a relatively low-cost option compared with moving. In prime property areas, such as London and the South East, there has also been a trend for digging out basements to create larger properties.

Basement conversions are often favoured by planners: they’re hidden away, so unlikely to be an eyesore. Jason Wakefield, waterproofing specialist from Which? Trusted trader Timberwise, told us about a recent example where planning for an extension in a new-build was refused, but granted instead for a basement.

Find out more about whether your project needs planning permission, or read on for more about how to waterproof your basement.

Approaching the project: what should you think about?

  1. Budget – your budget will influence the works and what’s possible. Which? has worked with the Royal Institute of Chartered Surveyors (RICS) to bring you more detailed costs for dealing with waterproofing. Bear in mind that these costs do not refer specifically to basements, where you will need to waterproof all four walls and floor; you can’t treat one wall alone.
  1. The desired outcome – you need to know what you want to use the space for. This will influence how your trader approaches the project.

For example, if you want to install a gym, this requires heavy equipment, so you need to ensure the floor screed (concrete under the floor) is strong enough.  Is it going to be a bedroom? If so, then ensuring the room is warm enough will be important. Or if you decide later to install a kitchen area, then any drilling into the walls could affect the waterproofing. So have a clear idea of the room’s future use and let your trader know.

  1. Finding the right trader – it is essential to work with a properly qualified and experienced trader when it comes to waterproofing. They need to abide by British Standard BS8102 (2009), which refers to below-ground structures. If your trader hasn’t heard of this, find another one.

Ensure that your surveyor is a Certified Surveyor in Structural Waterproofing (CSSW). A CSSW-qualified surveyor will be up to date with the latest guidelines, including the need to use two different forms of waterproofing in new-build properties. See below for the different methods.

Which? Trusted Traders has endorsed damp-proofing and waterproofing specialists, who will all have the appropriate qualifications. Plus you can check the reviews from other customers on their profile pages to find out more about what they’re like to work with. 

Different methods of waterproofing

There are three different methods of waterproofing, known as types A, B and C.

Type A, also known as tanking, involves coating internal walls and floors with a waterproof slurry and/or renders to create a physical barrier to keep moisture out. It’s not possible to tank a single wall – it has to create an impenetrable ‘tank’ to hold the water out.

Type B is similar to type A, in that it uses a physical barrier. But in this case the concrete structure itself is waterproof. This is typically used in new-builds rather than being applied retrospectively in older properties.

Type C is quite different in that it manages the way water comes into the property, rather than seeking to exclude it altogether. This method uses membranes applied to the internal face of the wall and floor, combined with a series of channels and pumps.

While intuitively it might feel right to try to keep water out completely, Refresh PSC’s Andy Barber told us that a Type C method can be less risky for older properties, as it won’t put additional stress on the structural fabric of the building. But it really depends on the structure of your home, which is why an initial survey is so important (see below).

Property types and surveys

Properties with existing spaces are always going to be easier to work with, as they won’t require digging out from scratch. Many Victorian properties were built with space for a coal cellar, which you can turn into a gym, a living space, a bedroom, or whatever you would like. Depending on what you intend to use it for, you may need additional planning permission and sign-off from your local council’s Building Control, particularly if you’re creating a bedroom space.

Before starting the project, you will need a site survey to determine the details of:

  • your property type
  • the age and construction of your property
  • the depth below ground of the basement
  • whether there are tree roots in the area
  • the level of local water tables
  • the local soil types and the likelihood of structural movement.

The type of property may influence the method of waterproofing that your trader installs but, generally speaking, it’s possible to create a basement extension in any property, as long as you have access to the ground floor. The site survey should bring to light any possible complications with the type of property, depth of the basement or tree roots – these are usually easy to overcome.

After the survey, the waterproof design specialists will also carry out an inspection, before making the decision about which method to use. A new-build would also involve geology reports on the surrounding soil, and hydrology reports on the local water table. 

There’s a lot of preparation involved in waterproofing your basement, but it’s all to ensure that your trader gets the process right. Once the designs are finalised, a straightforward installation of waterproofing in an average basement can take as little as a week to complete, providing you with valuable extra space in your home.

If you’d like to convert your basement, you can find Which? Trusted Traders’ endorsed damp-proofing specialists in your area.

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