Multiwall polycarbonate sheets are widely used for pergolas, canopies, garden rooms, walkways, and similar outdoor structures. While the material is straightforward to work with, many issues that occur after installation are typically related to incorrect fixing methods, unsuitable accessories, or missing components rather than the sheets themselves.

This guide outlines common installation mistakes and practical steps that can help support a suitable and long-lasting polycarbonate roof installation.

You can view compatible accessories here:
👉 https://polycarbonatex.co.uk/collections/multiwall-accessories

1. Not Allowing for Thermal Expansion

The Issue

Polycarbonate naturally expands and contracts with temperature changes. If the sheets are fixed too tightly, or holes are drilled too small, the material may have limited room to move.

How to Help Avoid It

• Pre-drill fixing holes slightly larger than the screw diameter

• Use fixings designed for multiwall polycarbonate

• Avoid overtightening screws

These steps support natural movement during temperature changes.

2. Using Incorrect Fixings

The Issue

Standard screws are not designed for multiwall polycarbonate. Using them can place excessive pressure around fixing points.

How to Help Avoid It

• Use fixings made specifically for polycarbonate roofing

• Ensure fixings include spacers or buttons to distribute pressure

• Match fixings to the support type (timber or metal)

Example:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-super-fixings-buttons-pack-of-10

Appropriate fixings help support a secure and consistent installation.

3. Incorrect or Missing End Tapes

The Issue

The internal flutes of multiwall sheets must be sealed correctly. Incorrect taping can allow dust or debris to enter.

How to Help Avoid It

• Apply solid aluminium tape to the top edge

• Apply breather (vented) tape to the bottom edge

• Use end closures to finish the edges where required

Breather tape example:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-breather-tape-10m

Correct taping supports ventilation and helps prevent contamination inside the flutes.

4. Installing Sheets with the Wrong Side Facing Outwards

The Issue

Polycarbonate roofing sheets have a UV-protected surface. Installing the sheet with the wrong side outward can reduce its effectiveness.

How to Help Avoid It

• Check the manufacturer’s protective film for orientation

• Ensure the UV-protected side faces outward

• Remove the protective film only when installation is complete

Correct orientation ensures the sheet is installed as intended.

5. Using Glazing Bars That Don’t Match Sheet Thickness

The Issue

Glazing bars must be the correct size for the sheet thickness. Using mismatched components may affect sheet alignment and sealing.

How to Help Avoid It

• Select glazing bars designed for your chosen sheet thickness

• For larger structures, consider aluminium glazing bars

• Ensure bars are fixed securely and spaced correctly

Example glazing bar:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-with-1-end-cap-1

Appropriate glazing bars help support sheet edges and joints.

6. Insufficient Roof Pitch

The Issue

A roof that is too flat may experience slow water run-off and increased debris collection.

How to Help Avoid It

• Follow the recommended minimum pitch for your sheet type or system

• Increase the roof pitch where possible to encourage water movement

Adequate pitch helps support cleaner water run-off.

7. Unsupported or Excessively Overhanging Sheet Edges

The Issue

Polycarbonate sheets require suitable support along the structure. Unsupported edges may be prone to movement in certain weather conditions.

How to Help Avoid It

• Follow support-centre recommendations for your sheet thickness

• Avoid large overhangs without structural support

• Use closures or end bars at exposed edges

Correct support helps maintain consistent alignment across the roof.

8. Incorrect Cutting Techniques

The Issue

Unsuitable cutting tools can cause uneven edges or rough cuts, making installation more difficult.

How to Help Avoid It

• Use a fine-tooth circular saw or a sharp hand saw

• Support the sheet fully while cutting

• Remove swarf from flutes after cutting

Clean edges help ensure a neat installation.

Summary

Polycarbonate roofs can be installed effectively when the correct components and techniques are used. Many common issues relate to:

• Allowing space for thermal movement

• Using suitable fixings and glazing bars

• Installing sheets with the correct orientation

• Sealing sheet ends appropriately

• Providing adequate support and pitch

Following these principles helps ensure that the sheets and accessories function as intended.

Browse Polycarbonate Sheets & Accessories

polycarbonatex.co.uk supplies multiwall polycarbonate sheets and accessories suitable for roofing systems, including glazing bars, tapes, closures, and fixings.

👉 Sheets: https://polycarbonatex.co.uk/collections/multiwall-polycarbonate-sheets
👉 Accessories: https://polycarbonatex.co.uk/collections/multiwall-accessories