January 2026

How to Install Multiwall Polycarbonate Roofing Sheets (Complete Step-by-Step Guide)

The Ultimate Installation Guide for Pergolas, Lean-To Roofs, Carports & Canopies

Multiwall polycarbonate roofing sheets are one of the most popular choices for UK homeowners thanks to their strength, insulation, lightweight construction, and long lifespan. But to get the best performance out of your roof, installation must be done correctly.

This guide will walk you through every step of installing polycarbonate sheets — from preparation and measuring, to fixing, sealing, and finishing. Whether you're a DIYer or a trade professional, this complete guide will help you achieve a strong, watertight, professional-quality roof.

1. Tools & Materials You Will Need

Before starting, make sure you have the correct tools:

Tools

Fine-tooth handsaw, circular saw, or jigsaw
Drill + drill bits
Tape measure
Silicone roller or firm hand roller
Utility knife
Spirit level
Clamps (optional)
Rubber mallet

Materials

Multiwall polycarbonate sheets (correct thickness)
Glazing bars
Fixing buttons
Breather tape
End closures
Side flashings
Wall flashing tape (for lean-to roofs)

All accessories are available at polycarbonatex.co.uk — ensure they match your sheet thickness.

2. Understanding the Structure of Polycarbonate Sheets

Multiwall sheets have internal flutes (channels) running in one direction. These flutes must run vertically or downhill so that:

Condensation can drain
Moisture escapes correctly
Structural strength is maintained

Never install sheets with flutes running horizontally.

3. Planning Your Roof Layout

Correct planning prevents leaks, gaps, and costly mistakes.

Determine the Roof Pitch

Your roof must have a minimum 5° pitch (approx. 100mm drop per metre).

This ensures:

Rainwater drains efficiently
No pooling occurs
The sheets stay clean and clear

Calculate Sheet Coverage

Allow for:

Glazing bar width
Edge flashing coverage

4. Cutting Polycarbonate Sheets (If Needed)

Polycarbonate is easy to cut when using the right method.

Best Cutting Tools

Fine-tooth handsaw
Circular saw with fine blade
Jigsaw with fine-tooth blade

Cutting Tips

Leave sheet film ON during cutting
Clamp sheet to prevent vibration
Cut slowly and evenly
Blow out swarf (plastic dust) from flutes

Clear Swarf From Flutes

Use:

Compressed air
Vacuum with nozzle
Soft brush

This step is essential — trapped swarf can cause condensation marks.

5. Taping the Sheet Ends

Each sheet has a top and a bottom.
You must tape both ends differently:

Bottom End — Breather Tape

Allows:

Moisture vapour to escape
Internal condensation to drain
Prevents insects/dust entering

Top End — Solid Aluminium Tape

Creates a permanent, watertight seal.

After Taping — Fit End Closures

End closures:

Protect tape
Improve weather resistance
Create a clean finish

Tap them on gently with a rubber mallet.

6. Installing the Glazing Bars

You’ll use either:

Snap-Fix Glazing Bars
Screw-Down Glazing Bars

Both create watertight sheet joints and allow thermal expansion.

General Installation Steps

Fix glazing bar base securely to rafters.
Position sheets leaving a 3–5mm expansion gap between sheets.
Snap or screw the top cap into place.
Ensure caps are fully seated along entire length.

Correct glazing bar installation is essential for:

Leak prevention
Sheet stability
Proper thermal movement

7. Fixing the Sheets Down (If Required)

Most roofs use glazing bars for fixing.
For additional fixing points (e.g., on purlins):

Using Polycarbonate Fixing Buttons

Fixing buttons:

Allow movement
Prevent cracking
Create watertight seals

Fixing Rules

Always drill oversize holes (2–3mm larger than the shank).
Do NOT overtighten — the sheet must move freely.

8. Installing Side Flashings

Side flashings stop:

Wind-driven rain
Debris ingress
Draughts
Uplift from wind

Fixing Tips

Fix to timber or steel supports
Ensure flashing sits snugly against sheet

Side flashings give the roof a clean, professional finish.

9. Flashing Against a Wall (Lean-To Roofs)

If your roof connects to a house or wall, you need wall flashing tape.

How to Install Wall Flashing Tape

Clean wall surface thoroughly.
Apply tape firmly to the wall above the sheet.
Overlap tape onto sheet by 20–40mm.
Roller the tape for strong adhesion.

This creates a watertight seal between the roof and wall.

10. Allowing for Thermal Expansion

Polycarbonate expands and contracts more than many roofing materials.

Expansion Rules

Leave 3–5mm gaps at each edge
Drill oversize holes for fixings
Never trap sheets too tightly
Avoid rigid clamping

Ignoring thermal expansion will cause:

Cracking
Creaking noises
Buckled sheets

11. Final Checks

Before removing the protective film:

Check:

All sheets aligned
All fixings secure (but not overtightened)
Flashings installed
No swarf left inside flutes
Tapes fully adhered
End closures properly fitted

Remove Film

Remove protective film immediately after installation — leaving it on too long makes it difficult to remove later.

12. Ongoing Maintenance

Polycarbonate roofs are low-maintenance.

Cleaning