November 2026

How to Match Replacement Polycarbonate Sheets to an Existing Roof

If you need to replace a damaged, discoloured or ageing polycarbonate roofing sheet, finding the correct replacement isn't always as straightforward as ordering a panel that looks similar.

This is particularly true if your roof was installed several years ago, you've recently moved into the property or you don't know who originally supplied the roofing materials.

You might know that your roof has clear polycarbonate sheets, for example, but are they 10mm, 16mm or 25mm thick? What width do you need? And will a new sheet fit your existing glazing bars?

The good news is that you don't necessarily need to know the original manufacturer to find a suitable replacement. However, you do need to identify the existing sheet specification and confirm compatibility with the roofing system.

In this guide, we'll explain how to identify your existing polycarbonate sheets, what measurements to take and how to order replacement panels that suit your roof.

How Do You Identify Existing Polycarbonate Roofing Sheets?

Before ordering replacement polycarbonate, there are five main things to establish:

Sheet thickness: Is the existing panel 10mm, 16mm, 25mm or another thickness?
Sheet type: Is it multiwall, solid or corrugated polycarbonate?
Sheet dimensions: What width and length does the replacement need to be?
Sheet colour: Is the existing material clear, bronze, opal or another finish?
Roofing system compatibility: Will the replacement fit the existing glazing bars and supports?

Getting these details right can save considerable time and avoid ordering materials that won't fit.

If your existing roof uses multiwall polycarbonate, you can explore our multiwall polycarbonate roofing sheets to compare the available options.

Step 1: Identify the Type of Polycarbonate Sheet

The first step is establishing which type of polycarbonate your roof uses.

Not all polycarbonate roofing sheets have the same construction.

Multiwall Polycarbonate

Multiwall polycarbonate contains internal chambers, often called flutes.

When viewed from the exposed end, you can see the internal walls that separate these chambers.

Multiwall sheets are commonly used for pergolas, lean-tos, verandas, carports and conservatory roofs.

At PolycarbonateX, our main multiwall range includes 10mm, 16mm and 25mm thicknesses.

Solid Polycarbonate

Solid polycarbonate is a continuous sheet without internal chambers.

It has a different construction and may require a different glazing system.

A solid polycarbonate panel shouldn't automatically be replaced with multiwall material simply because the overall dimensions are similar.

Corrugated Polycarbonate

Corrugated polycarbonate has a raised, repeating profile.

It is commonly installed using fixing arrangements that differ from those used for flat multiwall sheets.

If your existing roof is corrugated, you'll need to identify the profile and compatible fixing system before ordering replacements.

Important: PolycarbonateX's standard multiwall sheets are not direct replacements for every solid or corrugated roofing system.

Step 2: Measure the Existing Polycarbonate Thickness

Sheet thickness is one of the most important measurements when ordering replacements.

Common multiwall roofing thicknesses include:

Thickness	Typical applications
10mm	Suitable smaller roofing projects with appropriate support
16mm	Pergolas, lean-tos, verandas and carports
25mm	Roofing applications where greater rigidity or insulation is required

Other thicknesses also exist, particularly on older roofing systems.

How to Measure Sheet Thickness

If you can safely access an exposed sheet edge, measure the overall thickness from the top surface to the bottom surface.

A ruler may provide an initial indication, although callipers are more accurate.

Measure the actual polycarbonate rather than including any end closures or glazing components.

If the edge is covered by a trim, you may need to refer to the original installation documentation or have a competent installer identify the sheet.

Never walk directly on polycarbonate roofing to take measurements.

If the sheet thickness cannot be confirmed safely, don't guess. Incorrect thickness can affect how securely the panel fits into the existing glazing system.

Step 3: Check the Internal Structure of the Sheet

Two polycarbonate sheets can have the same overall thickness but different internal constructions.

For example, multiwall sheets may contain different numbers or arrangements of internal walls.

This can affect their appearance, rigidity, weight and thermal performance.

If you're replacing just one sheet, it's worth examining the exposed end of an existing panel.

Look at:

The number and arrangement of internal chambers.
Whether the flutes run straight along the sheet.
The overall thickness.
The appearance of the internal walls.

Take a clear photograph of the sheet's cross-section if possible.

Does the Internal Structure Have to Match Exactly?

Not necessarily, but the replacement must meet the relevant performance and installation requirements.

A different internal structure may also look noticeably different when installed beside older sheets.

For a single-panel replacement, matching the original construction as closely as practical can help maintain a consistent appearance.

However, visual similarity alone doesn't establish structural or glazing-system compatibility.

If the original specification is unknown, the replacement should be checked against the roofing system's requirements.

Step 4: Identify the Existing Polycarbonate Colour

Polycarbonate roofing is available in several colours and finishes.

The three main options in the PolycarbonateX range are clear, bronze and opal.

