October 2026

Can You Install Polycarbonate Roofing Over an Existing Roof?

If you're planning to upgrade an existing shed, pergola, carport or lean-to roof, you might be wondering whether you can install polycarbonate roofing sheets directly over the existing roof covering.

Perhaps your current roof is covered with felt, corrugated sheets or another material that's starting to deteriorate. Rather than removing everything, installing polycarbonate over the top might seem like a quicker and easier solution.

But is it actually a good idea?

In most cases, removing the existing roof covering before installing polycarbonate is the better approach.

Although it may be technically possible to construct a separate supporting system above an existing roof, installing multiwall polycarbonate directly over old roofing materials can create problems with support, drainage, ventilation and weatherproofing.

In this guide, we'll explain when installing polycarbonate over an existing roof might be possible, why replacing the original covering is usually preferable and what to check before ordering your new roofing materials.

Can You Put Polycarbonate Sheets Over an Existing Roof?

It depends on the existing roof construction, the condition of the supporting structure and the type of polycarbonate roofing system you intend to install.

Multiwall polycarbonate sheets are normally installed as part of a suitable roofing system, with correctly positioned supports, compatible glazing bars and adequate allowance for thermal expansion.

Simply laying new polycarbonate sheets over an existing roof covering isn't generally an appropriate installation method.

Potential problems include:

Inadequate support beneath the polycarbonate sheets.
Difficulty securing glazing bars correctly.
Moisture becoming trapped between roof coverings.
Poor drainage and water accumulation.
Restricted ventilation.
Additional loading on the existing structure.
Problems allowing for thermal expansion.
Difficulty inspecting or repairing the original roof.

The important distinction is between installing polycarbonate over an existing roof covering and installing polycarbonate onto an existing roof structure.

The second option is often much more practical.

If the existing rafters, beams and supports are in good condition and suitable for the proposed roofing system, you may be able to retain them while replacing the old covering with new polycarbonate sheets.

Can You Install Polycarbonate Roofing Over a Felt Roof?

Felt roofing is commonly found on sheds, garages, garden buildings and some flat or low-pitched roofs.

If the felt is deteriorating, covering it with polycarbonate might seem like an easy way to stop leaks.

However, we wouldn't generally recommend installing multiwall polycarbonate directly over an existing felt roof.

Why Is Installing Polycarbonate Over Felt Problematic?

Felt roofs are often installed over continuous timber decking, such as plywood or OSB.

Multiwall polycarbonate roofing systems, on the other hand, typically require an appropriate arrangement of supporting rafters, glazing bars and sheet clearances.

Installing polycarbonate directly over felt-covered decking can introduce several problems.

Moisture and condensation

Moisture may become trapped between the existing roof and the new covering, particularly where ventilation and drainage haven't been properly designed.

Incompatible fixing arrangements

Standard multiwall glazing bars are not intended to be installed indiscriminately over old felt roofing.

They require suitable structural support and appropriate fixings.

Insufficient roof pitch

Many felt roofs have relatively shallow pitches, while multiwall polycarbonate systems often require a greater minimum fall.

Hidden deterioration

If the existing roof has been leaking, the timber decking or supporting structure may already have suffered moisture damage.

Covering the roof without investigating its condition could conceal an underlying problem.

What's the Better Alternative?

If you want to replace a felt roof with polycarbonate, the better approach is generally to remove the existing covering and assess the structure underneath.

Depending on the design, you may also need to remove the decking or modify the supporting framework.

The aim is to create a roof structure suitable for the chosen polycarbonate sheets and glazing system.

Remember that replacing an opaque felt roof with polycarbonate will also change the amount of light entering the space, which may be an important reason for considering the upgrade.

Can You Install Polycarbonate Over Corrugated Roofing Sheets?

Corrugated roofing is commonly used on carports, sheds, shelters and agricultural buildings.

Existing roofs may be covered with corrugated plastic, metal, bitumen or fibre-cement sheets.

Although it might be tempting to place new polycarbonate over the existing corrugated material, this isn't generally the preferred approach.

Why Shouldn't You Simply Cover Corrugated Roofing?

