How to Build a Pergola for Polycarbonate Roofing (Step-by-Step DIY Guide)
Building a pergola for polycarbonate roofing is one of the best ways to create a covered outdoor space that’s bright, durable and usable all year round. Whether you’re covering a patio, decking area or garden seating space, a properly built pergola provides the perfect structure for installing polycarbonate sheets.
In this guide, we’ll walk you through how to build a pergola step by step, including sizing, timber selection, rafter spacing and preparing the structure for polycarbonate roofing.
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 a Pergola for Polycarbonate Roofing?
A pergola is a timber frame structure consisting of:
• Vertical posts
• Horizontal beams
• Rafters supporting the roof
When fitted with polycarbonate sheets, it becomes a covered outdoor space that protects against rain while allowing natural light through.
Step 1 — Plan Your Pergola Size
Before building, decide:
• Width (left to right)
• Projection (front to back)
• Height
Typical Pergola Sizes
• Small: 2m x 2m
• Medium: 3m x 2.5m
• Large: 5m x 3m
Your size will determine:
• Number of rafters
• Sheet sizes
• Overall cost
Step 2 — Choose the Right Timber
Using the correct timber is essential for strength and durability.
Recommended Timber Sizes
• Posts: 100mm x 100mm (or larger)
• Beams: 150mm x 50mm (or stronger)
• Rafters: 100mm x 50mm
Timber Tips
• Use pressure-treated timber
• Ensure timber is straight and dry
• Seal or treat exposed edges
Step 3 — Set Your Posts
Posts form the foundation of your pergola.
Installation Options
• Concrete into the ground
• Use post shoes or anchors
Spacing
• Typically 2m–3m apart
• Depends on overall structure size
Ensure posts are:
• Level
• Square
• Secure
Step 4 — Install the Main Beam
The main beam runs across the front of the pergola.
It supports the rafters and provides structural strength.
Important
• Ensure beam is level
• Secure firmly to posts
• Use strong fixings
Step 5 — Set the Roof Pitch
Polycarbonate roofing requires a minimum pitch:
👉 5° or greater
How to Achieve Pitch
• Fix rear beam higher than front beam
• Or angle rafters slightly downward
This ensures:
• Proper water drainage
• Better performance
Step 6 — Install Rafters
Rafters run from the wall (or rear beam) to the front beam.
They support the polycarbonate sheets.
Rafter Spacing (Important)
Spacing must match sheet width.
Typical spacing:
• 700mm – 1050mm centres
Example
If using 1000mm sheets:
👉 Rafters at 1000mm centres
Step 7 — Prepare for Polycarbonate Sheets
Before installing sheets, ensure:
• Rafters are aligned
• Spacing is consistent
• Structure is square
Step 8 — Install Glazing Bars
Glazing bars sit on top of rafters and hold sheets in place.
They:
• Secure the sheets
• Allow expansion
• Create a watertight seal
Browse accessories here:
https://polycarbonatex.co.uk/collections/multiwall-accessories
Step 9 — Install Polycarbonate Sheets
Place sheets between glazing bars.
Important Installation Rules
• UV-protected side faces outward
• Channels run top to bottom
• Allow for expansion
• Do NOT screw through sheets
Use Cut-to-Size Sheets
This ensures:
• Perfect fit
• Easier installation
• Professional finish
Order here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet
Step 10 — Apply Sealing Tape
Each sheet needs:
• Aluminium tape at the top
• Breather tape at the bottom
This prevents:
• Water ingress
• Condensation
Step 11 — Fit End Closures and Trims
End closures:
• Protect sheet edges
• Improve drainage
Glazing bar end caps:
Step 12 — Add Guttering (Optional but Recommended)
Guttering helps:
• Direct water away
• Protect structure
• Improve usability
Common Mistakes to Avoid
❌ Incorrect rafter spacing
Leads to poor sheet fit
❌ No roof pitch
Causes water pooling
❌ Screwing through sheets
Leads to cracks and leaks
❌ No sealing tape
Causes condensation
❌ Incorrect overhang
Water misses gutter
How Much Does It Cost to Build a Pergola?
Typical costs:
• Small: £300–£600
• Medium: £600–£1,200
• Large: £1,200+
Depends on:
• Size
• Materials
• Sheet thickness
Accessories Needed for a Full Roof
A complete system includes:
• Polycarbonate sheets
• 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
Building a pergola for polycarbonate roofing is a straightforward DIY project when planned correctly. By choosing the right timber, setting proper rafter spacing and installing sheets with the correct accessories, you can create a durable, weatherproof outdoor space.
Using cut-to-size sheets and a complete roofing system will make installation easier and ensure a professional finish that lasts for years.
