Rafter Spacing Guide for Polycarbonate Roofing (Complete UK Guide)
Rafter spacing is one of the most important factors when installing polycarbonate roofing sheets. Get it right, and your roof will be strong, stable and long-lasting. Get it wrong, and you can experience sagging sheets, noise, leaks and long-term damage.
Whether you’re building a pergola, lean-to roof, veranda or carport, understanding the correct rafter spacing is essential.
In this guide, we explain the ideal rafter spacing for polycarbonate roofing, how it affects sheet selection, and how to ensure your structure performs properly.
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 Rafter Spacing?
Rafter spacing refers to the distance between the centres of each rafter supporting your roof.
Polycarbonate sheets are installed between glazing bars, which are fixed directly on top of these rafters.
👉 This means:
Rafter spacing = sheet width
Why Rafter Spacing Is So Important
Correct rafter spacing ensures:
• Sheets are properly supported
• Reduced flexing and movement
• Lower noise during rain
• Better resistance to wind
• Longer lifespan
Incorrect spacing can cause:
• Sagging sheets
• Cracking
• Leaks
• Poor installation fit
Standard Rafter Spacing for Polycarbonate Roofing
Typical rafter spacing depends on sheet width.
Common setups include:
• 700mm centres
• 900mm centres
• 1000mm centres
• 1050mm centres
These align with standard polycarbonate sheet widths.
Matching Rafter Spacing to Sheet Width
Polycarbonate sheets sit between glazing bars.
Example:
Rafter centres: 1000mm
Sheet width: ~980mm
The slight difference allows:
• Expansion gap
• Easy installation
• Proper fit within glazing bars
Recommended Spacing by Sheet Thickness
6mm Polycarbonate Sheets
👉 Recommended spacing:
600mm – 700mm centres
Used for:
• Small pergolas
• Lightweight structures
10mm Polycarbonate Sheets
👉 Recommended spacing:
700mm – 1000mm centres
Used for:
• Pergolas
• Lean-to roofs
• Medium structures
16mm Polycarbonate Sheets
👉 Recommended spacing:
900mm – 1050mm centres
Used for:
• Larger roofs
• Verandas
• Carports
25mm Polycarbonate Sheets
👉 Recommended spacing:
1000mm – 1250mm centres
Used for:
• Conservatories
• High-end structures
What Happens If Rafters Are Too Far Apart?
Spacing rafters too wide can cause:
• Sheet sagging
• Increased movement
• Loud noise during rain
• Stress on glazing bars
• Potential cracking
This is one of the most common installation mistakes.
What Happens If Rafters Are Too Close Together?
Spacing rafters too closely can:
• Increase material cost
• Add unnecessary structure
• Limit sheet width options
While not dangerous, it’s inefficient.
How to Measure Rafter Spacing Correctly
Measure from the centre of one rafter to the centre of the next.
This ensures:
• Accurate sheet sizing
• Correct glazing bar placement
Rafter Spacing for Different Structures
Pergolas
• Typically smaller structures
• Spacing: 700mm – 900mm
• Often use 10mm sheets
Lean-To Roofs
• Medium to large structures
• Spacing: 900mm – 1000mm
• Use 10mm or 16mm sheets
Carports
• Larger spans
• Spacing: 1000mm+
• Usually 16mm sheets
How Rafter Spacing Affects Sheet Choice
Rafter spacing directly determines:
• Sheet width
• Sheet thickness
• Number of sheets needed
This is why calculators rely heavily on spacing.
Using Cut-to-Size Sheets with Your Rafters
Cut-to-size sheets ensure:
• Perfect fit between rafters
• Minimal waste
• Faster installation
Order here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
Glazing Bars and Rafter Alignment
Glazing bars must sit:
👉 Directly on rafters
This ensures:
• Proper support
• Secure fixing
• Watertight sealing
Browse accessories here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
Other Factors That Affect Rafter Spacing
Wind Exposure
Exposed areas require:
• Closer rafter spacing
• Thicker sheets
Roof Pitch
A steeper pitch improves:
• Strength
• Water runoff
Minimum recommended pitch: 5°
Snow Load (UK Consideration)
In the UK, snow loads are usually manageable, but:
• Larger spans = more load
• Thicker sheets help
Common Rafter Spacing Mistakes
Avoid these:
❌ Guessing spacing
Always measure accurately
❌ Not matching sheet width
Sheets must align with spacing
❌ Ignoring thickness requirements
Thin sheets need closer spacing
❌ Misaligned glazing bars
Bars must sit directly on rafters
How a Roof Calculator Uses Rafter Spacing
A good calculator:
• Matches sheet width to spacing
• Calculates number of sheets
• Selects correct accessories
• Prevents errors
This removes guesswork entirely.
Accessories Needed for Proper Installation
A full 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
Rafter spacing is a critical part of any polycarbonate roofing system. It determines sheet size, structural strength and overall performance.
By matching your rafter spacing to the correct sheet width and thickness, you can ensure your roof is strong, quiet and long-lasting.
For best results, always measure accurately, use compatible accessories and consider cut-to-size sheets for a perfect fit.