Corrugated sheets have raised profiles and valleys rather than a continuous flat supporting surface.

This can make it difficult to provide suitable support for multiwall polycarbonate and its glazing bars.

Can You Reuse the Existing Roof Frame?

Potentially, yes.

If the existing roof has suitable rafters or other structural supports, it may be possible to remove the old corrugated sheets and install a compatible polycarbonate roofing system.

However, the frame must be assessed for structural suitability, including wind uplift and snow loading.

You should also check whether the existing supports are positioned correctly for the new glazing bars.

For more information, read our polycarbonate roof rafter spacing guide.

Important: Some older corrugated roofs contain asbestos cement. If the material is unidentified or asbestos is suspected, do not disturb, drill or remove it until it has been appropriately assessed. Specialist handling or removal may be required.

Can You Replace an Existing Shed Roof with Polycarbonate?

Yes, replacing an existing shed roof with polycarbonate may be possible, provided the structure and roofing system are suitable.

This can be particularly appealing if you want to introduce more natural light into a garden workshop, potting shed or storage building.

However, the existing roof design needs careful consideration.

Many traditional sheds have relatively shallow roof pitches or continuous timber decking.

These arrangements aren't automatically suitable for multiwall polycarbonate roofing.

Before proceeding, check:

Whether the roof has sufficient pitch.
Whether the rafters can support the proposed roofing system.
Whether additional structural supports are required.
How the glazing bars will be secured.
Whether the shed walls and connections can resist the new roof loads.
How rainwater will drain away from the roof.

You should also consider whether replacing the entire opaque roof with transparent or translucent panels is appropriate for the building.

For some projects, incorporating a properly designed rooflight may be more suitable than replacing the whole covering.

Can You Add Polycarbonate Roofing to an Existing Pergola?

Yes, an existing pergola may be suitable for polycarbonate roofing, but the structure must be assessed before adding a solid roof covering.

Unlike a pergola designed purely to provide shade, a roofed pergola needs to withstand additional environmental loads.

Wind uplift is particularly important.

A solid roof can experience substantial upward forces during windy conditions, even when the roofing material itself is relatively lightweight.

Before installing polycarbonate, check:

The condition of the existing timber or metal frame.
Whether the structure was designed to carry roofing.
The spacing and size of the rafters.
The suitability of the beams and posts.
The strength of structural connections.
The adequacy of the foundations.
Whether the roof has sufficient pitch.

If the structure is suitable, polycarbonate sheets can be installed using an appropriate glazing system.

Our guide to installing a polycarbonate pergola roof explains the installation process in more detail.

Can You Install Polycarbonate Over an Existing Metal Roof?

Installing polycarbonate directly over existing metal roofing sheets is generally not recommended without a purpose-designed supporting system.

Metal roofing expands and contracts with temperature changes, and its profile may not provide the support required by multiwall polycarbonate.

The two materials also have different movement characteristics.

Attempting to fix polycarbonate directly over metal sheets could create problems with:

Thermal expansion.
Fixing security.
Moisture accumulation.
Drainage.
Glazing bar support.
Weatherproofing.

However, replacing existing metal roofing with polycarbonate may be possible if the supporting framework is suitable.

Steel and aluminium structures can support polycarbonate roofing when the glazing system, fixings and structural design are compatible.

For more information, read our guide to installing polycarbonate roofing on metal frames.

Is It Better to Remove the Existing Roof Before Installing Polycarbonate?

In most domestic applications, yes.

Removing the old covering allows you to inspect the structure and install the new roofing system correctly.

There are several advantages.

1. You Can Inspect the Existing Structure

Removing the old covering provides an opportunity to check for damaged timber, corrosion, loose fixings and other problems.

These issues may otherwise remain hidden beneath the new roof.

2. You Can Position the Glazing Bars Correctly

Multiwall polycarbonate requires suitable support along the sheet edges.

Starting with an exposed framework makes it easier to position rafters and glazing bars correctly.

3. You Can Improve the Roof Pitch

If the existing roof doesn't have sufficient fall, removing the covering may allow the structure to be modified.

