What lies beneath

Extra space is something that many home owners desire but often that means adding a conservatory or a loft conversion. David Symes of Delta Membrane Systems looks at the basement alternative.

While it may not be the most obvious option for extending a home, basements can be a cost-effective, spacious and comfortable solution to space demands, and it is easier to achieve than many imagine.

Basements fall into three categories: those that exist within the structure, those designed-in to a new-build property and those created as a retro-fit to an existing house. What all three have in common is that they can provide inviting living areas that deliver the extra space required by a household.

The most common impression people have of basements is the dark, damp-prone space that perhaps houses the boiler, the Christmas decorations or a plethora of creepy-crawly wildlife. Not particularly inviting.

In refurbishment projects, the main problem to solve is damp. The solution in such projects is not so much preventing the damp, but managing it. Damp proofing a basement area can be an involved and expensive process. The alternative ‘management’ option is much simpler.

Here, damp is allowed to penetrate the walls and floor, where it then meets a barrier that directs the moisture to a drainage channel which – in turn – sends it to a sump. This unwanted water is then pumped away to a suitable drainage outlet.

The damp will not penetrate the water-proof barriers, allowing the room side to be plastered on the walls and floor screeded prior to decoration. This results in the ability to create a warm, inviting living space that can be used for a variety of applications – bedrooms, living room, personal cinema or even a home gymnasium.

Converting a cellar into habitable accommodation may need a ‘change of use’ planning application. Basements that have been designed into a new-build will have been approved if the rest of the plans are approved. Retro-fitting basements to existing properties (excavating and creating new rooms under the house) will often be covered by permitted development rights, as the basement is unlikely to alter the building’s appearance. It is worth speaking to the local planning authority for advice.

Building Regulations approval, on the other hand, is an essential requirement for both refurbishment and new construction, and the Basement Information Centre offers the guidance document Basements for Dwellings. This is an essential read for those new to this type of work, as it provides practical guidance in helping to meet the relevant requirements in Schedule 1 to the Building Regulations.

It also gives good practice advice for matters not directly or precisely covered by the Regulations. Indeed, the Basement Information Centre is a useful source of information, along with fully qualified CSSW staff (a recognised industry qualification) at relevant product companies.

Terraced houses have their own particular challenges with party walls, but they are not unsurmountable, and should not put anyone off a project. Neighbours and landlords need to be consulted, but it is worth referring to the Party Wall etc. Act 1966 which provides a framework for preventing and resolving disputes in relation to party walls, boundary walls and excavations near neighbouring buildings.

A building owner proposing to start work covered by the Act must give adjoining owners notice of their intentions, and this is set down in the Act. If the neighbours disagree with what is proposed, the Act provides a mechanism for resolving disputes.

Once all the legislation has been assessed, and the necessary permissions obtained, it is then down to the construction and the selection of the right materials for the job. Obviously there is far more construction work involved with a new-build basement compared with the refurbishment of a cellar – however, that may be an advantage as the area will be designed and built for purpose. Once the structure has been created, the work is much the same in either situation.

Tanking below ground level commonly involves the application of a layer of cementitious waterproof render system on the walls, linked to a waterproof screed on the floor. Tanking can also be carried out using a sheet membrane, asphalt or other liquid-applied waterproofing material.

Hydrostatic pressure – the external water pressure around the basement – is also a critical factor that needs to be considered. It is crucial that the tanking is securely fixed to the substrate as the pressure from the water table around the basement can be significant. Hydrostatic pressure will force water through tiny gaps very quickly, so great care should be taken at this stage to ensure the waterproofing will meet the demands made of it.

As mentioned earlier, cavity membranes are a suitable alternative to tanking. Membranes with a studded profile can be used to form an inner waterproof structure. The studded side is placed against the wall, creating an area that allows water to flow down to the floor and into a drainage channel to a sump. It is then pumped out to a suitable drainage outlet.

This method is also the number one choice in refurbishments, as the amount of construction work needed is greatly reduced compared with the alternatives.

The floor must not be overlooked, and a suitable high capacity drainage membrane should be specified. Again, using this solution the moisture will find its way to the drainage channel.

Taking the basement route to creating extra space may, to many, seem to be a much tougher option than loft extensions or conservatories, but that really is not the case. Basement construction can be quick and the benefits for the future of the occupiers of the property are substantial. Designed correctly, the end result is additional living space that is warm, comfortable, free of damp and – most importantly to the home owner – capable of adding value to the property.

Many residents in the more affluent areas of London are recognising how basements create the required space and greatly improve the value of their individual properties. It seems that ‘going down’ is the way to go.

David Symes is technical director at Delta Membrane Systems.