Can You Fit Polycarbonate Roofing Sheets to a Metal Frame?

If you have a steel pergola, aluminium veranda, metal carport or garden shelter, you might be wondering whether you can install polycarbonate roofing sheets directly onto the existing structure.

The answer is yes. Polycarbonate roofing can be installed on suitable metal frameworks, provided the structure, glazing system and fixings are compatible.

Multiwall polycarbonate is lightweight, durable and available in different thicknesses and colours, making it an attractive roofing option for many outdoor structures.

However, installing polycarbonate onto metal rafters requires some different considerations compared with a traditional timber roof.

In this guide, we'll explain what to check before ordering, how glazing bars can be fitted to metal supports, which fixings you may need and how to avoid common installation mistakes.

Can You Install Polycarbonate Roofing on Steel or Aluminium?

Yes. Multiwall polycarbonate sheets can be installed on suitably designed steel and aluminium roof structures.

Common applications include:

• Steel-framed pergolas

• Aluminium verandas

• Metal carports

• Garden shelters

• Steel-framed canopies

• Outdoor seating areas

• Covered walkways

The most important consideration is whether the existing frame provides adequate support for the chosen roofing system.

Although polycarbonate is considerably lighter than glass, the framework must still withstand wind, snow and other loads.

You should also check that the metal rafters are correctly positioned to support the glazing bars and roofing sheets.

An existing metal structure isn't automatically suitable for roofing simply because it is strong enough to stand on its own.

Open pergolas and decorative metal frames may need additional structural assessment or modification before a solid roof covering is added.

Is Polycarbonate Suitable for a Metal Pergola?

Polycarbonate is a popular option for covering pergolas because it provides weather protection while allowing natural light into the space.

For a metal pergola, multiwall polycarbonate offers several advantages:

• Lightweight compared with glass

• Available in clear, bronze and opal finishes

• Resistant to normal outdoor weather exposure

• Available in different thicknesses

• Can be cut to suit individual roof dimensions

• Compatible with appropriately designed glazing systems

However, the metal structure must have a suitable roof pitch, adequate supports and secure connections.

Adding a solid roof also changes how wind acts on the structure. A pergola originally designed with open rafters may experience considerably greater wind uplift once roofing sheets are installed.

If you're still deciding which sheets to use, read our guide to choosing the best polycarbonate thickness for a pergola roof.

What Thickness Polycarbonate Should You Use on a Metal Frame?

The best thickness depends on the roof dimensions, supporting structure, required insulation and glazing system.

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

10mm Polycarbonate

10mm multiwall polycarbonate can be suitable for certain smaller roofing projects where the support spacing and glazing system meet the manufacturer's requirements.

It's a relatively lightweight option, but it generally provides less insulation and rigidity than thicker multiwall sheets.

16mm Polycarbonate

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

It offers a useful balance of strength, thermal performance and affordability.

For many domestic roofing projects, it's a sensible thickness to consider, provided the intended support arrangement is suitable.

25mm Polycarbonate

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

It may be worth considering for larger projects or where improved insulation is important.

However, thicker sheets don't remove the need for correctly positioned structural supports.

The chosen thickness must be suitable for the roof's design and the glazing system being used.

Explore our multiwall polycarbonate roofing sheets to compare the available thicknesses and colours.

Can You Fit Polycarbonate Glazing Bars to Metal Rafters?

Potentially, yes, but the glazing bars and fixings must be suitable for the particular metal structure.

Many polycarbonate roofing systems use glazing bars that run along the rafters and retain the edges of the sheets.

At PolycarbonateX, we supply PVC top-cap and aluminium-base glazing bars for suitable rafter-supported roofing applications.

These consist of an aluminium base and a PVC top cap.

On a typical timber installation, the aluminium base is secured to the supporting timber rafter using suitable screws.

For a metal installation, the fixing method will be different.

The installer must establish whether the glazing bar can be securely attached to the metal rafters without compromising the glazing system or the supporting structure.

Important considerations include:

• The metal rafter's width and shape

• The thickness of the metal

• Whether the rafter is hollow or solid

• The required screw type and fixing method

• The glazing bar manufacturer's instructions

• Corrosion compatibility between metals

• The loads acting on the completed roof

A hollow aluminium box section, for example, may require a different fixing arrangement from a thick steel support.

Do not assume that screws supplied for timber installation are suitable for metal rafters.

Where necessary, consult the glazing system manufacturer or a competent installer to confirm compatibility before purchasing.

For more information about glazing systems, read our guide to polycarbonate roofing accessories.

What Screws Should You Use for Polycarbonate Roofing on Metal?

The correct screws depend on the metal type, thickness, fixing arrangement and roofing system.

Fixings used with timber rafters are not necessarily suitable for steel or aluminium.

Metal roofing installations may require appropriately specified self-drilling screws, threaded fixings or another approved fastening method.

