The Hidden Risks of Placing Waterproofing Design in the Structural Engineer’s Scope

Direct answer

Placing waterproofing design within the structural engineer’s scope is one of the most common and most consequential procurement errors on commercial basement projects. Structural engineers are expert in the structural behaviour of concrete and can legitimately design Type B (structurally integral) waterproofing systems. What they are not typically trained to do is produce the full waterproofing strategy, system selection across all types, drainage coordination, grading assessments, interface detailing, and contractor accountability, that an independent waterproofing specialist provides. When these two scopes are conflated, the result is a design gap that is invisible on the programme and expensive to discover on site.

Full explanation

The decision to include waterproofing in the structural engineer’s scope usually happens for straightforward reasons: the structural engineer is already on the team, their scope already includes the substructure, and it seems efficient to give them the waterproofing design as well. The logic is understandable. The result is frequently a project with a design gap at its centre that nobody has noticed.

What structural engineers legitimately design

Structural engineers design reinforced concrete substructures. This includes the specification of concrete mix design, waterproofing admixtures, reinforcement cover, joint design, and construction methodology for Type B (structurally integral) waterproofing systems. This is real waterproofing work, and structural engineers with experience in basement construction are often genuinely competent at it.

The problem arises when the structural engineer’s Type B design is treated as a complete waterproofing strategy. On most commercial basement projects, Type B is one layer in a multi-system approach. The question of whether Type B is sufficient, whether Type A external membrane is required, whether Type C drainage is needed, and how all three interact at junctions and penetrations, these are questions that require specialist waterproofing knowledge that most structural engineers do not have.

What falls through the gap

When waterproofing is placed in the structural engineer’s scope, the following elements are typically either absent or inadequate. The BS 8102:2022 risk assessment, the formal assessment of water risk, consequences of failure, and required grade of protection for each basement zone, may be addressed superficially or not at all. The grading schedule, which specifies the required moisture condition in each basement space based on its intended use, is often missing. The drainage strategy, particularly where Type C cavity drain membranes are used, requires specialist knowledge of sump design, pump specification, and maintenance access that falls outside structural engineering competence. The interface between the waterproofing and the building envelope above ground level, a critical zone for water ingress, is rarely addressed by the structural engineer because it falls outside their scope boundary.

Expert witness evidence

CLW has reviewed waterproofing failures as expert witness where the root cause was a design error stemming from specialist knowledge that the structural engineer did not have and was not expected to have. In one category of case, the structural engineer correctly designed the concrete structure to Type B standard, but the project required a Type A external membrane in addition to the concrete, due to the hydrostatic conditions. The external membrane was not specified because nobody with the relevant expertise reviewed the system selection decision. The Type B structure alone was not adequate for the groundwater conditions. The failure was not the structural engineer’s fault, it was the procurement structure’s fault for placing a responsibility in a scope it did not belong to.

In another pattern, the structural engineer specified the waterproofing system but did not address the basement grading requirements. Spaces that were designed as Grade 3 (dry) environments were specified with waterproofing appropriate for Grade 1 (basic utility). The client’s intended use required a higher grade of protection, but the mismatch was not identified until finishes were being installed and moisture was affecting the fit-out.

The NHBC and PCA position

NHBC guidance is explicit on this point: waterproofing design should be led by qualified waterproofing professionals. The PCA maintains a register of Waterproofing Design Specialists, individuals who have demonstrated competence in structural waterproofing design to a recognised standard. The purpose of this register is precisely to distinguish waterproofing design competence from the general construction competence of the structural engineer or architect.

BS 8102:2022 requires that a waterproofing specialist be appointed as part of the design team. This is not a recommendation, it is a requirement of the standard against which waterproofing designs are measured in disputes. A structural engineer producing a waterproofing strategy without specialist waterproofing qualifications is not complying with the standard’s intent, even if their structural design is entirely sound.

The correct procurement structure

The structural engineer should retain their Type B design responsibility. An independent waterproofing consultant should be appointed alongside them to produce the overall waterproofing strategy, covering system selection, grading, drainage, interfaces, and performance specification. The two professionals work together, the structural engineer coordinates their concrete specification with the waterproofing strategy, the waterproofing consultant coordinates the external systems with the structural form. Neither can do the other’s job, and neither should be asked to.

The additional cost of this structure is modest. The structural engineer’s fee is not reduced, their Type B scope remains. The waterproofing consultant’s fee covers the elements that the structural engineer was never equipped to deliver. The procurement saving from not appointing the waterproofing consultant is illusory: the risk does not disappear, it simply goes unmanaged.

Related: What Makes a Waterproofing Design Worse Than Useless, and How to Spot One.