Clear Polycarbonate

Clear polycarbonate allows high levels of natural daylight through the roof.

It's commonly chosen for pergolas, lean-tos and garden structures where maintaining brightness is important.

Bronze Polycarbonate

Bronze polycarbonate has a tinted appearance.

It can help reduce glare and solar heat transmission compared with clear sheets of a similar specification.

Opal Polycarbonate

Opal polycarbonate has a white, light-diffusing appearance.

It provides softer, more evenly distributed daylight and can reduce glare.

Why Colour Matching Can Be Difficult

An older polycarbonate sheet may have changed appearance over time.

Surface dirt, weathering and differences between manufacturers can make a new sheet look noticeably different from the original.

Even when two products are both described as bronze or opal, their appearance and light transmission may vary.

If you're replacing one panel in a larger roof, consider whether a slight colour difference would be acceptable.

If appearance is particularly important, requesting a sample or comparing product specifications before ordering may help.

For more information, read our guide to clear, bronze and opal polycarbonate sheets.

Step 5: Measure the Correct Replacement Sheet Width

Getting the width right is essential, particularly when retaining existing glazing bars.

One of the most common mistakes is measuring the visible gap between the glazing bars and ordering a sheet to that exact dimension.

The visible opening isn't necessarily the correct finished sheet width.

Multiwall polycarbonate sheets usually extend beneath the retaining sections of the glazing bars.

The roofing system also requires suitable allowance for thermal expansion.

What's the Best Way to Establish the Width?

If the original sheet is available and was correctly installed, measuring its full width can provide a useful starting point.

However, don't automatically copy the dimensions of a panel that has previously slipped, bowed or been incorrectly fitted.

You should also identify:

The existing glazing bar system.
The distance between the supporting rafters.
The manufacturer's required sheet engagement.
The necessary thermal expansion clearances.

If you cannot confirm the correct width, consult the glazing system manufacturer or a competent installer before ordering.

Our guide to measuring a roof for polycarbonate sheets provides additional advice.

Step 6: Measure the Required Sheet Length

Once you've established the correct width, you'll need to determine the finished sheet length.

For a typical pitched roof, polycarbonate sheets run down the slope towards the gutter.

Measure the required length along the roof slope rather than simply measuring the horizontal projection.

You'll also need to account for the roofing system's requirements at the upper and lower ends.

These may include:

Required engagement beneath upper glazing components.
Clearance for thermal expansion.
The intended overhang at the eaves.
Sheet end closures.
Gutter positioning.

If you're replacing an existing panel that was correctly installed, its original full length can be a useful reference.

However, if the old roof has suffered drainage problems or incorrect overhang, the replacement dimensions may need adjusting.

For more information, read our guide to polycarbonate roof overhang.

Step 7: Check Whether the New Sheets Will Fit Your Glazing Bars

Even when you've identified the correct thickness and dimensions, you must confirm compatibility with the existing glazing system.

Different manufacturers use different glazing bar designs.

Some systems are intended for specific sheet thicknesses, while others can accommodate a range of compatible panels.

The important considerations include:

Correct sheet thickness.
Adequate sheet engagement.
Suitable gasket compression.
Secure sheet retention.
Required thermal movement.
Compatibility with the existing supporting structure.

What If You Don't Know the Glazing Bar Manufacturer?

Start by checking whether you have any original invoices, installation instructions or product labels.

If these aren't available, photographs of the glazing bars and their visible profiles may help identify the system.

However, don't assume that glazing bars from different manufacturers are interchangeable simply because they look similar.

If you're hoping to retain your existing system, read our dedicated guide:

Can You Replace Polycarbonate Sheets Without Replacing the Glazing Bars?

This explains what to check before deciding whether the original glazing components can be reused.

Step 8: Check the UV-Protected Side

Polycarbonate roofing sheets are manufactured with UV protection to help protect the material during outdoor exposure.

Depending on the product, UV protection may be applied to one or both sides.

Where a sheet has one-sided UV protection, the protected surface must face outwards towards the sun.

New sheets are generally supplied with protective film carrying installation information.

Follow the manufacturer's instructions carefully.

Can You Identify the UV Side on an Old Sheet?

Not always.

Once the original protective film has been removed, identifying the UV-protected side may be difficult without reliable manufacturer information.

If you're ordering new sheets, the important thing is to install them with the correct orientation.

Do not assume that the UV protection arrangement on an unidentified older sheet is the same as that of a new product.

What If Your Existing Polycarbonate Sheets Are No Longer Available?

This is a common problem with older roofs.

The original supplier may no longer trade, or the exact sheet specification may have been discontinued.

Fortunately, that doesn't automatically mean you need to replace the entire roof.

A suitable alternative may be available, provided it meets the requirements of the existing glazing system and supporting structure.