A suitable pitch helps rainwater drain towards the gutter and reduces the likelihood of standing water.

Read our polycarbonate roof pitch guide for more information.

4. You Can Install the Correct Sealing Components

Multiwall polycarbonate sheets require suitable sealing tapes, end closures and glazing components.

These are easier to install correctly when the roofing system is accessible.

5. You Avoid Concealing Existing Problems

Installing new roofing over an old covering may hide leaks or deterioration without resolving the underlying cause.

Removing the existing material gives you a better opportunity to identify and address those issues.

Can You Keep the Existing Rafters When Replacing a Roof with Polycarbonate?

Yes, provided the existing rafters are structurally suitable and compatible with the proposed roofing system.

This is often one of the most important considerations for customers replacing an old roof.

If the existing framework is sound, retaining it may reduce the amount of reconstruction required.

However, several measurements need to be checked.

Rafter Spacing

The distance between rafters must suit the selected sheet widths and glazing system.

For example, a 980mm-wide polycarbonate sheet may be used with rafters at approximately 1,000mm centres in a compatible system.

However, the exact spacing depends on the glazing bar design, sheet fitting allowances and manufacturer's instructions.

This is a layout example rather than confirmation that the spacing is structurally suitable.

Rafter Width

The rafters must provide an appropriate fixing surface for the selected glazing bars.

If the existing rafters are too narrow or incorrectly positioned, modifications may be necessary.

Rafter Condition

Timber should be checked for rot, splitting or other deterioration.

Metal rafters should be inspected for corrosion, damaged connections or deformation.

Structural Suitability

The rafters must be suitable for the loads acting on the completed roof.

The fact that they supported the previous covering doesn't automatically mean they're suitable for a new roofing system.

If you're planning a timber roof, our guide to choosing timber sizes for polycarbonate roofing explains the main structural considerations.

What Thickness Polycarbonate Should You Use When Replacing an Existing Roof?

The correct thickness depends on the roof dimensions, intended use, support spacing and glazing system.

At PolycarbonateX, we supply 10mm, 16mm and 25mm multiwall polycarbonate roofing sheets.

10mm Polycarbonate

10mm multiwall polycarbonate can be a practical choice for suitable smaller roofing applications.

It's lightweight and offers a cost-effective way to introduce natural light into a structure.

However, the support arrangement must meet the sheet manufacturer's requirements.

16mm Polycarbonate

16mm polycarbonate is a popular choice for pergolas, carports, lean-tos and verandas.

It offers a balance of rigidity, insulation and affordability.

For many domestic roof replacement projects, it's a useful option to consider.

25mm Polycarbonate

25mm multiwall polycarbonate generally offers improved thermal insulation and greater rigidity compared with thinner sheets of otherwise comparable specification.

It may be worth considering where insulation or additional stiffness is important.

However, thicker sheets still require suitable structural support and compatible glazing bars.

You can compare our available options in the multiwall polycarbonate roofing sheets collection.

Can You Use Cut-to-Size Polycarbonate for an Existing Roof?

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

Unlike a new structure, where rafter positions can often be planned around standard sheet widths, an existing roof may have fixed dimensions.

Ordering sheets cut to your required measurements can help reduce on-site cutting and unnecessary waste.

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

Before ordering, you'll need to establish:

The overall roof width.
The roof slope length.
The distance between glazing supports.
The correct sheet width for the glazing system.
The required thermal expansion clearances.
Any necessary eaves overhang.

Remember that the gap between rafters isn't necessarily the finished width of the polycarbonate sheet.

The glazing system and its fitting requirements must be taken into account.

For help with measurements, read our guide to measuring a roof for polycarbonate sheets.

What Accessories Do You Need When Replacing a Roof with Polycarbonate?

A complete multiwall polycarbonate roofing installation normally requires more than the sheets themselves.

Depending on the roof design, you may need several accessories.

Glazing Bars

Glazing bars secure the edges of the polycarbonate sheets and help create weather-resistant joints.

The correct type depends on the supporting structure and chosen sheet thickness.

