Considering a concrete basement project? Here’s what you need to know

When beginning a basement construction or renovation project, it’s important to know that any work undertaken should meet the BS 8102:2009* standard for waterproofing. With plenty of recommendations and guidance on methods of dealing with and preventing water ingress, it is the go-to guide when it comes to this type of work.

When it comes to basement construction, concrete is the main material in the structure, mainly due to its proven durability and strength. When choosing the right concrete, it needs to have watertight status, as even regular reinforced concrete designed to BS EN 1992 can still allow cracks to form.

Making concrete watertight

When it comes to making sure your concrete is suitable for residential basements, you or your builder should look to add something called an admixture. There are various products on the market that do different jobs, but, in a nutshell, you are looking to reduce the amount of mixing water needed as well as the quantity of capillaries in the hardened concrete. (Capillaries are microscopic voids which are left behind by excess water during the mixing process).

You can achieve this by adding a superplasticiser to the fresh concrete mix as well as capillary-blocking admixtures which lower the water/cement ratio and waterproof the remaining paths for water. The hydrophobic layer that is created within the capillaries performs well when there is variable water pressure, something common in below-ground and basement construction.

You can also turn to Sika WT200P admixture which has crystalline technology. Working slightly differently, it creates a non-soluble structure in unwanted capillaries. This is produced by chemicals reacting in the presence of calcium hydroxide and moisture. It then fills and blocks the capillary structure, as well as small voids and micro-cracks. The reaction is reactivated in the presence of moisture or water, offering high performance in conditions where water is permanently present.

Basement sealing

Using crystalline or hydrophobic technology will ensure a watertight concrete, however, you still need to pay particular attention to the joints between the sections in the basement. A PVC waterbar is one option that will create a fully watertight barrier for construction and movement joints.

Other alternative joint-sealing products, such as those in the SikaSwell range, can make the construction process easier because they are simple, versatile and reliable. These hydrophilic strips allow the sealant to expand when it is exposed to water and stops it in its tracks. These types of products are designed for joints exposed to low-to-medium groundwater pressure, making it a ideal for most basement work.

Dual/multiple systems

Dual systems are recommended by BS 8102:2009 where the risk of water ingress are high. More than one waterproofing protection system is used, such as watertight concrete and an external waterproofing membrane. With the right selection of membrane, these can have the advantage of making the building watertight but also to stop below ground gases.

Basements are at risk of being exposed to below ground gases, such as radon, methane and carbon dioxide. This is particularly the case in construction projects being delivered on brownfield sites due to the lack of land availability across the country. These gases are more harmful than water because they can be a silent killer. The application of waterproofing/gas membranes, such as SikaProof and Sikaplan VOC membranes, can contribute to reducing the risk of various gases entering into the building.


The simplest component when constructing any basement or underground structure is watertight concrete, but it is imperative that you pay particular focus to the selection of the correct dual waterproofing system for that project and ultimately the finished environment.

Defects can occur in any waterproofing system, but the risk can be minimised through design, planning and a waterproofing specialist who can offer expert specification advice and a high level of site support throughout the build.

As the UK’s only manufacturer to provide a complete range of structural waterproofing systems, including tanking membrane, cementitious render, watertight concrete and cavity drain, Sika is ideally placed to provide the impartial advice on the creation of the optimum waterproofing strategy.

* Source: British Standards Institution, BS 8102. Code of practice for protection of below ground structures against water from the ground. BSI, London, 2009