If your home has an existing cellar, you may want to take advantage of the added space and turn it into a usable room. Even if you only plan to use the room for storage, as opposed to living space, you still need it to be properly waterproofed (or tanked) to prevent your belongings from getting damp.

The waterproofing process you choose will depend on the type of ground your home is built on. However, it is also important to consider what you intend to use the cellar room for, and what your budget is, before starting any work.

How much does it cost to tank a cellar?

Providing there is adequate headroom, converting an existing cellar into additional living space costs around £750-1,200/m².

However, if the existing ground floor is concrete slab as opposed to floor joists suspended from the walls, or you have to divert the existing drains, or if you live in an area with solid rock, clay, peat or marshy ground, or if the local water table is high, then costs could rise to more like £2-3,000/m².

To waterproof a damp cellar through tanking can cost as little as £40-80/m². However, do bear in mind that lowering the floor level to increase headroom costs around £200-300/m².

Whilst a rough cost for underpinning the walls will come in at around £500-1,000/m². Further information on the costs involved can be found on the website of the Basement Information Centre.

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Waterproofing Effectively

Even if you do not intend to carry out a full conversion to your cellar, waterproofing it will still add value to your home, providing a useful storage area and giving future buyers the option of turning it into another room if they wish.

Being below ground, the earth surrounding the cellar acts as a route for water to enter through the walls, and in order to prevent this from happening, the walls must be fully waterproofed. There are several ways in which to do this, either by sealing the walls using a surface coating, known as wet basement tanking, or through the use of membrane systems.


Tanking Systems

The way these systems work is by applying a waterproof coating directly to the inside of the porous cellar walls to prevent moisture from seeping in. They do not remove the water, they simply create a barrier.

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For a tanking system to work well, the walls that the tanking product – these range from cementitious coatings, bituminous coatings, membranes, paints and sealants – is being applied to must provide a good ‘key’ (i.e. a scratch coat). Tanking systems also require the brickwork of the house to be stable due to the fact that tanked walls need to resist water pressure that will build up.

In older homes, preparing walls to take whatever coating is being used can be quite time-consuming, involving hacking off old plaster, raking out old mortar, repointing and applying salt-neutralising products.

Waterproof coatings can be applied using a trowel or are sometimes sprayed on, with the aim of forming a bond with the masonry substrate, to create a completely waterproof barrier. Plaster can then be applied on top of these coatings.

Attention must also be applied to weak points in the structure – typically the wall/floor junction – where water is most likely to enter.


Cavity Drain Membrane Systems

Membrane systems are used on most cellar conversions, particularly in areas with high water tables and on older properties, as they do not rely on sound substrates for their effectiveness.

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The majority of these systems involve studded membranes, often made from high-density polyethylene, being fixed to the walls with plastic plugs, fitted with mastic seals behind the head. Water is allowed to flow down within the cavity created to the floor – which will also be covered with a membrane – and into a drainage system.

The floor can be covered with screed, or a layer of insulation and chipboard. The stud profile also means an air gap is created, acting as a depressurisation zone for any water that comes in through the walls, so that as the water enters, under great pressure, it loses its force and falls behind the membrane, down to a drainage conduit, which channels water out, often with the assistance of a pump.

This can then be covered with plasterboard. The floor can be sealed with either a membrane and concrete, or a studded membrane, but you should seek professional advice regarding which will be best in your area.

Inevitably all this will add to the floor height of the cellar, reducing headroom. Unless the remaining headroom is not lower than that used for new, modern homes – around 2.35m – a dig out and underpinning of the walls will be required before any waterproofing work can begin.


Drainage

Although it is occasionally possible to drain the water that collects behind cavity membrane walls away using natural drainage – where the water flows out to an open, non-earth-retaining elevation – it is usually necessary to have a sump pump system to take water out — in fact, many basement conversion companies will not guarantee a project unless one has been installed.

A sump pump is a pump used to remove water that accumulates in a ‘sump pit’, funnelled in from the drains of a basement waterproofing system. They send water away from the house and into a main drain elsewhere, such as a storm drain. Sump pumps are usually run off the main electricity supply and should have battery backup.

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