If your polycarbonate roof is leaking, dripping, or showing signs of moisture, you’re not alone. Even though polycarbonate roofing systems are designed to be completely watertight, leaks can still happen — but the root cause is almost always installation issues, worn accessories, or incorrect tapes, not the polycarbonate sheets themselves.

This complete guide will help you identify exactly why your roof is leaking, where the water is entering, and how to fix the problem permanently using the right accessories.

Why Polycarbonate Roofs Leak

Polycarbonate panels themselves do not leak. The leak comes from the system around the panels:

• Tapes

• Glazing bars

• Trims

• Flashing

• Screws and fixings

• Incorrect pitch

• End closures

Once you identify the source, the fix is usually very simple and low cost.

1. Incorrect Tape on the Top or Bottom of the Sheet

This is the number one cause of moisture, fogging and leaks.

Top of the sheet (high end):

Must use aluminium (solid) tape to block rain and wind.

Bottom of the sheet (gutter end):

Must use breather tape to allow moisture out.

If these are reversed or missing, you will experience:

• Water in chambers

• Fogging

• Drips at the gutter edge

• Internal condensation

Fix:

1. Remove trims/end closures

2. Remove incorrect tape

3. Clean the surface

4. Apply correct tape

5. Re-fit drip trim/end closure

Replacement tapes:
https://polycarbonatex.co.uk/collections/multiwall-accessories

2. Missing or Damaged Glazing Bar End Caps

Water often enters at the top of glazing bars if the end cap is missing.

End caps prevent:

• Water running into the glazing bar

• Wind-driven rain entering

• Sheet movement causing gaps

Fix:

Replace glazing bar end caps with the correct type:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-end-cap

This alone fixes most top-edge leaks.

3. Glazing Bars Not Sealed or Clipped Down Properly

If you see water between sheets, the glazing bar is usually the issue.

Common causes:

• Bar not fully snapped down

• Bar warped from heat

• Gasket cracked or shrunk

• Screws overtightened

• Cheap PVC bar twisting

Fix:

• Check the top cap is fully clipped

• Replace worn gaskets

• Ensure screws are not over-tightened

• Replace warped or old glazing bars

• Add new end caps

All replacement components:
https://polycarbonatex.co.uk/collections/multiwall-accessories

4. Poorly Installed Flashing Against a Wall

Lean-to roofs often leak at the wall junction.

Causes:

• Flashing is missing

• Flashing is sitting on the sheet

• Flashing is sealed incorrectly

• Sealant failure over time

• Sheet expansion pushing flashing

Fix:

1. Fit new flashing above the sheet

2. Use low-modulus roofing sealant (not silicone)

3. Ensure a 20–40mm overlap over the roof

4. Never clamp flashing directly onto polycarbonate

This fixes most lean-to leaks.

5. Leaks Around Fixings

If water comes through screw holes, the issue is always the fixing method.

Causes:

• No rubber washer

• Over-tightened fixings

• Holes drilled too tight

• Screws installed at an angle

• Incorrect type of screw used

Fix:

1. Replace screws with rubber-washer roofing fixings

2. Pre-drill holes slightly oversize for expansion

3. Install screws straight, not angled

4. Use LM roofing sealant only if needed

If the sheet has cracked, replace it using our cut-to-size service:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

6. Water Running Behind the Drip Trim

This often appears as a leak but is simply incorrect drainage.

Causes:

• Drip trim fitted too high

• No drip trim installed

• Sheet overhang too short

• Gutter misaligned

Fix:

• Lower or reinstall drip trim

• Increase sheet overhang to 50–70mm

• Ensure gutter sits below drip trim

• Replace worn or loose trims

Correct trims available here:
https://polycarbonatex.co.uk/collections/multiwall-accessories

7. Roof Pitch Too Shallow

A polycarbonate roof should have a minimum 5° pitch (around 10cm fall per metre).

If pitch is too low:

• Water travels sideways

• Wind pushes water under joints

• Water sits on glazing bars

• Condensation increases

A flat or low-pitch roof is far more likely to leak.

Fix:

• Increase pitch if possible

• Use wider glazing bars

• Seal all trims carefully

• Ensure flashing is deeper

If increasing the pitch is impossible, upgrading glazing bars and tapes is essential.

8. Condensation Misdiagnosed as a Leak

This is extremely common.

Condensation appears when:

• Wrong tape is used

• Breather tape is blocked

• Cold air enters chambers

• Gutter end isn’t ventilating

It can look like a leak because water appears inside the structure.

Fix:

• Replace top tape with aluminium

• Replace bottom tape with breather tape

• Add drip trim for drainage

• Improve airflow

Replacement tapes and trims:
https://polycarbonatex.co.uk/collections/multiwall-accessories

9. Water Getting into Multiwall Chambers

If water enters the internal chambers of a sheet, the sheet becomes cloudy and stained.

Causes:

• Wrong tape

• Damaged tape

• Sheet edge cut incorrectly

• No end closure protecting tape

Fix:

Once water enters the chambers, the only solution is replacement.
Order an exact-size replacement sheet here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

10. Structural Movement or Incorrect Fixing Spacing

If the frame moves or rafter spacing is incorrect, glazing bars can lift slightly — causing leaks.

Fix:

• Ensure rafters are spaced correctly for sheet width

• Ensure glazing bars run centre of rafters

• Add additional fixings where required

• Replace warped timber or aluminium support

Correct accessories:
https://polycarbonatex.co.uk/collections/multiwall-accessories

How to Diagnose the Leak Location

To find the source quickly, use this simple method:

1. Follow the water backwards

Water rarely leaks exactly where it drips. Trace the path upwards.

2. Check the top edge first

Most leaks begin at the top of the roof.

3. Inspect glazing bars next

Look for gaps, lifted caps or missing end caps.

4. Check the wall flashing

Any crack or gap above the roof will cause leaks.

5. Check bottom-end tapes and trims

Incorrect breather tape causes condensation and drips.

With these steps, you can diagnose 99% of leak issues.

When to Replace the Sheets

Replacement is recommended if:

• Water has entered multiwall chambers

• Sheets are scratched or UV-faded

• Sheets are bowed due to heat or overtightening

• Fixings have cracked the sheet

• The wrong thickness was used originally

Order exact-size replacements here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

Where to Get Verified Replacement Accessories

For guaranteed compatibility, use accessories specifically designed for multiwall polycarbonate roofing:

Accessories (tapes, trims, fixings)

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

Cut-to-Size Sheets

https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

Delivery Policy

https://polycarbonatex.co.uk/pages/delivery-policy

Conclusion

Polycarbonate roof leaks can almost always be fixed without replacing the entire roof — you simply need to locate the fault in the system. With correct tapes, glazing bars, drip trims, flashing and fixings, your roof will stay watertight for decades.

Fix the leak at the source and your polycarbonate roof can easily last 20+ years without further issues.