Choosing the correct polycarbonate sheet thickness is one of the most important decisions when building a pergola, lean-to roof, veranda, or carport. The thickness you choose affects strength, insulation, durability, and how well the roof performs over time.

Many people assume thicker sheets are always better, but the best thickness actually depends on the structure, rafter spacing, and how exposed the roof is to wind or snow.

In this guide, we explain how to choose the right polycarbonate thickness for your project and when each option works best.

Why Polycarbonate Thickness Matters

Polycarbonate roofing sheets are available in a range of thicknesses, typically from 4mm to 25mm in multiwall designs.

The thickness determines:

• Structural strength
• Resistance to wind and snow
• Thermal insulation
• Noise reduction
• Overall lifespan of the roof

Choosing a sheet that is too thin can lead to flexing or movement during strong winds, while choosing a thickness that is unnecessarily heavy can increase costs without providing extra benefits.

Common Polycarbonate Thickness Options

For most roofing projects, multiwall polycarbonate sheets are available in these standard thicknesses:

• 4mm
• 6mm
• 10mm
• 16mm
• 25mm

Each thickness is designed for different roofing applications.

4mm Polycarbonate Sheets

4mm sheets are the thinnest commonly used polycarbonate sheets.

They are primarily used for:

• Cold frames
• Small greenhouses
• Shed windows
• Non-structural panels

They are not usually recommended for roofing structures such as pergolas or lean-to roofs.

Advantages

• Lightweight
• Affordable
• Easy to cut and handle

Limitations

• Limited strength
• Minimal insulation
• Can flex easily in wind

For outdoor roofing structures, thicker sheets are usually more suitable.

6mm Polycarbonate Sheets

6mm twinwall polycarbonate sheets are commonly used for small garden structures.

Typical applications include:

• Small pergolas
• Porch roofs
• Lightweight canopies
• Small greenhouse roofs

They offer better rigidity than 4mm sheets while still remaining lightweight.

Advantages

• Lightweight and easy to install
• Improved insulation compared with 4mm
• Suitable for small structures

Limitations

• Less rigid than thicker sheets
• Not ideal for large spans

If your pergola has widely spaced rafters or sits in an exposed location, thicker sheets are usually recommended.

10mm Polycarbonate Sheets

10mm twinwall polycarbonate sheets are one of the most versatile roofing options.

They are commonly used for:

• Pergola roofs
• Carports
• Lean-to roofs
• Verandas
• Patio covers

10mm sheets offer a good balance between strength, insulation and cost.

Advantages

• Stronger than thinner sheets
• Good insulation properties
• Suitable for medium-sized roofs
• Good light transmission

When to use 10mm sheets

10mm sheets work well when:

• Rafters are spaced correctly
• The roof pitch is adequate
• The structure is moderately sheltered

If your structure is large or heavily exposed to wind, thicker sheets may be more suitable.

16mm Polycarbonate Sheets

16mm polycarbonate sheets are one of the most popular choices for permanent roofing structures.

They are frequently used for:

• Garden rooms
• Verandas
• Large pergolas
• Carports
• Conservatory roofs

Because the multiwall structure contains more air chambers, 16mm sheets provide improved insulation and rigidity.

Advantages

• Strong and durable
• Excellent insulation
• Reduced flexing
• Suitable for larger spans

This thickness is often recommended for homeowners who want a long-term roofing solution.

25mm Polycarbonate Sheets

25mm polycarbonate sheets are the thickest commonly used multiwall sheets.

They are generally used for:

• Conservatory roofs
• Large outdoor structures
• Garden rooms
• High-performance insulated roofs

Advantages

• Maximum insulation
• Very rigid structure
• Excellent strength

However, for most pergolas or lean-to roofs, 25mm sheets are usually unnecessary unless insulation is a priority.

Factors That Determine the Right Thickness

Choosing the correct sheet thickness depends on several factors.

Roof Size

Larger roofs generally require thicker sheets because the panels span greater distances.

Small pergolas can often use 6mm or 10mm sheets, while larger structures usually benefit from 16mm sheets.

Rafter Spacing

Rafter spacing plays a major role in determining the correct sheet thickness.

If rafters are spaced further apart, the sheet needs to be stronger to prevent movement.

Glazing bars are typically used to support sheets between rafters.

Wind Exposure

If your structure is located in an exposed area, such as an open garden or elevated position, thicker sheets provide better resistance to wind.

Thicker sheets also reduce vibration and noise during storms.

Insulation Requirements

Multiwall polycarbonate contains air chambers that improve insulation.

Thicker sheets contain more chambers, which means:

• Better temperature regulation
• Reduced heat loss
• Improved comfort in covered areas

If you are building a garden room or enclosed structure, thicker sheets may be beneficial.

Why Multiwall Polycarbonate Is Popular

Multiwall polycarbonate is widely used for roofing structures because it combines strength, insulation and light transmission.

Benefits include:

• High impact resistance
• Lightweight construction
• UV protection
• Excellent durability
• Good insulation properties

These characteristics make it ideal for pergolas, lean-to roofs and carports.

Accessories That Improve Roof Performance

The performance of a polycarbonate roof also depends on the accessories used during installation.

Essential accessories include:

• Aluminium sealing tape
• Breather tape
• Glazing bars
• End closures
• Drip trims

Using compatible accessories ensures the roof remains watertight and properly ventilated.

Conclusion

Choosing the right polycarbonate sheet thickness ensures your roof remains strong, durable and weather resistant.

For most pergolas, lean-to roofs and carports:

• 6mm sheets suit small structures
• 10mm sheets work well for medium roofs
• 16mm sheets are ideal for larger or permanent structures

By selecting the correct thickness and installing the appropriate accessories, you can create a polycarbonate roof that performs reliably for many years.