How to Choose the Right Polycarbonate Sheet Thickness

Selecting the right polycarbonate sheet is the first step towards building a durable, attractive and long-lasting roof. This guide explains the different sheet thicknesses, colours and common applications, helping you choose the best option for your project with confidence.

At first glance, all polycarbonate sheets look very similar. They're all lightweight, weather resistant and designed to let natural daylight into the space below. So it's easy to wonder why choosing the correct thickness is so important.

The answer is simple.

The thickness of the sheet affects how strong it is, how well it insulates, how far it can span between roof supports and how your finished roof performs over many years.

A thicker sheet is generally stronger and provides better insulation, but that doesn't automatically make it the right choice. Choosing a sheet that's thicker than your project actually needs can simply increase the cost without providing any real benefit.

The best polycarbonate roof isn't built using the thickest sheet available. It's built using the sheet that's best suited to the design of your roof, the spacing between your rafters and how you plan to use the space beneath it.

In the next sections, we'll explain the differences between each sheet thickness and help you decide which option is the best fit for your project.

Each one has been designed for different types of projects and roof structures. While they may look similar from the outside, each thickness offers different levels of strength, insulation and spanning capability.

10mm Polycarbonate Sheets

10mm is the minimum thickness we recommend for general roofing applications. It's lightweight, allows the highest level of natural daylight to pass through and is ideal for smaller roofs with closely spaced supports. It's often chosen for pergolas, canopies and simple lean-to roofs where keeping costs down is an important consideration.

16mm Polycarbonate Sheets

16mm is our most popular and most versatile roofing sheet. It offers a significant improvement in strength and insulation compared with 10mm, making it suitable for the majority of domestic projects. For many homeowners, it provides the best balance between performance, appearance and value for money.

25mm Polycarbonate Sheets

25mm is designed for larger or more demanding roofing projects where improved insulation and greater structural performance are important. It's often chosen for premium pergolas, larger verandas and garden rooms where comfort beneath the roof is just as important as weather protection.

Choosing the right sheet thickness isn't about buying the strongest or most expensive option. It's about selecting the sheet that best matches your roof and how you plan to use it.

If you're building a typical garden pergola, canopy or lean-to roof, you'll usually be choosing between 10mm and 16mm sheets. For larger structures, or projects where improved insulation is important, 25mm may be the better option.

As a general guide:

Choose 10mm if...

• You're building a smaller pergola, canopy or lean-to roof.
  
• Your roof has closely spaced rafters or glazing bars.
  
• Keeping the project cost down is important.
  
• You want to maximise the amount of natural daylight entering the space.
  

Choose 16mm if...

• You're looking for the best all-round solution.
  
• You want a stronger roof with improved insulation.
  
• You're building a typical domestic pergola, veranda or carport.
  
• You want an excellent balance between strength, appearance and value.
  

Choose 25mm if...

• Your roof is larger or has wider spans between supports.
  
• Thermal insulation is an important consideration.
  
• You're creating a space that you'll use throughout the year.
  
• You're happy to invest a little more for improved comfort and performance.
  

If you're still unsure, don't worry. In the following sections we'll explain each thickness in much more detail, including where it works best, its advantages and the situations where another option may be more suitable.

If you're building a typical garden pergola, canopy or lean-to roof, there's a good chance you've already considered 10mm polycarbonate sheets. They're the most affordable roofing option we recommend for general roofing applications and are often the perfect choice for smaller projects.

One of the biggest advantages of 10mm sheets is that they allow plenty of natural daylight to pass through while remaining lightweight and easy to handle. This makes them ideal for DIY installations where lifting large roofing sheets into position can be challenging.

Although 10mm sheets are the thinnest roofing sheets we recommend, they're still extremely strong when installed correctly using suitable glazing bars and the correct spacing between supports. Like every roof, the supporting structure does much of the work, so the sheet and roof frame should always be designed to work together.

Because 10mm sheets contain less trapped air than thicker options, they don't provide the same level of thermal insulation as 16mm or 25mm sheets. For an open pergola or garden canopy this often isn't important, but if you're hoping to create a more comfortable outdoor room that you'll use throughout the year, a thicker sheet may be a better investment.

