The 7 Sins of Waterproofing (And How to Avoid Them)

The 7 Sins of Waterproofing (And How to Avoid Them)

Waterproofing is one of the most critical stages of any new construction or remediation project. When done correctly, it’s an invisible guardian, protecting a structure’s integrity from the relentless threat of moisture. When done poorly, it can lead to catastrophic failures, from crumbling foundations and rampant mould growth to costly, disruptive repairs.

Steering clear of common pitfalls is key to a successful outcome. This guide breaks down the seven most common (and costly) sins of waterproofing and provides clear, actionable advice on how to get the job right the first time. By avoiding these mistakes, you can ensure a durable, leak-proof finish that stands the test of time.

[A seamless application of TREMproof 250GC]

Sin #1: Terrible Surface Preparation

Just as you wouldn’t paint on a dirty, dusty wall, you can't apply a waterproofing membrane to an unprepared surface and expect it to work. This is arguably the most common and damaging sin. Contaminants like dust, grease, loose debris, or previous coatings will prevent the new membrane from bonding correctly.

How to Avoid It:
Surface preparation is non-negotiable. Your substrate must be clean, sound, and dry.

• Clean: Substrate shall be properly cleaned so that the surface to receive the coating, sealant or liquid-applied flashing is free of mould, paint, sealers, coatings, curing agents, loose particles, and other contamination or foreign matter that may interfere with the adhesion.

• Repair: Fill all cracks, holes, and voids with a suitable, and compatible product. The surface should be smooth and free of sharp edges that could puncture the membrane.
• Prime: Most waterproofing systems require a primer. This crucial step seals the porous surface, mitigates against pinholes in the membrane, and dramatically improves adhesion. Always use the primer recommended by the membrane manufacturer.

Sin #2: Choosing the Wrong Material for the Job

The world of waterproofing is vast. There are sheet membranes, liquid-applied systems, and cementitious coatings, just to name a few. Using a product designed for a below-grade foundation on an exposed rooftop balcony is a recipe for disaster. Each material has specific properties - like UV stability, flexibility, and chemical resistance, that make it suitable for certain applications and completely wrong for others.

How to Avoid It:
Do your homework and match the product to the project.

• Assess the Area: Is the area exposed to sunlight? Will it experience foot traffic? Is it subject to building movement (expansion and contraction)? Is it a positive-side (exterior) or negative-side (interior) application?

• Read the Data Sheet: Every reputable waterproofing product comes with a technical data sheet (TDS). This document is your best friend. It details what the product is for, its limitations, and how to apply it.
• When in Doubt, Ask: Don't guess. Consult with the manufacturer or a technical expert at your supplier. Explaining your specific project will help them recommend the perfect system for your needs.

[Areas exposed to sunlight, such as a rooftop, require a UV-resistant product that helps extend the useful life of the structure]

Sin #3: Ignoring the Details

Water will always find the path of least resistance. In waterproofing, this path is almost always a poorly handled detail. Corners, joints, seams, and penetrations (like pipes or drains) are the most vulnerable points in any system. Simply painting a thick coat over these areas isn't enough; they require special attention and detailing.

How to Avoid It:
Treat every detail with precision.

• Reach Out To An Expert: Fully developed and detailed waterproofing design by a specialist.
• Properly Seal Penetrations: Use appropriate flange details, bond breakers, or sealant around all pipes, drains, and posts. The goal is to create a seamless transition between the penetration and the membrane.
• Mind the Joints: Construction joints and expansion joints are there for a reason. Don't simply coat over them. Use a system and detail specifically designed for jointing that will accommodate movement while maintaining a waterproof seal.

Sin #4: Applying Too Little (or Too Much) Product

More is not always better, and less is definitely not enough. Waterproofing membranes are engineered to perform at a specific thickness, measured in Wet Film Thickness (WFT) and Dry Film Thickness (DFT). Applying too little product creates a weak, thin barrier that can easily fail under pressure or wear. Conversely, applying too much in a single coat can lead to improper curing, cracking, and a waste of material.

How to Avoid It:
Measure and follow the manufacturer's or supplier’s specifications.

• Calculate Coverage: The product's data sheet will specify the required coverage rate (e.g., 1 litre per square metre). Calculate the total area you need to cover and ensure you have enough product.

• Use a Thickness Gauge: A wet film thickness gauge is an inexpensive and essential tool. It allows you to check your application depth as you work to ensure you're hitting the target.
• Always follow the manufacturer/supplier’s instructions.