Sheet End Closures

End closures protect the exposed ends of multiwall polycarbonate and provide a neat finish.

Sealing Tapes

Suitable sealing tapes help prevent dirt and insects entering the internal chambers of the sheets.

For a typical pitched installation, impermeable foil tape is used at the upper end and breathable anti-dust tape at the lower end, subject to the manufacturer's instructions.

Wall Flashing

Where the roof meets an existing wall, an appropriate flashing detail is needed to prevent water entering the junction.

Fixing Buttons

Fixing buttons may be used where specified by the roofing system to provide additional sheet restraint.

They must be installed using suitable fixings and the required allowances for thermal movement.

Explore our polycarbonate roofing accessories collection for glazing bars, sealing tapes, closures and fixings.

Common Mistakes When Replacing an Existing Roof with Polycarbonate

Installing Sheets Directly Over Damaged Roofing

Covering an existing problem doesn't necessarily resolve it.

Damaged or deteriorating roofing materials should be properly assessed before work begins.

Assuming the Existing Frame Is Suitable

An existing roof structure may need modification before it can support a new polycarbonate system.

Ignoring Roof Pitch

A roof designed for one covering may not have sufficient pitch for another.

Always check the new roofing system's requirements.

Using Incompatible Glazing Bars

Glazing bars must suit the sheet thickness, supporting structure and installation arrangement.

Forgetting Thermal Expansion

Polycarbonate expands and contracts with temperature changes.

Incorrectly fitted sheets can experience stress, distortion or movement-related problems.

Ordering Sheets Before Checking Measurements

Existing roof dimensions don't always correspond to standard polycarbonate sheet sizes.

Measure carefully and allow for the glazing system before ordering.

Frequently Asked Questions

Can you put polycarbonate roofing directly over felt?

It's generally not recommended to install multiwall polycarbonate directly over felt roofing. Removing the old covering and preparing a suitable supporting structure is usually preferable.

Can you install polycarbonate over corrugated sheets?

Installing multiwall polycarbonate directly over corrugated roofing isn't generally advisable without a purpose-designed supporting system. Replacing the old sheets may be a better option.

Can you put polycarbonate over a shed roof?

You may be able to replace an existing shed roof with polycarbonate, provided the structure, pitch and glazing system are suitable.

Can you install polycarbonate over metal roofing?

A separately designed system may be possible, but directly fixing multiwall polycarbonate over existing metal roofing isn't generally recommended.

Can you reuse existing roof rafters?

Yes, if the rafters are structurally suitable, correctly positioned and compatible with the new roofing system.

Do you need to remove old roofing before installing polycarbonate?

In most domestic roof replacement projects, removing the existing covering is the preferable approach. It allows the supporting structure to be inspected and the new roofing system to be installed correctly.

Can you replace an existing roof with 16mm polycarbonate?

Yes, 16mm multiwall polycarbonate can be suitable for many roofing applications, provided the structural supports, glazing system and installation meet the relevant requirements.

Can you order polycarbonate sheets to fit an existing roof?

Yes. PolycarbonateX supplies cut-to-size multiwall polycarbonate sheets in 10mm, 16mm and 25mm thicknesses, allowing customers to order suitable rectangular sheets to their required dimensions.

Replacing an Existing Roof with Polycarbonate? We Can Help

Installing polycarbonate over an existing roof covering might initially seem like a convenient solution, but it isn't always the most practical or reliable approach.

For most domestic projects, removing the existing covering and installing polycarbonate onto a suitable supporting structure is the better option.

This allows the new roofing system to be correctly supported, secured and weatherproofed.

At PolycarbonateX, we supply a range of roofing materials for suitable pergolas, lean-tos, carports, canopies and other outdoor structures.

Explore our:

You can also use our polycarbonate roof calculator to help estimate the materials required for your project.

Not sure whether your existing roof structure is suitable?

Contact the PolycarbonateX team with your roof measurements, photographs and details of the existing framework.

We'll be happy to advise on suitable sheets and accessories from our range. Where structural modifications or specialist roofing arrangements are required, these should be assessed by a competent professional before installation.