Why basement waterproofing design must be driven by clear drawings

Over many years, one of the recurring frustrations we see is how often clients are left with waterproofing “designs” that offer little more than broad narrative, lacking the detail a contractor needs in the trench. That gap between design intent and site execution is where risk, shortcuts and failures lurk.

Here’s what too many waterproofing reports get wrong, and how the industry standards expect better.

The problem? Words without usable drawings

Designers often bury generic descriptions, product data sheets, or typical details in 50 pages of narrative, leaving installers to hunt for critical information and frequently find it missing.

As a result, on site people must “fill in the blanks” by guesswork. How do membranes interface at service penetrations, around slab edges, beneath raised slabs, in internal box-outs, or at expansion joints? Which layer laps over which? What tolerances/movements are permitted?

That ambiguity leads to shortcuts, improvisation, and, ultimately, failure.

In contrast, a proper waterproofing design isn’t just a report. It must be followed by a project-specific package of drawings including but not limited to layout plans, cross-sections, and detail interfaces showing exactly how the waterproofing system, its ancillaries, and structure interrelate.

What the standards demand (and what’s being ignored)

BS 8102:2022

The revised standard reinforces that a waterproofing specialist should be part of the design team from the early stages. The scope is expanded: ingress from sources beyond groundwater (e.g. flood, surface water, through openings) must be considered.

It continues to define three protection types Type A (barrier), Type B (structurally integral), Type C (drained) and allows combinations thereof, subject to compatibility, to meet the project’s environmental grade.

The standard explicitly recognises that defects are inevitable. Repairability and maintainability must be built into the system from the outset. Importantly, BS 8102:2022 places greater emphasis on the construction phase and expects designers and contractors to maintain communication as changes occur.

Yet I often see “BS 8102-compliant” design reports that lack the very drawings and interface clarity that the standard presumes will exist.

NHBC Chapter 5.4 – “Waterproofing of basements and other below ground structures”

Chapter 5.4 is NHBC’s go-to when assessing waterproofing in warranty projects.

It sets out that waterproofing design must be rigorous and capable of preventing ingress from ground water and other sources, for all elements (walls, slabs, foundations).  The chapter emphasises provision of information. Design documents, drawings and specifications must be clear, coordinated, and communicated to all relevant parties (site teams, sub-contractors, suppliers).

NHBC also expects continuity of waterproofing across junctions (e.g. between DPC, DPM, tanking) and that details of penetrations, insulation, junctions, trench fill, service entries etc. are included in the design.

I’ve lost count of the times a client asked, “where in the design does it show the detail of the seal at the pipe penetration, or how the slab membrane laps to the wall membrane?” and the answer is, nowhere. That’s a major red flag under NHBC scrutiny.

PCA / Industry Guidance

The Property Care Association’s guidance and codes (especially for existing structures) call for full system design, specification of interfaces and risk assessment.

Best-practice publications (e.g. on Type C systems) emphasise that continuity, detailing and coordination of ancillary products are as critical as the membrane itself.

What good looks like (and what we at EPG demand)

When we accept a waterproofing design commission or take over a responsibility, our minimum requirements are:

• Full design drawings
• Plan showing waterproofing extents and transitions
• Sections and details showing all interfaces (membrane to structural elements, membranes to penetrations, to insulation, junctions and terminations either at DPC level or between alternate graded areas)
• Layouts showing drainage routing, sump, pumps, falls, access points
• If a dual or hybrid system is used, the sequencing and mechanical compatibility must be explicit
• Clear hierarchy of laps, overlaps, tolerances, movement joints

Maintainability / repair strategy

Where water might reach the void, how it will be collected and removed (particularly in Type C systems)

• How repairs or diagnostic measures would be carried out mid-life
• Coordination package
• Interfaces with structural, services, drainage, insulation designers
• Site reference levels, levels of fittings, site tolerances
• Design responsibility and warranty
• A named designer with accountability
• A design warranty (or retrospective assumption when called in)

If you’re a client or project lead, ask your waterproofing ‘designer’ if their deliverables include the above. If their answer is “we’ll leave that to the contractor,” you’re walking into risk.

Why these matter

Without proper drawings, contractors are forced to guess or improvise. That’s where weak spots go unnoticed until water finds them.

Defects in waterproofing are expensive, disruptive, and often hidden until long after handover.

Clients often assume the original designer holds responsibility; but too often they discover there is no one accountable, and then we (EPG) are asked to salvage design and liability post installation.

The industry must raise the bar. We should call out, not people but deficient practices. We need to flush out those who advertise themselves as “experts” or “specialists” but don’t deliver real, installable design.

If you’re in the waterproofing supply, contracting, or client side, demand clarity, demand drawings, demand accountability. Let’s quit tolerating “report-only” designs that leave the site to fend for itself.

Find out more about our structural waterproofing service HERE