Best suited for

• Garden pergolas
  
• Canopies
  
• Lean-to roofs
  
• Covered seating areas
  
• Projects where maximum natural daylight is important
  

Things to consider

Before choosing 10mm sheets, make sure your roof has adequate support. The distance between your rafters or glazing bars has a direct effect on how the sheet performs. Wider roof spans usually require thicker sheets or additional roof supports. We'll explain this in more detail later in this guide.

If you're unsure which sheet thickness to choose, 16mm is usually the option we recommend. It offers the best balance between strength, insulation, light transmission and value, making it the most popular choice for the majority of domestic roofing projects.

Compared with a 10mm sheet, a 16mm sheet has a more substantial internal structure. This makes it stronger, more rigid and better at reducing heat loss, while still allowing plenty of natural daylight to pass through. If you're planning to spend time beneath your roof throughout the year, these improvements can make a noticeable difference to comfort.

Another advantage of 16mm sheets is their ability to span greater distances than 10mm sheets when installed correctly. This can provide more flexibility when designing your roof structure, although the exact spacing between supports should always be based on the expected roof loading and the manufacturer's spanning guidance.

For many homeowners, 16mm represents the "sweet spot". It offers a significant improvement in performance without the additional cost and weight of a 25mm sheet, making it an excellent all-round choice for both new builds and replacement roofs.

Best suited for

• Pergolas
  
• Verandas
  
• Lean-to roofs
  
• Carports
  
• Garden canopies
  
• Most domestic roofing projects
  

Things to consider

Although 16mm sheets provide improved rigidity and insulation, they still rely on the roof structure being designed correctly. The glazing bars, rafters and supporting framework all work together to support the roof, so choosing the correct roof design is just as important as choosing the correct sheet thickness.

If you're looking for the highest level of performance, 25mm polycarbonate sheets are our premium roofing option. They provide the greatest level of thermal insulation, excellent rigidity and are designed for projects where comfort beneath the roof is just as important as keeping the rain out.

Unlike a standard pergola that may only be used during warmer months, many customers choose 25mm sheets when creating a space they'll use throughout the year. Whether it's an outdoor seating area, a garden room, a covered entertaining space or a large veranda, the additional insulation can help make the area more comfortable in both summer and winter.

The thicker multiwall construction also provides excellent structural performance when installed as part of a correctly designed roofing system. While 25mm sheets are capable of spanning similar distances to 16mm sheets under some published loading conditions, they offer noticeably better thermal insulation and a more substantial feel. Choosing 25mm should therefore be based on the overall performance you want from your roof, not simply because it's the thickest sheet available.

Because the sheet contains more internal walls, it allows slightly less natural daylight to pass through than thinner sheets. Most homeowners don't find the difference significant, but it's worth considering if creating the brightest possible space is your main priority.

Best suited for

• Premium pergolas
  
• Large verandas
  
• Garden rooms
  
• Outdoor living spaces
  
• Roofs where improved insulation is important
  

Things to consider

A 25mm sheet isn't automatically the best choice for every project. If your roof has relatively short spans and you're building a simple open pergola, a 16mm sheet may provide everything you need at a lower cost. We always recommend choosing the sheet that best suits your project rather than simply selecting the thickest option available.

One of the biggest misconceptions about polycarbonate roofing is that choosing the right sheet thickness is all you need to think about.

In reality, your roof structure is just as important as the sheet itself.

A polycarbonate sheet doesn't support itself. Instead, it's supported by glazing bars and roof rafters, which carry the weight of the roof and transfer it safely into the supporting framework.

The distance between each support is known as the span.

The larger the span, the harder the sheet has to work. That's why thicker sheets are generally able to span greater distances than thinner sheets.

For example, imagine placing a piece of timber between two chairs.

If the chairs are close together, the timber stays strong and doesn't bend very much.

Move the chairs further apart and the timber begins to flex.

Polycarbonate sheets behave in exactly the same way.

This is why two customers building similar pergolas may need different sheet thicknesses. One roof may have rafters every 500mm, while another has rafters every 900mm. Even if both roofs are the same overall size, the sheet has to bridge a much greater distance in the second example.

The expected loading on the roof also plays an important part. Wind, snow and the overall roof design all affect how much support the sheet requires. Because of this, published span figures should always be treated as design guidance rather than a universal rule.

