Why Do Polycarbonate Sheets Expand and Contract? (Complete Guide)
If you’ve ever installed or researched polycarbonate roofing, you’ve probably come across the term “thermal expansion.” Polycarbonate sheets expand in heat and contract in cold — and this movement is completely normal.
However, if expansion isn’t accounted for during installation, it can lead to problems such as bowing, slipping, cracking or leaks.
In this guide, we explain why polycarbonate sheets expand and contract, how much movement to expect, and how to install them correctly to avoid issues.
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 Expand and Contract?
Polycarbonate is a thermoplastic material.
This means:
👉 It reacts to temperature changes
When temperatures rise:
• The material expands
When temperatures drop:
• The material contracts
This is a natural physical property and happens in all plastic roofing materials.
How Much Do Polycarbonate Sheets Expand?
Polycarbonate can expand by:
👉 Approximately 3mm per metre
Example
A 3 metre sheet may expand:
👉 Up to 9mm in hot weather
This is why expansion gaps are essential.
Why Expansion Matters in Roofing
If expansion isn’t allowed for, it can cause:
• Bowing or warping
• Sheets slipping
• Cracking around fixings
• Noise (creaking or popping)
• Pressure on glazing bars
• Leaks over time
Signs Expansion Is Causing Problems
Look out for:
• Sheets pushing against glazing bars
• Cracks near fixings
• Visible bending
• Clicking or popping sounds in heat
• Sheets moving out of position
These are all signs that expansion hasn’t been properly managed.
How to Allow for Expansion (Correct Installation)
1. Use Glazing Bars (Essential)
Glazing bars are designed to:
• Hold sheets securely
• Allow movement
• Prevent stress
They are critical for managing expansion.
Browse accessories here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
2. Do NOT Screw Directly Through Sheets
Screwing directly into sheets:
• Restricts movement
• Causes stress points
• Leads to cracking
Correct Method
Use:
• Glazing bars
• Fixing buttons (if required)
3. Allow Expansion Gaps
Sheets should not be forced tightly into place.
Always allow:
👉 A small gap at edges
This gives the sheet room to expand.
4. Use Correct Sheet Sizes
Oversized sheets can:
• Push against glazing bars
• Bow under pressure
Undersized sheets can:
• Move excessively
Best Solution
Use cut-to-size sheets:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
5. Avoid Overtightening Fixings
Overtightening:
• Restricts movement
• Creates stress points
• Leads to cracks
Correct Approach
• Tighten securely but not excessively
• Allow slight flexibility
6. Install Sheets in the Right Direction
Channels must run:
👉 Top → bottom
This ensures:
• Proper drainage
• Even expansion
7. Ensure Proper Roof Pitch
Minimum pitch:
👉 5°
This helps:
• Water runoff
• Reduces pressure on sheets
• Supports structural performance
How Temperature Affects Polycarbonate in the UK
In the UK, temperatures can vary significantly:
• Cold winters
• Warm summers
• Sudden temperature changes
This constant fluctuation means:
👉 Expansion and contraction happens regularly
Why This Is Not a Fault
Many people assume movement means poor quality.
👉 It doesn’t
Polycarbonate is designed to:
• Expand safely
• Return to shape
• Withstand temperature cycles
Problems Caused by Ignoring Expansion
❌ Cracking
From restricted movement
❌ Warping
From pressure build-up
❌ Slipping
From forced expansion
❌ Noise
From stress release
❌ Leaks
From distorted seals
How to Fix Expansion Issues
If sheets are bowing or cracking:
• Check spacing
• Check fixings
• Replace damaged sheets
If sheets are too tight:
• Remove and reinstall
• Allow proper gaps
If accessories are failing:
• Replace glazing bars or fixings
Browse parts here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
When to Replace Polycarbonate Sheets
Replacement is needed if:
• Cracks have formed
• Sheets are permanently warped
• Movement has caused damage
Order replacements here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
Accessories That Help Manage Expansion
A proper system includes:
• Glazing bars
• Fixings
• End caps
• Sealing tapes
Delivery Information
Check delivery here:
https://polycarbonatex.co.uk/pages/delivery-policy
Conclusion
Polycarbonate sheets expand and contract due to temperature changes — this is completely normal and part of how the material works. However, if this movement isn’t allowed for during installation, it can lead to serious issues such as cracking, bowing and leaks.
By using glazing bars, allowing expansion gaps and installing sheets correctly, you can ensure your polycarbonate roof performs properly in all weather conditions.