The most important factors are:

Compatible thickness.
Correct finished dimensions.
Suitable structural and performance specifications.
Appropriate UV protection.
Acceptable colour and appearance.
Compatibility with the existing glazing components.

If you cannot establish compatibility, replacing or modifying the glazing system may be necessary.

Can You Order Replacement Polycarbonate Sheets Cut to Size?

Yes.

Cut-to-size polycarbonate can be particularly useful when replacing sheets on an existing roof.

Older pergolas, lean-tos and carports don't always have dimensions that match standard sheet widths.

At PolycarbonateX, we offer cut-to-size multiwall polycarbonate sheets in 10mm, 16mm and 25mm thicknesses.

This can help when:

Replacing an individual damaged panel.
Matching an unusual sheet width.
Working with fixed rafter positions.
Replacing panels on an older roof.
Reducing unnecessary cutting during installation.

However, the finished dimensions must still account for the glazing system and required fitting clearances.

Cut-to-size sheets solve a dimensional problem; they don't automatically solve a compatibility problem.

What Photographs Should You Take Before Ordering Replacement Sheets?

If you're unsure what type of polycarbonate you currently have, photographs can be extremely useful.

Ideally, take the following:

Photograph	What it helps identify
Overall roof	Roof layout, sheet arrangement and application
Exposed sheet end	Thickness and internal chamber structure
Glazing bar close-up	Type of retaining system
Sheet colour in daylight	Approximate colour and finish
Roof edge and gutter	End closures and overhang arrangement
Existing product markings	Potential manufacturer or specification

If possible, include a ruler in photographs of the exposed sheet edge.

You should also record the required width, length and thickness separately.

Only take photographs and measurements from a safe position. Never stand or walk directly on polycarbonate sheets.

Common Mistakes When Ordering Replacement Polycarbonate

Ordering by Appearance Alone

Two sheets may look similar but have different thicknesses, internal structures or performance specifications.

Measuring Only the Visible Opening

The correct sheet width may include material concealed beneath the glazing bars.

Assuming All 16mm Sheets Are Identical

Sheets of the same nominal thickness can have different internal constructions and performance characteristics.

Ignoring Existing Glazing Bar Compatibility

A correctly sized sheet still needs to be securely retained by the existing roofing system.

Forgetting Thermal Expansion

Polycarbonate requires suitable clearance for movement as temperatures change.

Expecting an Exact Colour Match

New sheets may look different from older panels, even when both are sold under the same colour name.

Replacing a Sheet Without Investigating the Original Problem

If the old panel slipped, cracked or bowed because of an installation issue, the same problem may affect the replacement.

Frequently Asked Questions

How do I know what thickness my polycarbonate roof is?

Measure the exposed edge of the sheet, excluding any trims or closures. If the edge cannot be accessed safely, consult the original documentation or an installer.

Can I replace a 16mm polycarbonate sheet with another manufacturer's 16mm sheet?

Potentially, provided the replacement meets the glazing system's compatibility, dimensional and performance requirements. Matching thickness alone isn't sufficient.

Do replacement polycarbonate sheets need the same internal structure?

Not necessarily, but the replacement must meet the roofing system's requirements. Different internal structures may also create a visible difference between panels.

Can I replace just one polycarbonate roof panel?

Yes, in many cases. However, the new sheet must be compatible with the existing roof, and there may be a noticeable difference in appearance.

What if I don't know who manufactured my original roof?

You can start by identifying the sheet thickness, dimensions, construction and glazing system. Original paperwork or product markings may also help.

Can I replace clear polycarbonate with bronze or opal?

Potentially, provided the new sheet meets the required specification and is compatible with the existing glazing system. Changing colour will also affect the appearance and daylight entering the space.

Can I order replacement sheets to an exact width?

Yes. PolycarbonateX supplies cut-to-size multiwall sheets within the available product dimensions and specifications.

What if my replacement sheet doesn't match the existing glazing bars?

Don't force it into position. The correct sheet specification or a compatible glazing system must be established before installation.

Need Help Matching Replacement Polycarbonate Sheets?

Replacing an old polycarbonate roof panel doesn't always mean purchasing an entirely new roofing system.

In many cases, you can find suitable replacement sheets by identifying the original thickness, dimensions, colour and glazing requirements.

At PolycarbonateX, we supply British-made multiwall polycarbonate roofing sheets in 10mm, 16mm and 25mm thicknesses, with standard and cut-to-size options available.

Explore our:

Not sure which replacement sheets you need?

Contact the PolycarbonateX team with your measurements and photographs of the existing sheets and glazing bars.

We'll be happy to help identify suitable products from our range and explain what information is needed before ordering.

Where the existing roofing system is unidentified or uses proprietary glazing components, final compatibility may need to be confirmed by the original manufacturer or a competent installer.