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.
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.
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.
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.