The correct fixing must provide adequate holding strength without damaging the glazing bar or metal framework.

When selecting screws, consider:

• Steel or aluminium substrate

• Metal thickness

• Required fixing penetration or engagement

• Corrosion resistance

• Screw head compatibility

• Required clamping force

• The roofing manufacturer's installation requirements

Stainless steel fixings may be appropriate in some applications, but contact between dissimilar metals can introduce corrosion concerns, particularly in exposed environments.

The complete fixing arrangement should be selected for the specific installation.

Do PolycarbonateX Glazing Bars Come with Metal Screws?

Our standard glazing bar installation arrangements are intended for suitable timber-rafter applications.

If you're installing the system onto a metal structure, you will need to confirm that the glazing bar is compatible and obtain suitable metal fixings separately.

We recommend checking the required screw specification with your installer or fixing supplier rather than substituting screws without considering the metal thickness and fixing requirements.

Can You Use Polycarbonate Fixing Buttons on Metal?

Fixing buttons can be used in suitable polycarbonate roofing installations to provide additional sheet restraint at specified fixing positions.

However, they must be installed in accordance with the sheet and fixing system requirements.

At PolycarbonateX, our fixing button packs include caps, washers and screws intended for timber.

If you're fixing into metal, the supplied timber screws will need to be replaced with suitable fixings for your structure.

The caps and washers may still be usable where the fixing arrangement is compatible, but the replacement screws must be suitable for both the substrate and the button assembly.

The screw head must work correctly with the button, and the completed fixing must retain the sheet without restricting its required thermal movement.

Fixing buttons should not be treated as a substitute for an appropriate glazing system or adequate structural support.

For more information, see our guide to installing polycarbonate fixing buttons.

How Far Apart Should Metal Rafters Be for Polycarbonate Roofing?

Rafter spacing depends on the chosen sheet thickness, glazing system and structural design.

The same basic principles apply whether your rafters are timber, steel or aluminium.

On a typical rafter-supported polycarbonate roof, the glazing bars run along the rafters, with the roofing sheets fitted between them.

The rafter spacing must therefore accommodate the sheet width and the required glazing bar clearances.

For example, a 980mm-wide polycarbonate sheet may be used with rafters at approximately 1,000mm centres in an appropriate glazing system, subject to the manufacturer's fitting specifications.

However, this is only a sheet-layout example.

It doesn't establish that the chosen metal rafters are structurally adequate or that the sheet span is suitable for the project's loads.

The final spacing must satisfy both the glazing system requirements and the structural design.

Read our polycarbonate roof rafter spacing guide for more information.

Can You Install Polycarbonate on Metal Purlins?

Some metal roof structures include horizontal purlins running across the rafters.

These can provide additional support beneath the roofing sheets where the roof system is designed to use them.

However, purlins don't automatically replace the need for glazing bars along the sheet edges.

Multiwall polycarbonate must be supported and retained in accordance with the manufacturer's installation requirements.

The position and height of the purlins also matter.

For example, purlins that sit flush with the top of the rafters may provide intermediate support, but they must not interfere with the glazing bars or create unwanted pressure points beneath the sheets.

Where metal purlins are used, consider the contact surfaces, required support spacing and allowance for thermal movement.

Do Polycarbonate Sheets Expand on a Metal Frame?

Yes. Polycarbonate expands and contracts as temperatures change.

This is particularly important on outdoor roofs exposed to direct sunlight.

Steel, aluminium and polycarbonate have different thermal expansion characteristics.

As a result, the roofing sheets and metal framework won't necessarily expand by the same amount.

A correctly designed glazing system must accommodate this movement.

Potential problems can occur if sheets are fitted too tightly or fixed in a way that prevents normal expansion.

These can include:

• Bowing or distortion

• Unwanted stress around fixings

• Movement-related noises

• Damage around fixing holes

• Pressure against glazing components

Always follow the manufacturer's requirements for expansion gaps, fixing holes and sheet engagement.

For a detailed explanation, see our guide to thermal expansion in polycarbonate roofing.

What Roof Pitch Do You Need for Polycarbonate on Metal?

A suitable roof pitch is essential for rainwater drainage.

Many multiwall polycarbonate roofing systems specify a minimum pitch of around 5°, although the exact requirement depends on the sheet and glazing system.

At 5°, the roof falls approximately 87mm for every metre of horizontal run.

For example, a roof with a 3m horizontal projection would require approximately 262mm of fall to achieve a 5° pitch.

This is a geometric calculation, not a substitute for the manufacturer's installation specification.

If your metal frame is almost flat, you may need to alter the support arrangement before fitting polycarbonate.

Do not assume that a low-pitch metal shelter is suitable for polycarbonate without checking the roofing system's minimum pitch.

You can find more information in our polycarbonate roof pitch guide.

Can You Cut Polycarbonate Sheets to Fit a Metal Frame?

