Why Surface Preparation Matters in Roof Coating Projects
| Bitesize CPD
Introduction
Surface preparation is one of the most important stages of any industrial roof coating project.
It is often treated as a straightforward preliminary operation: clean the roof, allow it to dry and apply the coating. In practice, cleaning frequently reveals defects and substrate conditions that could not be fully assessed during the original survey.
The extent of preparation can therefore affect:
- the suitability of the roof for coating
- the repair work required
- the project programme
- the final cost
- the long-term performance of the coating system
This applies whether the work involves a complete roof coating, cut edge corrosion treatment or a liquid-applied gutter lining.
Existing Finish
Cleaning may reveal loose plastisol, previous coatings or repairs that must be completely removed.
Weather Conditions
Rain, dew and contamination can require prepared surfaces to be cleaned and dried again.
Hidden Defects
Corrosion removal may expose pitting, perforations or sections requiring localised replacement.
Cost and Programme
Additional stripping, treatment, drying and repair work can materially alter the original scope.
Cleaning Is Also an Investigation
Cleaning does more than remove dirt and surface contamination.
High-pressure washing may expose poorly adhered plastisol, previous coatings or repair materials that appeared sound during the initial inspection. If the existing finish begins to detach during cleaning, it cannot simply be left beneath the new coating.
All loose and unsound material must be removed until a stable surface is reached.
The true preparation requirement may therefore only become apparent after work has started. A roof that initially appeared to require routine cleaning may need substantial stripping before it is suitable for coating.
When the Original Finish Comes Away
Where the original factory-applied finish is removed, the galvanised steel beneath it may become exposed.
Depending on the specified coating system, the exposed surface may require additional preparation, including:
- removal of loose material and residues
- treatment with a mordant or T-wash solution
- thorough rinsing with clean water
- sufficient time to dry
- application of the appropriate primer
These are separate operations, each requiring labour and suitable weather conditions.
What initially appeared to be a cleaning operation can consequently become a much more extensive preparation process.
Preparation Is Time-Sensitive
A clean surface does not necessarily remain suitable for coating indefinitely.
If several rainy days occur between cleaning and coating, areas of the roof may become contaminated again by airborne dirt, standing water, debris or foot traffic.
Morning dew and condensation can also delay application.
Previously cleaned areas may therefore need to be inspected, re-cleaned and allowed to dry before coating can begin.
This is particularly relevant where cleaning and coating are programmed as separate operations or undertaken by different teams.
Weather delays do not only affect application days. They can also cause completed preparation work to be repeated.
Delays are not inevitable, but should be considered at the point where a project is tendered to contractors.
The specification and contract documentation should therefore establish how weather-related re-cleaning, drying and associated delays will be treated.
Preparation May Reveal Hidden Defects
Corrosion can conceal more serious deterioration.
At sheet ends and cut edges, cleaning back the corrosion may reveal pitting, thinning or complete perforation of the steel. The appropriate repair will depend on the extent and location of the damage.
Possible treatments may include:
- a reinforced tape repair
- a localised splice repair
- replacement of an isolated roof sheet
- more extensive sheet replacement where deterioration is widespread
The purpose of preparation is not to hide these defects beneath a coating. It is to establish whether a sound and durable substrate remains.
Similar issues can arise when preparing gutters. Removal of debris, corrosion products and previous repairs may reveal perforations, failed joints or sections that require repair before a lining system can be installed.
Implications for the Specifier
The extent of preparation that might be required cannot always be established with certainty before cleaning begins.
A specification should ideally make clear:
- the required standard of surface preparation
- how loose or poorly adhered existing finishes are to be treated
- what happens if the galvanised substrate becomes exposed
- how additional repairs will be assessed and authorised
- whether provisional allowances are required
- how weather-related re-cleaning will be managed
- who carries the commercial risk of unforeseen additional preparation
A simple instruction to “clean and coat” may be inadequate where the condition of the underlying roof is uncertain.
Realistic allowances for preparation help reduce disputes between the client, contractor and coating manufacturer once work is underway.
Preparation Affects More Than Adhesion
Poor preparation is commonly associated with coating adhesion failure, but its consequences are wider.
It may also result in:
- corrosion continuing beneath the new system
- premature breakdown at sheet edges and details
- defective material being concealed rather than repaired
- unexpected programme delays
- additional costs arising after commencement
- reduced service life of the completed refurbishment
A coating system can only perform as intended when it is applied to a suitably prepared and structurally adequate substrate.
Key Learning Points
- Cleaning is also an investigative stage of the project.
- The full preparation requirement may not be known until cleaning begins.
- Loose or poorly adhered existing finishes must be removed.
- Exposed galvanised steel may require additional treatment before coating.
- Weather can cause previously prepared surfaces to require re-cleaning.
- Cut edge corrosion may conceal perforated or severely thinned steel.
- Surface preparation should be considered when establishing both cost and programme.
- A coating should not be used to conceal defects that require repair.