Why Is Water Inside My Polycarbonate Sheets? (Causes & Fix Guide)
If you’ve noticed water, condensation or dirt inside your polycarbonate sheets, you’re not alone. This is one of the most common issues with polycarbonate roofing, especially on pergolas, lean-to roofs and verandas.
While it might look like a serious problem, it’s usually caused by incorrect sealing or ventilation — and in most cases, it can be fixed.
In this guide, we explain why water gets inside polycarbonate sheets, how to fix it, and how to prevent it from happening in the future.
Helpful resources mentioned throughout this guide:
Cut-to-size polycarbonate sheets
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
Multiwall accessories
https://polycarbonatex.co.uk/collections/multiwall-accessories
Glazing bar end caps
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-end-cap
Delivery policy
https://polycarbonatex.co.uk/pages/delivery-policy
Why Do Polycarbonate Sheets Have Chambers Inside?
Multiwall polycarbonate sheets are designed with internal channels (chambers).
These chambers:
• Improve insulation
• Reduce heat transfer
• Keep the sheet lightweight
However, they must be:
👉 Properly sealed and ventilated
Otherwise, moisture can enter.
What Does Water Inside the Sheet Look Like?
You may notice:
• Fog or condensation inside the sheet
• Water droplets trapped in channels
• Green algae or dirt build-up
• Cloudy appearance
This usually means moisture has entered the sheet.
Is It Normal to Have Moisture Inside?
A small amount of condensation can occur naturally due to temperature changes.
However:
👉 Visible water or dirt inside the sheet is NOT normal
It usually indicates an installation issue.
Top Causes of Water Inside Polycarbonate Sheets
1. Missing or Incorrect Sealing Tape
This is the most common cause.
Each sheet requires:
• Aluminium tape on the top edge
• Breather tape on the bottom edge
❌ Common mistake:
• Using the wrong tape
• Not using tape at all
✅ Fix:
• Remove trims
• Clean edges
• Apply correct tapes
Browse tapes here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
2. Breather Tape Not Installed (or Blocked)
Breather tape allows:
• Moisture to escape
• Air to circulate
If it’s missing or blocked:
• Moisture becomes trapped
• Condensation builds up
✅ Fix:
• Install breather tape at bottom edge
• Ensure it is not blocked by dirt
3. Poorly Sealed Top Edge
The top edge (wall side) must be completely sealed.
If not:
• Rainwater can enter the sheet
• Moisture travels through channels
✅ Fix:
• Apply aluminium tape properly
• Ensure flashing is sealed
4. Missing End Closures
End closures protect sheet edges.
Without them:
• Water can enter directly
• Dirt and insects can get inside
✅ Fix:
• Install end closures (U-profiles)
• Ensure tight fit
5. Incorrect Sheet Orientation
Polycarbonate sheets must be installed with:
👉 Channels running vertically (top to bottom)
If installed horizontally:
• Water cannot drain
• Moisture becomes trapped
✅ Fix:
• Reinstall sheets correctly
6. Condensation from Temperature Changes
Warm air + cold sheet = condensation.
This is normal to a degree.
However, poor ventilation can make it worse.
✅ Fix:
• Ensure breather tape is installed
• Improve airflow
7. Damaged or Worn Tape
Over time, tape can degrade due to:
• UV exposure
• Weather
• Age
✅ Fix:
• Replace old tape
• Inspect regularly
8. Pressure Washing or Water Ingress
Using high-pressure washers can:
• Force water into the sheet
• Damage sealing tape
✅ Fix:
• Avoid pressure washing edges
• Use gentle cleaning methods
Can You Remove Water From Inside the Sheet?
In some cases, yes — but it depends on the severity.
Minor Moisture
• Remove end closure
• Allow sheet to dry
• Reapply tape
Severe Water or Dirt Build-Up
• Often requires replacement
• Cleaning inside chambers is very difficult
When Should You Replace the Sheets?
Replacement may be needed if:
• Water is heavily trapped
• Dirt or algae has formed
• Sheets appear permanently cloudy
• Performance is affected
Order replacements here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
How to Prevent Water Getting Inside
✔ Use correct tapes
Top = aluminium
Bottom = breather
✔ Install sheets correctly
Channels must run vertically
✔ Use end closures
Protect sheet edges
✔ Maintain the roof
Clean regularly
✔ Avoid pressure washing
Protect seals
Accessories Needed for Proper Sealing
A full system includes:
• Aluminium sealing tape
• Breather tape
• End closures
• Glazing bars
Browse here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
Delivery Information
Check delivery here:
https://polycarbonatex.co.uk/pages/delivery-policy
Conclusion
Water inside polycarbonate sheets is usually caused by incorrect sealing or ventilation. While small amounts of condensation can be normal, visible water or dirt inside the sheet indicates a problem that should be addressed.
By using the correct tapes, installing sheets properly and maintaining the roof, you can prevent moisture from entering and ensure your polycarbonate roofing system performs as intended.
