What Size Polycarbonate Sheets Should I Use? Complete Sizing Guide

Choosing the correct polycarbonate sheet size is an important part of planning a pergola, lean-to, veranda, canopy or carport roof.

The right size depends on more than the overall dimensions of the roof. You also need to consider the rafter centres, glazing-bar positions, roof projection, sheet thickness and allowance for thermal movement.

If you're building a new structure, planning the sheet layout before fixing the rafters can make installation considerably easier and help reduce unnecessary cutting and waste.

This guide explains how to choose the correct width and length for multiwall polycarbonate roofing sheets.

Understanding Polycarbonate Sheet Dimensions

Polycarbonate roofing sheets have two main dimensions:

Sheet width

The width runs across the roof, from one glazing-bar position towards the next.

The correct sheet width is closely related to your rafter centres and glazing system.

Sheet length

The length normally runs down the roof slope, from the high point of the roof towards the gutter or lower edge.

The internal flutes of multiwall polycarbonate should also run in this direction, allowing moisture within the channels to drain towards the lower end.

Which polycarbonate sheet sizes are easiest to transport and install?

How Do I Choose the Correct Polycarbonate Sheet Width?

Multiwall polycarbonate sheets are normally installed between glazing bars, with the glazing bars positioned over the supporting rafters.

The important point is:

Sheet width and rafter centres aren't necessarily the same measurement.

The sheet needs sufficient room to sit correctly within the glazing system while allowing for natural thermal expansion and contraction.

For example, with a typical PolycarbonateX arrangement:

980mm sheet width → approximately 1000mm rafter centres

The additional space accommodates the glazing system and required fitting clearance.

If you're constructing a new roof, it is therefore usually better to choose your intended sheet layout first and position the rafters accordingly.

How Do I Measure Existing Rafter Centres?

If the roof structure already exists, measure from the centre of one rafter to the centre of the next.

Don't simply measure the clear opening between the inside edges of two rafters.

Measure several positions across the roof, particularly on an older structure. Existing rafters aren't always perfectly evenly spaced.

Once you know the actual rafter centres, you can determine whether a standard sheet size is suitable or whether cut-to-size sheets would provide a better fit.

For more detail, see our Polycarbonate Rafter Spacing Guide.

What If My Rafter Centres Don't Match a Standard Sheet Size?

You don't necessarily need to rebuild the roof.

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

Cut-to-size sheets are particularly useful for:

• Existing roof structures

• Unusual rafter centres

• Replacement roofs

• Bespoke pergolas and lean-tos

• Projects where standard sheet widths would create significant waste

Where standard sheet sizes suit your roof, however, they can be a more economical option because they make efficient use of standard sheet dimensions.

The best option depends on the particular roof rather than cut-to-size automatically being better for every project.

How Do I Choose the Correct Polycarbonate Sheet Length?

Sheet length normally follows the roof projection down the slope.

For a lean-to roof, for example, this is the distance from the upper end near the wall to the lower roof edge, measured along the slope rather than simply using the horizontal projection.

The required sheet length will depend on the roof design and how the sheet finishes at both the upper and lower edges.

You should consider:

• The actual sloping roof measurement

• Wall or upper-edge detail

• Lower supporting beam

• Gutter position

• Required sheet overhang

• End closures

• Thermal movement

Don't automatically order a sheet using only the horizontal projection of the roof.

How Much Should Polycarbonate Overhang the Gutter?

The lower end of the sheet normally extends beyond its final support sufficiently for rainwater to discharge cleanly towards the gutter.

However, the appropriate overhang depends on the roof and gutter arrangement.

Avoid excessive unsupported overhang, as the end of the sheet still needs to remain properly supported.

When planning the sheet length, consider the position of the final support and gutter together rather than simply adding a fixed amount to every roof.

What Thickness Polycarbonate Should I Use?

For general roofing applications, PolycarbonateX supplies 10mm, 16mm and 25mm multiwall polycarbonate.

10mm Polycarbonate

A lightweight and economical option for applications such as pergolas, canopies and lean-to roofs where the supporting structure provides appropriate support.

16mm Polycarbonate

A popular all-round roofing option offering increased rigidity and improved thermal performance. Common applications include pergolas, verandas, lean-to roofs and carports.

25mm Polycarbonate

A thicker multiwall option providing increased rigidity and improved thermal performance where these characteristics are required.

Thickness alone doesn't determine how far a sheet can span.

