How to Waterproof Basement Walls & Floors

I want to convert my basement. How should I waterproof it?

Many properties have basements and cellars; converting them into habitable spaces can be a satisfying and cost-effective way of extending a home or office. As the title of this guide suggests, this article addresses how to waterproof a basement using a selection of waterproofing and tanking products that will ensure the living areas of the underground rooms remain free from water infiltration and damp.

Why is water seeping into my basement?

By their nature basements are below-ground structures that are vulnerable to high water-table levels.

Fluctuations in the water-table level are due to variations in rainfall between seasons, and its height can vary even within the same neighbourhood.  Water is relatively heavy, weighing one tonne per cubic metre. If the soil encasing a basement becomes saturated the mass of the groundwater may apply significant pressure against the walls and floors of the cellar.  The pressure that the water exerts can force it to seep through porous masonry or to leak via small gaps at the base of the walls, where they meet with the concrete slab.

How do I waterproof a basement beneath an existing building?

In contrast to a newly-built basement, in which the structure itself functions as a water-resistant shell, a cellar or vault beneath an existing building generally requires an internal sealing system.

There are two main techniques for waterproofing an existing basement.  The first method is known as ‘Type A’ waterproofing or tanking, which involves applying impermeable coatings or bonded coverings to the internal surfaces to hold back the water that is seeping through the structure. The second method is known as ‘Type C’ waterproofing which utilises a cavity drain system to control the intrusion of water and manage its discharge.  An array of membranes and conduits direct the inflow and a pumping assembly governs the outflow.

The combined use of both techniques offers a beneficial ‘belt and braces’ approach.

BS 8102 2022, the British Standard entitled “Protection of structures against water from the ground” suggests that Type C cavity drain membrane systems are likely to offer the most effective and trouble-free type of waterproofing for upgrading cellars or basements.  The structure of the building, with or without a tanking coating, provides primary resistance against water infiltration and the drained cavity system administers the collection and discharge of any water that seeps or leaks through the walls and floors or at the junction where they meet.

What is a Cavity Drain Membrane?

Moulded from high-density polyethene (HDPE), cavity drain membranes are incredibly strong and flexible; they incorporate integral stand-off studs in the shape of truncated cones, which create a continuous air-gap from which water can drain.  The membrane is water-resistant and has a high resistance to water vapour transmission.

The size of the studs determines the drainage capacity.  Waterproofing membranes formed with 8mm studs are usually used on walls to drain water seepage into channels placed at their base.  Membranes manufactured with 20mm protrusions are generally used across a concrete floor to collect water coming through the slab.  Baseline cavity membranes are capable of dealing with substantial volumes of water and provide a functional barrier to the transfer of salts or other contaminants from the substrate. Please refer to the relevant product datasheets for a full specification.

Rolls of membrane are easy to unwind against a straight wall or floor surface; fixing plugs, fitted with seals, secure it to a wall in horizontal lengths or vertical strips.  Butyl rubber tape, which is a chemically inert, water-repellent and impermeable to gases, seal the seams between adjoining membranes.

How is water discharged from a cavity drain waterproofing system?

BS 8102 2020 indicates most basement structures are likely to be subjected to groundwater pressure at some point in time. This assumption of hydrostatic pressure dictates the necessity for fitting a water removal system in conjunction with cavity-drain membranes, irrespective of the current or historical conditions in below-ground constructions.

A basement drainage system comprises three critical parts; ducts for managing the flow of water beneath the membrane, sump basins for collecting the water from channel-outlets, and float activated pumps for discharging the accumulated water from the sump and expelling it from the building.

A network of profiled drainage channels sits in trenches beneath the screed. The concealed conduits run along and around the perimeter of the cellar and feature many inlet ports that allow groundwater to enter the drain.  Special connectors, ‘T’-junctions and corner-pieces combine to ensure the drainage is adaptable to the shape of the cellar.  The sump chamber, which houses the pumps, also sits beneath the flooring slab.  The layout includes various essential accessories, such as jetting-eyes and removable sump-covers, which facilitate inspection and maintenance of the system components.

The composition of drains, sumps and pumps concealed behind the watertight cavity drain membrane is key to successfully waterproofing a basement and keeping its habitable zones dry.

The animated video shown below demonstrates how to waterproof cellar walls and floors under the current British Standard and how to conceal the assembly behind plastered finishes.  The result is a sealed watertight system covering the base and sides of a cellar with provision made for disposing of water build-up behind the membrane via a sump and pump.

This guide has been compiled to assist the homeowners, cellar tanking contractors, and specifying professionals. Property characteristics and environments vary; as such, each basement waterproofing specification needs a project-specific design.

Related documents:

How to waterproof cellar walls with Basement Mesh brochure How to waterproof cellar walls with Basement Mesh

How to convert your basement with Baseline 8 brochure How to convert your basement with Baseline 8

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