Best Thickness for Pergola Roofs: 10mm, 16mm or 25mm?

Choosing the right polycarbonate thickness is an important part of planning a pergola roof. The sheet needs to be suitable for the supporting structure, while factors such as insulation, rigidity, light transmission and the spacing between supports can also influence which thickness is most appropriate.

For most pergola roofing projects, 10mm, 16mm and 25mm multiwall polycarbonate sheets are the main options to consider.

At PolycarbonateX, 10mm is the minimum thickness we recommend for general pergola roofing, while 16mm is a popular choice where additional rigidity and thermal performance are wanted. 25mm provides a thicker, more insulating option for projects where thermal performance is a greater priority.

What Is the Best Polycarbonate Thickness for a Pergola Roof?

As a general guide:

• 10mm polycarbonate — a lightweight and economical option for pergolas with suitable supporting rafters.

• 16mm polycarbonate — a popular all-round choice offering increased rigidity and improved thermal performance.

• 25mm polycarbonate — a thicker option offering greater insulation and rigidity for suitable roof structures.

The correct choice shouldn't be based on thickness alone. Rafter spacing, roof design, sheet length, glazing system and expected loading all need to be considered.

If you're unsure which thickness is suitable for your structure, always check the appropriate support requirements before ordering.

10mm Polycarbonate for Pergola Roofs

10mm multiwall polycarbonate is the starting thickness we recommend for general pergola roofing applications.

It provides a good combination of low weight, natural light and durability and can be an economical choice where the supporting structure has been designed with appropriate rafter spacing.

10mm sheets are particularly suited to projects where thermal insulation isn't the main consideration and the pergola is primarily intended to provide protection from rain while retaining a bright, open feel.

PolycarbonateX 10mm sheets are available in Clear and Bronze, allowing you to choose between maximum natural light and a more tinted appearance with reduced glare.

16mm Polycarbonate for Pergola Roofs

For many pergola projects, 16mm multiwall polycarbonate provides an excellent balance of strength, rigidity and thermal performance.

Its thicker multiwall construction provides greater rigidity than 10mm sheet and improved insulation, making it a popular option for substantial pergolas, verandas, lean-to roofs and other covered outdoor areas.

PolycarbonateX 16mm sheets are available in:

• Clear — for maximum natural light.

• Bronze — for a tinted appearance and reduced glare.

• Opal — for softer, more diffused light and increased privacy.

If you're building a pergola that will be used regularly throughout the year, 16mm is often worth considering.

25mm Polycarbonate for Pergola Roofs

25mm multiwall polycarbonate offers increased rigidity and better thermal insulation than thinner multiwall sheets.

It can be a good option for substantial covered areas where improved thermal performance is important, although a thicker sheet isn't automatically necessary for every pergola.

The supporting structure must still be designed appropriately for the chosen sheet and roofing system.

25mm sheets are available in several colour options, allowing the amount and character of light entering the covered area to be tailored to the project.

Is 4mm or 6mm Polycarbonate Suitable for a Pergola Roof?

We don't recommend 4mm or 6mm multiwall polycarbonate for general pergola roofing.

These thinner sheets are more commonly associated with applications such as greenhouses, cold frames and lightweight glazing.

For a permanent pergola, canopy, veranda, lean-to or similar roof, PolycarbonateX recommends starting with 10mm multiwall polycarbonate and selecting the final thickness according to the structure and required performance.

Does Thicker Polycarbonate Mean a Stronger Roof?

Sheet thickness is important, but it isn't the only factor determining the performance of a polycarbonate roof.

A successful pergola roof also depends on:

• Correct rafter and support spacing

• Suitable glazing bars

• Appropriate roof pitch

• Allowance for thermal expansion

• Correct sheet orientation

• Secure fixings

• Correct sealing and breather tapes

• Properly fitted end closures and flashings

A thicker sheet should therefore never be used as a substitute for correctly designed supporting rafters.

Rafter Spacing Is Just as Important as Sheet Thickness

One of the most important considerations when choosing polycarbonate sheets is the distance between the supporting rafters.

Different sheet thicknesses and sizes require appropriate support, and the roofing system needs to allow for the glazing bars and natural thermal movement of the polycarbonate.

If you're designing the pergola structure before ordering the sheets, read our Polycarbonate Rafter Spacing Guide before setting out your rafters.

This is particularly important if you want to use standard sheet widths efficiently or are ordering sheets cut to size.

Clear, Bronze or Opal Polycarbonate for a Pergola?

Thickness isn't the only choice you'll need to make.

Clear

Clear polycarbonate provides the highest level of natural light and is ideal where you want the pergola to remain as bright as possible.

Bronze

Bronze polycarbonate has a tinted appearance and helps reduce glare while still allowing natural light through the roof. It can be particularly attractive on timber pergolas and outdoor seating areas.

Opal

Opal polycarbonate diffuses the incoming light, producing a softer appearance and offering increased privacy. Availability depends on the sheet thickness selected.

The best colour therefore depends on how you want the area beneath the pergola to feel rather than simply its appearance from above.

What Roof Pitch Should a Polycarbonate Pergola Have?

A polycarbonate roof needs sufficient pitch to allow rainwater to drain effectively.

As a general rule, we recommend a minimum roof pitch of 5°, subject to the requirements of the roofing system and structure being used.

Insufficient pitch can lead to poor drainage and increase the likelihood of water-related problems.

Allow for Polycarbonate Expansion

Polycarbonate naturally expands and contracts as temperatures change.

Sheets should therefore never be installed tightly between fixed points without the required allowance for thermal movement.

Correct expansion allowance is particularly important on longer pergola roofs because the amount of movement increases with sheet length.

Read our Polycarbonate Expansion and Contraction Guide before installation for more detailed guidance.

How Should the Ends of Multiwall Polycarbonate Be Sealed?

Multiwall polycarbonate contains internal channels, or flutes, running through the sheet.

These should normally run down the direction of the roof slope so that moisture can drain towards the lower end.

For PolycarbonateX multiwall roofing:

• The upper end of the sheet is sealed with foil/sealing tape.

• The lower end uses breather tape to help ventilation and drainage.

• Sheet end closures are then fitted to protect and finish the exposed ends.

PolycarbonateX supplies the sealing and breather tapes together as a sealing tape kit covering up to 10 metres.

Do not simply seal both ends of a multiwall sheet with silicone, as the internal channels need to be treated correctly.

Can Polycarbonate Pergola Sheets Be Cut to Size?

Yes.

PolycarbonateX offers 10mm, 16mm and 25mm multiwall polycarbonate sheets cut to your required dimensions, which can be particularly useful when designing a pergola around specific rafter centres or an existing structure.

Using cut-to-size sheets can also reduce the amount of cutting required during installation.

Which Thickness Should I Choose?

For most projects, the decision can be simplified to:

Choose 10mm if you want a lightweight, economical pergola roofing option and your supporting structure is suitable.

Choose 16mm if you want increased rigidity and improved thermal performance for a more substantial pergola or covered outdoor area.

Choose 25mm where greater insulation and rigidity are priorities and the roof structure has been designed accordingly.

For general pergola roofing, we don't recommend dropping below 10mm.

Planning a Complete Pergola Roof?

Choosing the sheet thickness is only one part of planning the roof. You'll also need to consider sheet sizes, rafter centres, glazing bars, end closures, tapes, flashings and fixings.

The PolycarbonateX Roof Kit Calculator can help you calculate the sheets and roofing components required from your roof dimensions.

If you're still planning the structure, our Polycarbonate Roof Planning Guide also covers sheet thickness, colour, measuring and the other decisions to make before ordering.