Choosing the correct size polycarbonate sheets is one of the most important steps when building a pergola, lean-to roof, veranda or carport. Getting the sizing right ensures your roof is structurally sound, watertight and easy to install.

If the sheets are too wide, they won’t fit your glazing bars properly. If they’re too narrow, you’ll end up with gaps, leaks or wasted materials. In this guide, we’ll explain exactly how to choose the right sheet size for your project.

Helpful resources mentioned throughout this guide:

Multiwall polycarbonate accessories
https://polycarbonatex.co.uk/collections/multiwall-accessories

Cut-to-size polycarbonate sheets
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

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

Why Sheet Size Matters

Polycarbonate roofing systems are designed around rafter spacing and glazing bars, not random sheet sizes.

Choosing the correct size ensures:

• Sheets fit perfectly between glazing bars
• Proper structural support
• Reduced movement and noise
• Correct water drainage
• Minimal cutting and waste

Incorrect sizing is one of the main causes of installation problems and leaks.

Understanding Polycarbonate Sheet Dimensions

Polycarbonate sheets are defined by two main dimensions:

1. Width

This is the distance across the sheet (left to right).

2. Length

This is the distance from the top of the roof (wall side) to the bottom (gutter side).

Standard Polycarbonate Sheet Widths

Common sheet widths used in roofing systems include:

• 700mm
• 900mm
• 980mm
• 1050mm
• 1220mm
• 1600mm
• 2100mm

However, the correct width depends entirely on your rafter spacing, not just preference.

How to Choose the Correct Sheet Width

The width of your sheet must match the distance between glazing bars, which are fixed to your rafters.

Key rule:

👉 One sheet sits between two glazing bars

Example:

If your rafters are spaced at 1000mm centres:

• You would typically use ~980mm wide sheets
• The glazing bars sit on the rafters
• The sheet fits neatly between them

Why You Should NOT Guess Sheet Width

Choosing the wrong width leads to:

• Sheets not fitting into glazing bars
• Gaps or forced fitting
• Water leaks
• Poor expansion allowance

Always measure your structure first.

How to Choose the Correct Sheet Length

The sheet length is determined by the projection of your roof (distance from wall to front beam).

Example:

If your roof projects 2.5m:

• Your sheet length should be around 2.5m + overhang

Allow for Overhang

Polycarbonate sheets should extend beyond the front beam slightly to allow water to drip into the gutter.

Recommended overhang:

👉 50mm – 70mm

Example:

Roof projection: 2500mm
Final sheet length: 2550–2570mm

Why Cut-to-Size Sheets Are the Best Option

Trying to manually cut sheets on-site can lead to:

• Incorrect sizing
• Rough edges
• Time-consuming installation
• Risk of cracking

Cut-to-size sheets eliminate these issues.

Order exact sizes here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

How Many Sheets Do You Need?

To calculate the number of sheets:

Step 1 — Measure total roof width

Step 2 — Divide by usable sheet width

Example Calculation

Roof width: 3000mm
Sheet width: 1000mm

👉 You need 3 sheets

You will also need:

• 4 glazing bars (including edges)
• End caps
• Fixings and tapes

Accessories available here:
https://polycarbonatex.co.uk/collections/multiwall-accessories

Matching Sheet Size to Structure Type

Different structures require slightly different sizing approaches.

Pergola Roofs

• Typically use 6mm–10mm sheets
• Common widths: 900–1050mm
• Smaller spans = narrower sheets

Lean-To Roofs

• Often use 10mm–16mm sheets
• Common widths: 980–1050mm
• Larger spans require thicker sheets

Carports

• Larger structures
• Usually 10mm–16mm sheets
• Wider sheets may be used depending on design

Sheet Thickness vs Size (Important)

Sheet size and thickness work together.

Thinner sheets (6mm):

• Better for smaller widths
• Require closer rafter spacing

Thicker sheets (10mm–16mm):

• Can span wider areas
• More rigid
• Better for larger roofs

Choosing the right combination improves performance and lifespan.

Common Sizing Mistakes to Avoid

Avoid these common errors:

❌ Measuring overall roof instead of rafter spacing

Always measure between rafters, not total width.

❌ Forgetting expansion gaps

Polycarbonate expands with heat — glazing bars handle this, but sheets must not be forced.

❌ No overhang

Without overhang, water can run behind the gutter.

❌ Guessing sheet sizes

Always measure accurately or use cut-to-size.

Do You Need to Cut Polycarbonate Sheets Yourself?

In most cases — no.

Cutting sheets yourself increases the risk of:

• Incorrect sizing
• Rough or uneven edges
• Installation delays

Using pre-cut sheets is faster, cleaner and more reliable.

Order pre-cut sheets here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

Accessories You’ll Need Alongside Sheet Sizing

Correct sizing is only part of the system.

You will also need:

• Glazing bars
• Aluminium tape (top edge)
• Breather tape (bottom edge)
• End closures
• Drip trims

These ensure your roof is watertight and ventilated.

Browse accessories here:
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 Information

When ordering sheets, it’s important to understand delivery times and handling.

View delivery details here:
https://polycarbonatex.co.uk/pages/delivery-policy

Conclusion

Choosing the correct polycarbonate sheet size is essential for a successful roofing project. By measuring your roof accurately, matching sheet width to rafter spacing, and allowing for proper overhang, you can ensure a clean, professional installation.

Using cut-to-size sheets and compatible accessories makes the process even easier and helps avoid common mistakes.

With the right sizing, your polycarbonate roof will be strong, watertight and built to last for years.