Choosing the correct combination of sheet thickness, glazing bars and rafter spacing will help ensure your roof performs as intended for many years.

You'll often see manufacturers publish tables showing the maximum distance a polycarbonate sheet can span between supports. While these tables are extremely useful, they can also be confusing if you've never worked with polycarbonate roofing before.

The first thing to understand is that a span table doesn't tell you how to build your roof. Instead, it shows the maximum distance a sheet can safely span under different loading conditions when installed correctly as part of a complete roofing system.

Think of it as a safety guide rather than a set of instructions.

For example, a roof in a sheltered garden in the south of England may experience very different wind and snow loads compared with a roof in an exposed location or at higher altitude. Although the roofs may look identical, the structural requirements can be very different.

This is why the published span figures change depending on the design load. As the load on the roof increases, the maximum span between supports becomes smaller. In simple terms:

• More weight on the roof = more support required.
  
• Less weight on the roof = the sheet can span further.
  

The table below shows the published guideline spans for multiwall polycarbonate sheets installed with support on two sides. These figures should always be used alongside good roof design and the recommendations for your chosen roofing system. They shouldn't be treated as a one-size-fits-all solution.

Don't worry if the table above looks a little confusing at first. Most DIY customers aren't used to reading structural span tables, so let's simplify it.

The key thing to remember is that the further your roof supports are apart, the stronger your sheet needs to be.

If your roof has closely spaced rafters, a 10mm sheet may be perfectly suitable. As the distance between those supports increases, you'll normally need to consider a thicker sheet or redesign the roof so it provides additional support.

It's also important to remember that roof loading changes from one project to another. A roof exposed to stronger winds or heavier snowfall may require closer support spacing than a roof in a more sheltered location.

This is why we always recommend designing your roof first, then choosing the most suitable sheet thickness, rather than selecting a sheet and hoping it will work with any roof design.

If you're ever unsure, our technical team is always happy to help you choose the most appropriate sheet thickness and roofing system for your project.

Choosing the correct sheet thickness isn't difficult once you understand the basics, but there are a few common mistakes that we regularly see. Avoiding these can save you both time and money.

Choosing the Thickest Sheet "Just to Be Safe"

It's easy to assume that buying the thickest sheet available is always the best option. While thicker sheets do provide improved insulation and increased rigidity, they also cost more and aren't always necessary. A roof that's been designed correctly often performs perfectly with a thinner sheet that's suited to the application.

Ignoring the Roof Structure

The sheet is only one part of the roofing system. Your glazing bars, rafters, fixings and supporting framework all work together to create a strong roof. Even the best polycarbonate sheet won't perform as expected if the roof structure hasn't been designed correctly.

Focusing Only on Price

Choosing the cheapest sheet can sometimes end up costing more in the long run if it isn't suitable for the roof you're building. It's always worth considering the overall performance, expected lifespan and how you plan to use the space beneath the roof, rather than comparing prices alone.

Forgetting About Future Use

Many people build a pergola expecting to use it only in the summer, but later decide they'd like to spend much more time beneath it. Choosing a sheet with better insulation at the start can make the space far more comfortable throughout the year and may save you upgrading the roof later.

Not Asking for Advice

Every project is different. If you're unsure which sheet thickness is right for your roof, it's always better to ask before ordering. Our technical team can help you choose the most suitable roofing system for your project, giving you confidence that you're buying the right products the first time.

If you're still deciding which polycarbonate sheet thickness to choose, here's our advice based on the projects we help customers with every day.

• Choose 10mm if you're building a smaller pergola, canopy or lean-to roof with closely spaced supports and you're looking for the most cost-effective solution.
  
• Choose 16mm if you're looking for the best all-round option. It offers an excellent balance of strength, insulation, light transmission and value, making it our recommended choice for most domestic roofing projects.
  
• Choose 25mm if you're building a larger structure, creating an outdoor living space that you'll use throughout the year, or you'd like the highest level of thermal insulation and overall performance.
  

Remember, the sheet thickness is only one part of a successful roof. The glazing bars, supporting rafters and overall roof design are equally important. Choosing products that work together as a complete roofing system will always provide the best long-term results.

If you're ever unsure which option is right for your project, our team is always happy to help. We'd much rather help you choose the correct products before you order than have you buy something that's not ideally suited to your roof.