How Much Overhang Should a Polycarbonate Roof Have? (Complete Guide)
When installing a polycarbonate roof for a pergola, lean-to or carport, one small detail can make a big difference:
👉 The sheet overhang
Getting the overhang right ensures water drains properly into your gutter, prevents leaks and gives your roof a clean, professional finish.
In this guide, we explain exactly how much overhang a polycarbonate roof should have, why it matters, and how to get it right.
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 Is Roof Overhang?
Roof overhang refers to how far the polycarbonate sheet extends beyond the front beam or fascia of your structure.
This overhang allows water to drip cleanly into the gutter instead of running back underneath the roof.
Recommended Overhang for Polycarbonate Roofing
For most installations:
👉 50mm to 70mm overhang is ideal
This applies to:
• Pergolas
• Lean-to roofs
• Verandas
• Carports
Why Overhang Is Important
1. Directs Water Into the Gutter
Without sufficient overhang:
• Water can run behind the gutter
• Drips may fall in the wrong place
• Splash-back can occur
2. Prevents Leaks
Incorrect overhang can cause:
• Water tracking back under the sheet
• Moisture entering the structure
• Long-term water damage
3. Improves Appearance
Correct overhang gives your roof:
• A neat finish
• Consistent alignment
• Professional look
What Happens If Overhang Is Too Small?
If overhang is less than 50mm:
• Water may miss the gutter
• Drips may fall onto beams or walls
• Increased risk of leaks
What Happens If Overhang Is Too Large?
If overhang exceeds 70–100mm:
• Sheet can become unsupported
• Increased flexing
• Risk of bending or sagging
👉 Key rule:
Overhang should always remain supported and controlled
How to Measure Overhang Correctly
Step 1 — Measure roof projection
From wall to front beam
Step 2 — Add overhang allowance
Add 50–70mm
Step 3 — Order sheet length
Final sheet = projection + overhang
Example
Roof projection: 2500mm
Overhang: 60mm
👉 Sheet length: 2560mm
Overhang and Drip Trim Positioning
The drip trim sits at the edge of the sheet and helps direct water.
Correct positioning:
• Sheet slightly overhangs beam
• Drip trim sits below sheet edge
• Water flows into gutter
Overhang for Different Structures
Pergolas
• Typical overhang: 50–70mm
• Smaller structures = easier control
Lean-To Roofs
• Same overhang (50–70mm)
• More important due to wall connection
Carports
• May use slightly larger overhang
• Ensure sheet remains supported
Does Sheet Thickness Affect Overhang?
Yes — slightly.
Thin sheets (6mm)
• More flexible
• Avoid large overhangs
Thicker sheets (10mm–16mm)
• More rigid
• Can handle standard overhang easily
Common Overhang Mistakes
❌ No overhang at all
Water runs behind gutter
❌ Too much overhang
Sheet flexes and bends
❌ Uneven overhang
Looks unprofessional and drains poorly
❌ Not accounting for overhang when ordering
Sheets end up too short
Using Cut-to-Size Sheets for Perfect Overhang
Cut-to-size sheets allow you to:
• Add exact overhang
• Avoid cutting errors
• Ensure perfect fit
Order here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
Overhang and Roof Pitch
Roof pitch works together with overhang.
Minimum pitch:
👉 5°
This ensures:
• Water flows correctly
• Overhang functions properly
How Overhang Affects Gutter Performance
Correct overhang ensures:
• Water drops directly into gutter
• No splash-back
• Reduced maintenance
Incorrect overhang can:
• Cause overflow
• Create wet patches
• Lead to staining
Accessories That Help With Overhang
A proper system includes:
• Drip trims
• 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
Getting the overhang right is a small but crucial part of installing a polycarbonate roof. A recommended overhang of 50mm to 70mm ensures proper drainage, prevents leaks and improves the overall appearance of your structure.
By measuring accurately and using cut-to-size sheets, you can achieve a clean, professional finish and ensure your roof performs exactly as it should.
