Why Do Polycarbonate Sheets Expand and Contract? Complete Guide

If you've ever installed or researched polycarbonate roofing, you've probably come across the term thermal expansion.

Polycarbonate naturally expands as its temperature rises and contracts as it cools. This movement is a normal characteristic of the material and needs to be considered when designing and installing a polycarbonate roof.

If a sheet is fitted too tightly or its movement is unnecessarily restricted, temperature changes can place stress on the sheet and roofing system.

This can contribute to problems such as bowing, distorted sheets, stress around fixings and problems with the glazing system.

In this guide, we'll explain why polycarbonate expands, how much it can move and how a roofing system accommodates that movement.

Why Does Polycarbonate Expand and Contract?

Polycarbonate is a thermoplastic.

Like many materials, its dimensions change slightly as its temperature changes.

When the sheet becomes warmer:

It expands.

When it becomes cooler:

It contracts.

The important point is that the temperature of the sheet itself matters, not simply the outside air temperature.

A polycarbonate roof exposed to direct sunlight can become considerably warmer than the surrounding air, particularly during summer.

The roof therefore needs to be installed in a way that allows this normal movement to take place.

How Much Does Polycarbonate Expand?

The amount of movement depends on:

• The length of the sheet
• The change in sheet temperature
• The thermal expansion characteristics of the particular polycarbonate

A commonly quoted coefficient of linear thermal expansion for polycarbonate is approximately 0.065–0.07mm per metre for every 1°C change in temperature.

That means there isn't one fixed answer such as “polycarbonate expands 3mm per metre”.

The amount depends on the temperature change.

Example

Using approximately 0.065mm per metre per °C, consider a 3m sheet experiencing a 40°C change in temperature:

0.065 × 3 × 40 = 7.8mm

So the theoretical change in length would be approximately 7.8mm across that temperature difference.

This example is intended to demonstrate why installation clearance matters rather than prescribe a universal expansion gap.

Always follow the fitting requirements for the particular sheet and glazing system being installed.

Does Polycarbonate Expand in Width as Well as Length?

Yes.

Thermal expansion occurs across the dimensions of the sheet, not only along its length.

This is particularly important when fitting sheets between glazing bars.

The sheet shouldn't be cut so tightly that there is no room for it to move as its temperature changes.

The required sheet width therefore needs to account for the glazing system and appropriate fitting clearance.

This is one reason why sheet width and rafter centres aren't necessarily the same measurement.

See our Polycarbonate Rafter Spacing Guide for more information.

Why Does Thermal Expansion Matter on a Roof?

A correctly designed polycarbonate roofing system accommodates normal thermal movement.

Problems can arise when that movement is unnecessarily restricted.

For example, a sheet fitted tightly between fixed points can expand as it heats up but have nowhere for that additional length or width to go.

This can place pressure on:

• The polycarbonate sheet
• Glazing bars
• Fixing points
• End details
• Adjacent components

Over repeated heating and cooling cycles, an incorrectly installed roof may begin to show signs of stress.

What Problems Can Restricted Expansion Cause?

Potential symptoms include:

Bowing or distortion

A sheet that has insufficient room to expand may begin to bow rather than move naturally within the roofing system.

Stress around fixing points

Incorrectly positioned or overtightened through-sheet fixings can concentrate stress around the fixing.

Movement within the glazing system

Incorrectly sized sheets may not remain correctly positioned within the intended glazing arrangement.

Creaking or clicking

Some noise as materials heat and cool can occur on roofing systems.

Persistent or excessive noises may be worth investigating if accompanied by visible signs of restricted movement.

Problems with weatherproofing

If thermal movement causes components to distort or move outside their intended positions, it can potentially affect the roof's weather resistance.

However, leaks have many possible causes, so thermal expansion shouldn't automatically be assumed to be responsible.

How Does a Polycarbonate Roofing System Allow for Expansion?

Several parts of the installation work together.

1. Correct Sheet Width

The sheet needs sufficient room within the glazing system to accommodate normal movement.

It shouldn't be forced tightly between glazing bars.

For example, with a typical PolycarbonateX arrangement:

980mm sheet width → approximately 1000mm rafter centres

The difference accommodates the glazing system and appropriate fitting clearance.

The exact arrangement depends on the roofing system being used.

2. Correct Sheet Length

Thermal movement also needs to be considered along the length of the sheet.

Longer sheets can experience greater total dimensional change than shorter sheets exposed to the same temperature variation.

The upper and lower roof details therefore need to be planned so that the sheet isn't trapped tightly between fixed points.

See our Polycarbonate Sheet Sizing Guide before determining the final sheet dimensions.

3. Glazing Bars

Glazing bars join adjacent sheets while forming part of a roofing system designed to accommodate the sheets correctly.

PolycarbonateX uses PVC top cap and aluminium base glazing bars.

The aluminium base is fixed to the supporting rafter, while the polycarbonate sheets are positioned within the system with the appropriate clearance before the PVC top cap is fitted.

The sheet shouldn't simply be clamped so tightly that normal thermal movement is prevented.

4. Correct Use of Fixing Buttons

Fixing buttons may be used where additional sheet support is required.

Because these fix through the polycarbonate sheet, they need to be installed correctly.

A fixing through a polycarbonate sheet shouldn't unnecessarily restrict the sheet's ability to respond to temperature changes.

Follow the installation requirements for the fixing system, including any specified hole clearance and tightening requirements.

5. Don't Overtighten Fixings

Overtightening can place unnecessary pressure on the sheet or roofing components.

Fixings need to secure the roofing system correctly without crushing components or unnecessarily restricting the movement they are designed to accommodate.

