Can You Mix Waterproofing Products from Different Manufacturers in the Same System?

Direct answer

BS 8102:2022 requires all components of a waterproofing system — membrane, drainage, primer, screed, fixings, and related elements — to be co-ordinated in detail and designed to work together to meet the specified performance target. This principle, described in the standard as the “integrated whole” requirement, has direct implications for product selection. Products from different manufacturers can be used within the same system, but compatibility between them must be demonstrated, and the specification must address what happens to warranty and design liability at the interfaces between them. In practice, switching from one manufacturer’s system to another mid-specification creates interfaces that neither manufacturer will warrant, and a warranty gap that falls to the installer or the designer.

Full explanation

The ‘integrated whole’ principle

BS 8102:2022 opens its design section with the principle that a waterproofing system should be treated as an integrated whole. Every component — the membrane, the drainage layer, the concrete substructure, the primer, the protective elements — must be co-ordinated and must together achieve the specified performance grade. No single element should be considered in isolation.

This principle was central to the judgment in Outwing Construction Ltd v Thomas Weatherald Ltd [TCC, 1999], where Recorder Colin Reese QC found that “the waterproofing system consisted of both the tanking membrane and the sub-soil drainage” — neither could be considered in isolation, and a design deficiency in one element could not be remedied by demanding perfect performance from the other. The same logic applies at product level: the system is the sum of its components, and an interface between them that is neither designed nor warranted is a gap in the system.

In terms of product selection, “integrated whole” means that the specification must address how products interact at their interfaces, not just how each performs in isolation. A primer by Manufacturer A followed by a membrane by Manufacturer B creates an interface that both parties must warrant — and most will not, unless specific compatibility has been tested and documented.

Why manufacturers will not typically warrant mixed-product combinations

Manufacturers’ product warranties typically exclude use of their products in combination with products from other manufacturers, unless specific compatibility has been established. This is commercially rational: a manufacturer cannot control the quality, formulation, or application of another’s product, and cannot warrant what happens at the interface between them.

The consequence in practice is that a specification which uses, for example, a primer from Manufacturer A, a waterproofing membrane from Manufacturer B, a drainage composite from Manufacturer C, and a protective screed from Manufacturer D is likely to be a system without a warranty covering the whole. Each manufacturer will warrant their own product under their own conditions. None will warrant the interfaces.

When a defect occurs at one of those interfaces — and interfaces are the most common location of defects — there is no warranty obligation that extends to the failure. The investigation required to establish which manufacturer’s product failed at which interface is expensive and often inconclusive. The contractual outcome is a dispute between the client, the installer, and potentially the designer over who bears responsibility for a gap that could have been avoided.

When mixing manufacturers is unavoidable

There are circumstances where single-manufacturer solutions are not available, or where the design requires combinations that no single manufacturer produces. In these cases, the mixing should be deliberate, documented, and technically justified:

Obtain written compatibility confirmation from both manufacturers before specifying. This may involve site-specific testing on a trial area, or reliance on the manufacturers’ documented compatibility matrices where these exist. The confirmation should be in writing and retained in the project file.

Name the products in the specification. Generic descriptions (“proprietary waterproofing membrane to BS 8102”) without named products allow contractors to select products that may not be compatible with the remainder of the specification. Named products and named combinations are the starting point for managing compatibility.

Allocate warranty liability contractually. If neither manufacturer will warrant the combination, the specialist contractor who installs it should be required to provide a system warranty that covers the installed combination — not just the individual products they supplied. This is a negotiated position, but it is the correct mechanism: the installer is the party who made the decision to assemble the specific combination, and they should stand behind it.

Document the mixing as a considered design decision. The waterproofing design report should record that the combination was evaluated, that compatibility was confirmed, and the basis for that confirmation.

The mid-project substitution problem

A specific and common risk is mid-project product substitution: the contractor installs a different product from that specified without formal approval. This is most likely when:

• The specification uses generic descriptions rather than named products.
• There is no construction monitoring at critical points in the installation to verify compliance.
• There is commercial pressure on the contractor to substitute a cheaper or more readily available product.

Mid-project substitution that switches from one manufacturer’s system to another at a point partway through the installation creates an interface that was not designed and is not warranted by either manufacturer. This scenario occurs more commonly than many design teams expect, and it is one of the reasons independent construction monitoring at critical hold points is an essential element of a rigorous waterproofing specification.

The safeguard is a named specification with a defined approval process for substitutions, combined with independent monitoring that verifies the installed products match the specification before they are concealed.

What the specification should say

A waterproofing specification that addresses this issue should:

• Name the products to be used within each element of the waterproofing system.
• Require all products within each element to be from the same manufacturer unless explicitly agreed otherwise in writing.
• Require the contractor to provide written confirmation from each manufacturer that the specified combination is compatible and warranted.
• Require the installer to provide a system warranty covering the installed combination as a whole.
• State the approval process for any proposed product substitution — including who must approve, what information must be submitted, and that the approval must be received in writing before substitution occurs.
• Include the checking of product compatibility and correct installation as a hold point in the construction monitoring programme.