How To Waterproof a Cellar.

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How to waterproof a basement

The DryDirect guide to waterproofing cellars, damp proofing basements and creating extra space beneath your property.

Many older properties in the UK are built with cellars and basements that could provide extra usable space either for much needed dry storage or even as additional living areas like bedrooms.

The problem is that often cellars are damp, poorly insulated and subject to water ingress.

 With proper planning and consideration for the waterproofing design, cellars and basements can be transformed to create dry, light, pleasant spaces

Unfortunately we often speak to householders and contractors who have had a bad experience with basement waterproofing projects. Poorly designed waterproofing systems, low quality products or even using good waterproofing systems in the wrong place can all lead to disappointing results

Dampness, mould and even flooding can cause big problems for basements when things go wrong.

Basement needing waterproofing

This guide aims to give an overview of the process and methods we use, the British Standard recommendations as well some common pitfalls to avoid.

Step 1 – Planning and Design.

The first step in the process is to find out as much information about the project as possible. Some key questions:

What will the basement be used for?

The requirements for a dry storage area or bike store will be different to a habitable space such as a bedroom. Habitable spaces have requirements not only from a waterproofing perspective but also insulation, ventilation and a means of escape. The design requirements for different types of space a clearly set out in table 2 of the British Standard BS8102:2009. Other requirements are covered by the current building regulations.

What is the structure and condition of the basement?

The structure of the basement is an important part of waterproofing design. If we are to apply an impervious barrier to a wall or a floor to stop water coming in, the structure needs to be strong enough to resist water pressure that may build up behind without moving or cracking. Walls and floors may need to be repaired or upgraded. Different waterproofing systems work better with different building materials and surface preparation may need to considered. Our friends at Grindblasters can help with surface preparation for basements.

Basement Waterproofing
Basement Waterproofing Planning

Are any structural alterations required?

Structural alterations to the basement may be required if the basement needs to be made bigger in order to be usable (low ceiling height is a common problem) if a means of escape or lightwell needs to be created or if the floor needs to be upgraded in order to be strong enough for waterproofing.

Waterproofing Survey.

We do recommend that a professional waterproofing survey is carried out by a CSSW qualified waterproofing surveyor. A waterproofing survey would be able to assess these requirements and provide a specification that takes into account the site and project requirements. We can recommend suitable CSSW qualified surveyors for basement surveys.


2- System Selection.

The selection of the waterproofing system will be based on the information gathered above. 

There are two general methods of waterproofing an existing basement.

1- Internal Barrier Systems also know as tanking or Type A waterproofing. 

2- Internal Cavity Drainage Membranes, also known as Type C waterproofing

Further information on the types of waterproofing can be found here

Is it really waterproof?

To further confuse product selection. Some products that are labelled or described as being “waterproof” actually aren’t. Sounds crazy? What do we mean by this?

 Some waterproof paints or coatings are described as waterproof but this just means that they can be exposed to water without breaking down. It doesn’t mean that they can hold water back in a basement.

Some coatings are waterproof but can only be used on the outside of a structure. Bitumen coatings such as blackjack or self adhesive membranes simply peel off when used on the inside of a basement.

Product selection is key. Remember a watch or an umbrella can be waterproof but keeping one in your basement wont make it dry!

Koster KBE Liquid Film being applied

Common problems to look out for when waterproofing your basement:

When using Cavity Drainage Membranes like the Delta MS systems. Drainage is key. The studded membrane installed to the walls and floors wont stop water from coming through the structure behind. Cavity drainage membranes are normally installed with a groundwater pump as well as accessible drainage channels. Most manufacturers do not recommend connecting a cavity drainage membrane system to drains due to the risk of flooding, smells and rodent ingress (yuk!) Having a pump avoids these problems.

Delta MS500 and Delta Qwikseal Plugs

Barrier Systems like Koster NB1 Grey  need a mechanical and chemical bond in order to stuck to the walls and floors and resist water pressure. This means that the surface must be prepared to remove paint, salts and other nasties. Old renders and plasters will need to be removed too. We were once involved in a project where a waterproof render had been applied directly to wallpapered wall. An extreme example but the contractor involved was genuinely surprised that the waterproofing had failed.

Buried Roofs are common in basements and occur where the basement is bigger than the ground flood. Cavity Drainage membranes must never be used to waterproof the underside of a buried roof as water ponds on the membrane causing it to sag and fail. More information on buried roof and podium decks can be found here.

Delta Terraxx sheet - drainage system

Need more help with the design or specification of your basement waterproofing project?

For help with product selection, waterproofing design and installation contractors:

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