Does Polycarbonate Expansion Mean the Sheets Should Be Loose?

No.

Allowing for thermal movement doesn't mean sheets should be able to move freely around the roof.

The purpose of a correctly designed glazing system is to secure the sheets while accommodating their normal dimensional changes.

A sheet that is excessively undersized can create a different set of problems.

The aim is therefore the correct fit — neither tightly trapped nor inadequately engaged with the glazing system.

Why Cut-to-Size Dimensions Matter

Cut-to-size polycarbonate can be useful where an existing structure has rafter centres that don't conveniently match standard sheet dimensions.

However, “cut to size” doesn't mean cutting the sheet to exactly the clear opening with no allowance.

The dimensions still need to account for:

• Glazing bars
• Rafter centres
• Required engagement
• Thermal movement
• Outside-edge arrangement

PolycarbonateX offers 10mm, 16mm and 25mm multiwall polycarbonate cut to your required dimensions, within the available manufacturing limits.

For new structures, standard sheet sizes can sometimes be more economical if the rafters are designed around them.

Choosing between standard and cut-to-size sheets

Does Polycarbonate Expand More in Summer?

Generally, a sheet will be larger when it is hotter and smaller when it is colder.

However, the important factor is the actual temperature change experienced by the sheet.

A roof exposed to direct sunlight can heat substantially during the day and cool again later.

Over the course of a year in the UK, a roof may therefore experience repeated cycles of expansion and contraction.

The roofing system needs to accommodate this movement throughout its service life rather than only on the day it is installed.

Should I Install Polycarbonate Differently in Hot or Cold Weather?

Installation temperature can affect the starting dimensions of the sheet.

A sheet installed when it is particularly cold may subsequently expand as it becomes warmer, while a hot sheet may contract as it cools.

This is another reason to follow the manufacturer's specified installation clearances rather than attempting to make the sheet fit tightly during installation.

Do the Flutes Affect Expansion?

Multiwall polycarbonate sheets contain internal channels, commonly called flutes.

The flutes should normally run down the roof slope, from the high point towards the lower edge.

This orientation is important primarily for drainage of moisture within the channels and correct installation of the sheet.

It shouldn't be confused with providing the expansion allowance itself.

Does Roof Pitch Affect Thermal Expansion?

Roof pitch doesn't prevent polycarbonate from expanding or contracting.

Polycarbonate will respond to temperature changes regardless of the roof pitch.

However, correct pitch is important for a different reason: effective rainwater drainage.

PolycarbonateX generally recommends a minimum roof pitch of 5°, subject to the requirements of the particular roofing system and structure.

See our Polycarbonate Roof Pitch Guide for more information.

What Should I Do If My Polycarbonate Sheets Are Bowing?

Don't automatically assume thermal expansion is the cause.

Bowing can potentially result from several factors, including:

• Incorrect sheet dimensions
• Insufficient fitting clearance
• Supporting structure
• Incorrect rafter spacing
• Installation problems
• Excessive loading
• Damage or deformation

Inspect the complete roof rather than immediately cutting or modifying the sheets.

If the roof has only recently been installed and something appears wrong, check the installation instructions and measurements before making permanent alterations.

What If a Sheet Is Too Tight Between the Glazing Bars?

If the sheet has genuinely been installed without the required clearance, the correct solution may involve removing and refitting it to the appropriate dimensions.

Don't simply force the sheet further into the system or attempt to solve visible bowing by adding more restrictive fixings.

The cause should be identified first.

What If the Sheet Has Already Cracked?

A cracked roofing sheet should be inspected to establish the cause.

If the sheet has suffered significant structural damage, replacement may be necessary.

Before replacing it, check whether the original problem was caused by incorrect dimensions, inappropriate fixing, impact damage or another installation issue.

Otherwise, installing a replacement in exactly the same way may reproduce the problem.

Does Thickness Affect Thermal Expansion?

10mm, 16mm and 25mm polycarbonate all need appropriate allowance for thermal movement.

Choosing a thicker sheet doesn't remove the requirement to accommodate expansion and contraction.

Thickness should instead be selected according to the required roofing performance and structure.

See our Polycarbonate Sheet Thickness Guide for guidance on choosing between 10mm, 16mm and 25mm.

What Accessories Are Needed?

Depending on the roof design, a complete multiwall system may include:

• PVC top cap & aluminium base glazing bars
• Sealing tape kit
• Sheet end closures
• Suitable fixings
• Fixing buttons where required
• Side flashing
• Wall flashing where required
• Glazing-bar end caps

These components have different functions and shouldn't be viewed solely as expansion-control products.

See our Complete Polycarbonate Roof Kit Guide for an explanation of the complete system.

Use the PolycarbonateX Roof Kit Calculator

If you're planning a new roof, the PolycarbonateX Roof Kit Calculator can help calculate the sheets and roofing components required for your roof dimensions and configuration.

Planning the sheet layout, glazing bars and supporting structure together helps avoid dimensional problems later in the installation.

Conclusion

Polycarbonate naturally expands when its temperature rises and contracts as it cools.

The amount of movement isn't a fixed number per metre. It depends on the sheet dimensions and the temperature change it experiences.

As an illustration, using a thermal expansion coefficient of approximately 0.065mm per metre per °C, a 3m sheet experiencing a 40°C temperature change could change in length by approximately 7.8mm.

A correctly designed polycarbonate roofing system accommodates this normal movement through the appropriate sheet dimensions, glazing system, fitting clearances and fixing methods.

The key isn't to prevent polycarbonate from moving.

It's to install it so that normal thermal movement can occur without compromising the roof.