[It is always advised to read a product's data sheet before use]

Sin #5: Disregarding Cure Times

Patience is a virtue, especially in waterproofing. Once the membrane is applied, it needs time to cure - the chemical process where the polymer chains come together and the product achieves its physical properties. Rushing this stage is a critical error. Exposing a membrane to water, foot traffic, or covering it with tiles or soil before it has fully cured can cause irreversible damage.

How to Avoid It:
Respect the clock and the weather.

• Read the Specs: The TDS will state the required cure times, including time between coats, time until rain-resistant, and time until full cure (ready for traffic or tiling).

• Check the Weather: Temperature and humidity drastically affect curing. A cool, damp day can extend cure times significantly. Avoid applying products if rain is forecast or if temperatures are outside the recommended application range.
• Protect the Area: Cordon off the newly waterproofed area to prevent accidental foot traffic or damage from other trades on a job site.

Sin #6: Skipping Comprehensive Quality Assurance

You've prepped the surface, primed and detailed, and applied the membrane perfectly. But how can you be 100% certain it’s watertight before it's covered by tiles, soil, or other finishes? Simply hoping for the best or relying on an old-school flood test is a significant gamble. Modern construction demands robust Quality Assurance (QA). Skipping this final verification is a critical sin that can negate all your hard work, leading to hidden leaks that only reveal themselves after costly finishes are installed.

How to Avoid It:
Implement a multi-faceted QA strategy to verify the integrity of the membrane. While flood testing has its place, the industry is increasingly adopting more advanced, less intrusive methods. A proper QA plan should include:

• Thorough Visual Inspection: This is your first line of defence. Before any electronic testing, conduct a meticulous visual check of the entire surface. Look for any inconsistencies: pinholes, blisters, insufficient coverage, or gaps at seams and penetrations. This simple step can catch many application errors that can be fixed immediately.

• Electronic Leak Detection (ELD): This non-destructive testing method is a game-changer for QA. High-voltage ELD (or "spark testing") is used for dry membranes, where a technician sweeps a probe over the surface. Any pinhole or breach completes an electrical circuit, creating a spark and an audible alarm, pinpointing the exact location of the defect. It’s fast, incredibly accurate, and avoids introducing water into the building envelope.
• Electric Vector Field Mapping (EVFM): Also known as low-voltage testing, EVFM is ideal for horizontal surfaces and can even be performed after overburden (like soil or pavers) is installed. A low-voltage current is applied across the wet membrane surface. Because the membrane is an insulator, the current will only flow through breaches. A technician uses probes to detect these electrical paths, mapping out the precise location of any leaks without the need for flooding.
• Flood Testing (When Appropriate): While newer methods offer more precision, a traditional flood test can still be a valid part of QA, especially for shower pans or small contained balconies. If specified, ensure the membrane is fully cured (check the TDS), block all drains, and fill the area with water to a minimum depth of 50mm. Let it stand for at least 24 hours, then inspect the area and the ceilings/spaces below for any sign of moisture.

[Waterproofing gone wrong – a damaged ceiling with water stains and cracks]

Sin #7: Neglecting Moisture Management

A waterproofing membrane is your final line of defence, but it should never be the only one. A common sin is to rely solely on the membrane to hold back water, ignoring the principles of moisture management. This approach fails to address the root cause: water pooling against the structure. Without effective drainage, you create constant hydrostatic pressure and ponding water, which will relentlessly search for any weakness and dramatically shorten the lifespan of even the best-applied membrane. 

How to Avoid It:
Design a complete moisture management system that addresses both surface and subsurface water. The goal is to direct water away from the building, not just block it.

• Design for Falls on Both Substrate and Surface Levels: Proper drainage starts at the structural level. The substrate itself (the surface that is membraned) should be sloped to drains. Additionally, the finished floor level (FFL) must also be sloped, creating the primary path for surface water to flow away.
• Ensure Drains are Effective: Drains are the exit point of your entire system. Ensure they are positioned at the lowest point of both the substrate and the FFL. Use two-stage drains (weep-hole drains) in tiled applications, which allow water to drain from both the tile surface and the screed bed beneath, preventing water-logging. Always keep drains clear and protected from blockages. 

Waterproofing is a precise science, not a general maintenance task. By understanding and avoiding these seven sins, you move from guesswork to professional execution. Meticulous preparation, correct product selection, and a patient, detail-oriented approach are the cornerstones of a successful project. Taking the time to do it right ensures your work will protect the asset for years to come, saving you, and your clients, from the costly consequences of water damage.

Ready to ensure your next project is watertight and worry-free? Contact our Specification Team or Technical Representatives today for expert advice and tailored waterproofing solutions stand the test of time!