Waterproofing Membranes

A waterproofing membrane is a continuous, impermeable barrier applied to a structure to prevent water ingress, water vapour transmission and, where required, ground gas infiltration. In below-ground construction, waterproof membranes form the primary line of defence protecting basements, substructures, retaining walls, podium decks and tunnels from groundwater and soil moisture that will otherwise penetrate concrete over time. Without an effective waterproofing membrane, below-ground structures are vulnerable to water ingress that causes structural deterioration, loss of internal environment quality and costly remediation work after construction is complete.

Structural waterproofing membranes are specified under BS 8102:2009, the code of practice for protection of below-ground structures against water ingress. The standard defines three system types. Type A is barrier protection, delivered by an applied waterproof membrane that forms a continuous impermeable layer against the structure. All products in this range are suitable for use in Type A waterproofing systems as defined in BS 8102:2009 and cover the main product types used in basement waterproofing systems across residential and commercial construction.

The waterproofing membrane type required for any project depends on whether it is being applied to a new structure before the concrete is poured, or to an existing formed concrete structure after the pour. This is the primary specification decision, and choosing the wrong product type for the application is one of the most common and costly errors in structural waterproofing.

Pre-Applied Waterproofing Membranes

Pre-applied waterproofing membranes are positioned on the blinding concrete or against the formwork before the structural concrete is poured. As the concrete is placed, it bonds directly to the membrane, creating a fully bonded system that eliminates the risk of water tracking laterally behind the waterproofing layer if a point of failure occurs. They are the correct specification for new-build basements, raft foundations, ground-bearing slabs, pile caps and tunnels where no access to the external face will be available after the pour.

Our pre-applied range includes bituminous, polyolefin and bentonite geocomposite options from GCP, Sika and Visqueen, covering standard waterproofing applications through to dual-function membranes that provide combined waterproofing and ground gas resistance within a single layer.

Post-Applied Self-Adhesive Waterproofing Membranes

Post-applied waterproofing membranes are applied to the formed external face of a concrete structure after the formwork has been struck and the concrete has cured. Self-adhesive sheet membranes bond directly to the prepared substrate without open flame or solvent. Wall waterproofing membrane applications are the primary use case: basement walls, retaining walls and the external faces of substructures where the formed surface is accessible after construction.

Our post-applied range includes self-adhesive bituminous and polyolefin sheet membranes from GCP, Sika, RIW and Fosroc. The Visqueen Gas Resistant Self Adhesive Membrane also sits within this category, providing combined waterproofing and ground gas resistance where both are required within a single post-applied membrane layer.

Installing a Structural Waterproofing Membrane

Correct installation is as important as correct specification. A waterproofing membrane that is properly selected but poorly installed will fail, and the consequences of failure in a below-ground structure are expensive to remediate once the building is complete and backfilled.

Surface preparation is the most critical stage for post-applied membranes. The concrete substrate must be sound, clean and free from laitance, contamination, sharp protrusions and voids before the waterproof membrane is applied. Any irregularity in the substrate compromises adhesion and creates potential water ingress paths that will not become apparent until the structure is in service.

Laps between adjacent membrane sheets must be fully bonded and taken to the minimum overlap dimension specified by the manufacturer. Inadequate lapping at joints is one of the most common sources of waterproofing membrane failure on site and accounts for a significant proportion of post-construction leaks. The membrane must also be taken up to a minimum of 150mm above the finished external ground level to prevent water bridging the top of the waterproof membrane and entering the structure from above. Before backfilling, a protection board should be installed over the waterproofing membrane to prevent mechanical damage from stones, construction traffic and compaction plant.

Common waterproof membrane mistakes include inadequate surface preparation before application, poorly bonded laps at sheet joints, insufficient upstand height above ground level, failure to protect the membrane before backfill, and specifying a post-applied membrane for an application that requires a pre-applied system. All are avoidable at the specification and site supervision stage. A correctly specified and installed structural waterproofing membrane is designed to perform for the lifetime of the structure. Failures are almost always the result of installation or specification error rather than product failure.

Choosing the Right Waterproofing Membrane

The first decision when selecting a waterproofing membrane is whether a pre-applied or post-applied product is required. For new-build basement waterproofing systems and other below-ground structures where the membrane must be in place before the concrete is poured and where the external face will not be accessible after the pour, a pre-applied membrane is required. For applications where the concrete is already formed and the external face of the wall or slab is accessible for direct membrane application, a post-applied self-adhesive sheet membrane is the appropriate product type.

Where a self-healing waterproofing membrane for concrete is specified, or where ground conditions are aggressive or contaminated, the Sika Bentoshield Max LM pre-applied bentonite system is suited to those requirements. Where the application involves both waterproofing and gas resistance on the same structure, the Visqueen Gas Resistant Self Adhesive Membrane covers both within a single post-applied product.

Which waterproofing membrane is best for a given project depends on the structure type, the stage of construction at which waterproofing is applied, the ground conditions and any secondary performance requirements such as gas resistance.

Why Choose Construction Supplies UK for Waterproofing Membranes

Structural waterproofing membranes for below-ground applications: From pre-applied systems for new-build basement waterproofing to post-applied self-adhesive membranes for retaining walls and substructures, we stock a range covering the main product types used in Type A barrier waterproofing systems.

Trusted industry brands: Shop leading manufacturers including Fosroc, GCP, RIW, Sika and Visqueen.

Competitive trade prices: Waterproofing membranes at competitive trade prices with no minimum order value.

Fast UK-wide delivery: Reliable nationwide delivery with free shipping on orders over £300.

Expert support when you need it: Our team can help you choose the right waterproofing membrane for your project. Contact us at sales@constructionsuppliesuk.co.uk or call 03300 55 32 02.