Yes. Polycarbonate sheets can be cut to suit the dimensions of a suitable metal roofing structure.

This is useful for existing pergolas, shelters and carports where the rafter positions are already fixed.

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

Cut-to-size sheets can help reduce on-site cutting and minimise unnecessary waste.

Before ordering, measure:

1. The overall roof width.

2. The roof slope length.

3. The distances between glazing supports.

4. The required sheet widths.

5. The glazing system's fitting clearances.

6. Any necessary overhang at the eaves.

Remember that the distance between metal rafters is not necessarily the finished sheet width.

The glazing bar design and thermal expansion requirements must be taken into account.

If you're unsure how to measure, see our guide to measuring a roof for polycarbonate sheets.

How Do You Seal the Ends of Polycarbonate Sheets?

Multiwall polycarbonate contains internal chambers, sometimes called flutes.

These chambers need to be protected from dirt, insects and excessive moisture entering the sheet.

For a typical pitched installation, the upper end of the sheet is sealed with suitable impermeable foil tape, while the lower end uses breathable anti-dust tape to allow moisture to escape.

Sheet end closures are then fitted where required by the roofing system to protect the taped edges and provide a neat finish.

The lower end must be detailed so that drainage isn't obstructed.

The correct arrangement depends on the sheet orientation and manufacturer's installation instructions.

Explore our polycarbonate roofing accessories for compatible sealing tapes and end closures.

Common Mistakes When Installing Polycarbonate on Metal

Using Timber Screws in Metal Rafters

Timber screws aren't automatically suitable for steel or aluminium.

Use fixings specified for the actual metal substrate and glazing system.

Assuming Any Metal Frame Can Support Roofing

An open pergola or decorative metal structure may not have been designed to withstand the additional wind and snow loads of a covered roof.

Fitting Sheets Too Tightly

Polycarbonate requires allowance for thermal expansion.

Insufficient clearance can lead to distortion and stress around the glazing components.

Using Incompatible Glazing Bars

Not every glazing bar system is suitable for every metal profile.

Check the manufacturer's requirements before ordering.

Ignoring Roof Pitch

Insufficient roof pitch can contribute to poor drainage and water-related problems.

Incorrect Rafter Spacing

Metal rafters must be positioned to support the roofing system correctly, while also meeting the structural design requirements.

Forgetting Sheet End Protection

Leaving multiwall sheet flutes unprotected can allow dirt and insects to enter.

Use the appropriate sealing tapes and closures.

Frequently Asked Questions

Can you screw polycarbonate roofing directly onto metal?

Some roofing systems allow sheets to be mechanically fixed to metal supports using suitable fixings and sealing arrangements. However, multiwall polycarbonate must be installed according to the chosen sheet and glazing system's requirements. Direct fixing isn't a universal substitute for glazing bars.

Can you put polycarbonate on an aluminium pergola?

Yes, if the aluminium pergola is structurally suitable and the glazing system is compatible with its supporting profiles.

Can you install polycarbonate roofing on a steel frame?

Yes. Steel frames can support polycarbonate roofing when the structure, glazing bars and fixings are correctly specified.

Do you need special screws for metal rafters?

Yes. Fixings must be suitable for the type and thickness of metal and the roofing system being installed. Standard timber screws shouldn't be assumed suitable.

Can you use fixing buttons with metal rafters?

Potentially, provided the fixing system is compatible. PolycarbonateX fixing button packs include timber screws, so suitable replacement metal fixings must be sourced separately.

Do you need glazing bars on a metal frame?

For typical rafter-supported multiwall polycarbonate installations, suitable glazing bars are used to retain the sheet edges. Other purpose-designed roofing systems may use different arrangements.

Is 16mm polycarbonate suitable for a metal pergola?

16mm multiwall polycarbonate can be a suitable option, provided the support spacing, glazing system and overall structural design meet the relevant requirements.

Can you fit polycarbonate roofing to an existing metal carport?

Yes, potentially. The existing carport must first be assessed to confirm that it can safely support the proposed roof covering and resist the applicable wind and snow loads.

Planning a Polycarbonate Roof for a Metal Structure?

Polycarbonate roofing can be an excellent option for steel and aluminium pergolas, carports, verandas and garden shelters.

The key is ensuring that the metal framework, glazing bars and fixings are compatible before ordering your materials.

At PolycarbonateX, we supply a range of multiwall polycarbonate roofing sheets, cut-to-size sheets and roofing accessories for suitable installations.

You can also use our polycarbonate roof calculator to help estimate the roofing materials required.

Need advice before ordering?

Contact the PolycarbonateX team with your roof measurements, intended sheet thickness and details of your metal framework.

If possible, include the size and shape of your rafters, whether they're steel or aluminium, and the roof pitch.

We can help you identify suitable products from our range, while the structural suitability of the frame and any specialist metal fixing requirements should be confirmed by a competent installer or relevant manufacturer.