Buildings move.
Sometimes the movement is obvious.
Most of the time, it is not.
Concrete shrinks.
Timber moves with moisture.
Metal expands and contracts with temperature.
Different materials move at different rates.
And all of that movement can eventually reach the waterproofing system.
A membrane may be flexible, but that does not mean it can accommodate unlimited movement.
This is especially important around:
- Existing cracks
- Construction joints
- Expansion and movement joints
- Changes between different substrates
- Parapets and upstands
- Penetrations
- Large uninterrupted roof areas
One of the common mistakes is simply covering a crack and assuming the waterproofing product will deal with it.
But first we need to understand why the crack exists.
Is it stable?
Is it still moving?
Is it shrinkage?
Thermal movement?
Structural movement?
Is it actually a designed movement joint?
Because different situations require different details.
Reinforcement may help distribute local stress in some waterproofing systems.
But a genuine movement joint normally requires a detail specifically designed to accommodate that movement — not simply another coat of waterproofing over the top.
Movement joints cannot be considered as waterproofing details in isolation. Their geometry should also account for drainage, insulation continuity and thermal performance. A detail that accommodates structural movement may still create long-term risk if it forms a water-trapping recess or interrupts the thermal layer. On cold-climate projects, details capable of retaining water or snow should also be assessed for freeze–thaw and icing risk.
This is why substrate assessment is so important before installation.
A crack in the finished waterproofing may appear to be a membrane failure.
But sometimes the waterproofing is simply showing us movement that started somewhere underneath.
Buildings move. Waterproofing has to move with them.