Difference Between Waterproof & Protective Coatings
| Bitesize CPD
Waterproofing or Protective Coating? Understanding the Difference
Introduction
Industrial metal roofs are often described as being “waterproofed” when they are refurbished, but this is rarely the correct technical description.
Although a protective roof coating prevents rainwater reaching the steel beneath it, that alone does not make it a waterproofing system.
Understanding the distinction helps surveyors and specifiers choose the correct refurbishment strategy and avoid unrealistic performance expectations.
Waterproofing Systems
Waterproofing systems are designed to act as the primary weatherproof envelope.
Typical examples include:
- Single ply membranes
- Built-up felt systems
- GRP systems
- Liquid waterproofing systems incorporating reinforcement fleece or glass fibre
These systems are commonly installed on flat roofs where the membrane itself provides the waterproof barrier.
Their purpose is simple:
Prevent water reaching the structure below.
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Protective Roof Coatings
Protective coatings perform a different role.
On profiled steel roofs, the steel sheets already provide the weatherproof envelope.
The coating exists to:
- protect exposed steel from corrosion
- extend service life
- reduce future maintenance
- improve appearance
- delay expensive roof replacement
In other words, the roof provides the waterproofing.
The coating protects the roof.
That distinction is fundamental to correct specification.
Why Not Waterproof Every Metal Roof?
At first glance, installing a fully reinforced waterproofing membrane over an ageing metal roof may seem attractive.
Unfortunately, profiled steel roofs present engineering challenges that flat roofs do not.
Continuous Movement
Steel roof sheets expand and contract every day.
Movement occurs:
- with temperature changes
- under wind loading
- from maintenance foot traffic
Protective roof coatings formulated for profiled metal roofs are designed to accommodate this movement.
Reinforced waterproofing laminates are generally much less tolerant of repeated cyclic movement, particularly where movement is concentrated.
The Problem With Mid-Lap Joints
The greatest area of movement is normally the horizontal sheet overlap.
These joints are intentionally able to move.
Applying rigid reinforcement across them can create stress concentrations, often leading to cracking or debonding over time.
Ultimately, a rigid reinforced mid-lap joint is likely to trap moisture, which is the main cause of underside corrosion.
If this defect is not detected or designed out from the beginning, the roof could be permanently damaged or rendered irrepairable.
This is one reason why many experienced surveyors prefer to leave mid-lap joints unsealed unless there is a specific technical reason for sealing them.
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Complex Roof Details
Even if the main roof could be encapsulated successfully, numerous vulnerable details remain.
These include:
- ageing GRP rooflights
- corroded fixings
- flashings
- pipe penetrations
- ridge ventilation
- gutters
A waterproofing membrane cannot compensate for deteriorated components elsewhere on the roof.
Unless these details are repaired or replaced correctly, water ingress may still occur.
Longevity Matters
Perhaps the most important consideration is long-term maintainability.
Protective roof coatings are relatively straightforward to inspect, repair and overcoat during future maintenance cycles.
Fully reinforced waterproofing systems are significantly more difficult to repair once movement-related defects develop.
For industrial buildings expected to remain in service for decades, ease of future maintenance should form part of the original specification—not simply initial weatherproofing performance.
Comparison Table
Protective Roof Coating |
Waterproofing System |
|---|---|
| Protects existing roof | Becomes the roof |
| Designed for corrosion protection | Designed for waterproofing |
| Flexible | Reinforced membrane |
| Easy to overcoat | More difficult to repair |
Key Learning Points
- Waterproofing systems and protective coatings perform different engineering functions.
- Industrial metal roofs already provide the primary waterproof barrier.
- Protective coatings extend the life of that roof rather than replacing its waterproofing function.
- Steel roof movement makes fully reinforced waterproofing systems difficult to use successfully over profiled metal roofs.
- Long-term durability and future maintenance should always be considered alongside initial watertightness.