If your polycarbonate roofing sheets are bowing, bending or warping, it can be worrying — especially if your roof is newly installed. The good news is that in most cases, this issue is completely fixable and often caused by a few common installation or structural factors.

In this guide, we explain why polycarbonate sheets bow or warp, how to fix the problem, 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

What Does Bowing or Warping Look Like?

Bowing or warping typically appears as:

• Sheets curving upwards or downwards
• Uneven or wavy surfaces
• Movement between rafters
• Visible flexing in warm weather

This can affect both appearance and performance of your roof.

Is It Normal for Polycarbonate to Move?

Yes — to a certain extent.

Polycarbonate naturally:

👉 Expands and contracts with temperature changes

This is completely normal and must be allowed for during installation.

However:

👉 Excessive bowing or warping is usually a sign of an issue

Top Causes of Bowing or Warping

1. Rafter Spacing Too Wide

One of the most common causes is rafters being spaced too far apart.

If sheets are unsupported over large gaps, they can:

• Sag under their own weight
• Flex in heat
• Move during wind or rain

✅ Fix

• Reduce rafter spacing
• Add additional support
• Use thicker sheets

2. Sheet Thickness Too Thin

Thinner sheets (6mm) are more flexible and prone to movement.

If used on larger roofs, they may:

• Bow between rafters
• Vibrate in wind
• Warp over time

✅ Fix

Upgrade to thicker sheets:

• 10mm for most roofs
• 16mm for larger spans

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

3. No Expansion Gap Allowed

Polycarbonate expands in heat.

If sheets are installed too tightly:

• They have nowhere to expand
• Pressure builds up
• Sheets can bow or distort

✅ Fix

• Ensure proper spacing in glazing bars
• Avoid forcing sheets into tight gaps
• Allow movement within system

4. Incorrect Installation in Glazing Bars

If sheets are not seated correctly:

• They may sit unevenly
• Pressure points can form
• Warping can occur over time

✅ Fix

• Reinstall sheets correctly
• Ensure even positioning
• Check alignment with rafters

Browse accessories here:
https://polycarbonatex.co.uk/collections/multiwall-accessories

5. Excessive Heat Exposure

Darker sheets or poorly ventilated roofs can trap heat.

Heat causes expansion, which can lead to:

• Visible bowing
• Increased movement
• Stress on fixings

✅ Fix

• Improve ventilation
• Consider opal sheets for heat diffusion
• Ensure correct installation

6. Loose or Poor-Quality Glazing Bars

If glazing bars are loose or low quality:

• Sheets may move excessively
• Support is reduced
• Warping becomes more noticeable

✅ Fix

• Tighten glazing bars
• Replace worn components
• Use quality systems

End caps here:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-end-cap

7. Incorrect Roof Pitch

Flat roofs increase the risk of bowing.

Without proper slope:

• Water can pool
• Heat builds up
• Sheets experience more stress

✅ Fix

Ensure minimum pitch of:

👉 5° or greater

8. Poor Sheet Orientation

If sheets are installed incorrectly (UV side facing wrong direction):

• Material can degrade faster
• Warping may occur

✅ Fix

Ensure:

👉 UV-protected side faces outward

9. Water Inside the Sheet

If water enters the multiwall structure:

• It adds weight
• Causes uneven expansion
• Can lead to bending

✅ Fix

• Replace damaged tape
• Ensure breather tape is used
• Replace affected sheets if needed

When Should You Replace the Sheets?

Replacement may be needed if:

• Warping is severe
• Sheets are cracked
• Water is trapped inside
• Structural performance is affected

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

How to Prevent Warping in Future

✔ Use correct rafter spacing

Match sheet thickness

✔ Choose correct sheet thickness

Don’t under-spec

✔ Allow expansion

Never install too tightly

✔ Use proper accessories

Correct tapes and glazing bars

✔ Install with correct pitch

Minimum 5°

✔ Maintain the roof

Check regularly for issues

Accessories for Proper Installation

A complete system includes:

• Glazing bars
• Aluminium tape
• Breather tape
• End closures
• Fixings

Browse here:
https://polycarbonatex.co.uk/collections/multiwall-accessories

Delivery Information

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

Conclusion

Bowing or warping polycarbonate sheets are usually caused by structural or installation issues such as wide rafter spacing, thin sheets or lack of expansion allowance.

By identifying the cause and making the right adjustments, you can restore your roof and prevent future problems. With correct installation and materials, polycarbonate roofing should remain stable, strong and long-lasting.