Basement conversions and how to make them waterproof

What does a house owner you do when they need more room but do not have the outside space available to build a rear or side extension to their property?

Extending upwards or downwards may be a more cost-effective solution than moving to a new property.

Mega-basement conversions hit the headlines in late 2014 when Kensington & Chelsea Council won its battle to limit the scale of underground development in the Royal borough.

The council’s guidelines now state that basement conversions in the borough must be limited to a single floor and extend to no more than 50% of the space occupied by the garden.

Whether you are planning a mega-basement or a more modest conversion to give your home the extra space a family or business requires, it’s advisable to undertake a little research to ensure the new structure is waterproof.

Why? Basements and cellars are sunken into the moist ground, and so are even more susceptible to problems with damp than the rest of a building.

Your waterproofing options explained

1. Tanking

The traditional way of making a basement waterproof is cellar tanking. This involves coating the walls, ceiling and floor in multiple coats of render to act as a barrier against damp.

However, neighbours objecting to basement conversions often claim that these underground living areas will upset the subterranean status quo and send displaced groundwater seeping, or maybe even rushing, into nearby properties.

To allay these concerns, review the flood data for your local area and establish where the groundwater level is. This will dictate the type of waterproofing – or tanking – that will be required. 

Reviewing available data as to what has happened in the last couple of years may not represent the true picture, so it is advisable to arrange for a formal site investigation to be carried out. 

This will provide data in detailing the soil profile at depth for the site. This not only helps the architect establish foundation depths but will also confirm if groundwater is likely to be encountered. 

It should be noted that tanking must withstand the external water pressure – also known as hydrostatic pressure – around the basement conversion. The pressure from the water table around a basement can be enormous and unless the tanking is very securely fixed to the walls of the conversion it can fail.

Remember, hydrostatic pressure will force water through the tiniest fault very rapidly and once a leak occurs it can be very difficult to isolate and repair.

2. Cavity drain membranes

What happens if tanking will not provide the protection against damp required? A viable alternative to tanking is to use cavity drain membranes.

These membranes create an inner waterproof structure in the basement, behind which is a cavity (created by the studded profile of the membranes) that is fully drained, so any tiny leaks in the outer structure are diverted harmlessly away via a drain.

By constantly draining away any small leaks there is never any water pressure against the inner structure. Several reports consider cavity membranes to be the most reliable way to waterproof a basement.

Whatever system is used, householders should avoid putting screws through the waterproofing system when hanging shelves and pictures.

Further advice

Homeowners undertaking a basement conversion are advised to ensure their contractors and design team work under formal Construction Contracts and materials such as waterproof concrete and tanking applications are provided with an adequate 10-year insurance-backed guarantee covering both the materials
and workmanship.

Basement conversions will require a large number of skips to transport the earth dug up during building work away and dispose of it in an environmentally-friendly manner. Proskips has provided building contractors carrying out basement conversions with many hundreds of skips over the years.

To discover how much money we could save you, contact ustoday for an instant quote.