Sheet dimensions, supporting structure, glazing system, intermediate support and expected loading all need to be considered when designing the roof.

For a detailed comparison, see our Polycarbonate Sheet Thickness Guide.

What Size Polycarbonate Sheets Are Best for a Pergola?

There isn't one sheet size that suits every pergola.

For general pergola roofing, we recommend starting from 10mm multiwall polycarbonate, with 16mm or 25mm available where increased rigidity or thermal performance is wanted.

The correct sheet width should then be selected according to the rafter and glazing-bar layout.

If you're constructing the pergola from scratch, planning the sheets before permanently positioning the rafters can make it much easier to use standard sizes efficiently.

See our Best Thickness for Pergola Roofs Guide for more information.

What Size Polycarbonate Sheets Are Best for a Lean-To Roof?

Lean-to roofs commonly use 10mm, 16mm or 25mm multiwall polycarbonate, depending on the structure and required performance.

The sheet length runs down the roof slope, while the width is determined by the rafter and glazing-bar arrangement.

For an existing lean-to, accurately measure each rafter centre before ordering.

For a new structure, planning the sheet layout first can help keep the rafter centres consistent across the roof.

What Size Polycarbonate Sheets Are Best for a Carport?

Carports can be substantially larger than small pergolas or canopies, so the complete supporting structure needs to be considered rather than choosing sheet dimensions in isolation.

16mm multiwall polycarbonate is a popular option for many carport applications, but the appropriate sheet thickness, dimensions and support arrangement depend on the individual structure.

For larger or particularly exposed structures, appropriate consideration should also be given to wind and snow loading.

Standard Size or Cut to Size: Which Should I Choose?

Both can be good options.

Standard-size sheets

Standard sheets are often the most economical choice where their dimensions work efficiently with your roof design.

If you're building a new structure, you may be able to design your rafter layout around available standard sheet sizes.

Cut-to-size sheets

Cut-to-size polycarbonate is particularly useful when the structure already exists or when the required dimensions don't correspond conveniently with standard sheets.

It can reduce on-site cutting and make bespoke roof layouts considerably easier to plan.

Compare standard and cut-to-size sheet costs

Can I Cut Polycarbonate Sheets Myself?

Yes. Multiwall polycarbonate can be cut on site using suitable tools and the correct technique.

However, accurate cutting is important, particularly where the sheet needs to fit within a glazing system.

If you'd prefer not to cut the sheets yourself, PolycarbonateX offers a cut-to-size service for 10mm, 16mm and 25mm multiwall sheets.

See our How to Cut Polycarbonate Sheets Guide if you're planning to cut sheets yourself.

Don't Forget Thermal Expansion

Polycarbonate expands and contracts as temperatures change.

Sheets therefore shouldn't be ordered or installed on the assumption that they need to fit tightly between fixed points.

The glazing system needs to provide the appropriate fitting and expansion clearance.

This becomes increasingly important with longer sheets because the total amount of thermal movement increases with sheet length.

See our Polycarbonate Expansion and Contraction Guide before finalising your dimensions.

What Accessories Will I Need?

A complete multiwall polycarbonate roofing system may include:

• PVC top cap and aluminium base glazing bars

• Sheet end closures

• Sealing and breather tape

• Fixing buttons where required

• Side flashing

• Wall flashing where the roof meets a building

PolycarbonateX supplies the foil/sealing tape and breather tape together as a sealing tape kit covering up to 10 metres.

The exact accessories required depend on the roof design.

How Many Polycarbonate Sheets Do I Need?

The number of sheets required depends on the overall roof width and the chosen sheet/glazing-bar layout.

Don't simply divide the overall roof width by the nominal sheet width without allowing for the roofing system.

For example, a 980mm sheet may correspond to approximately 1000mm rafter centres in a typical PolycarbonateX arrangement.

The number of glazing bars also depends on how the outside edges of the roof are being finished.

This is why planning the complete layout is more reliable than calculating sheets independently.

Use Our Polycarbonate Roof Calculator

If you're planning a complete roof, the PolycarbonateX Roof Kit Calculator can make the process considerably easier.

Enter your roof dimensions and configuration and the calculator can determine the appropriate sheet layout and calculate the roofing components required.

This helps you plan the sheets, glazing bars and accessories as a complete system rather than working each component out separately.

Conclusion

Choosing the correct polycarbonate sheet size starts with understanding the complete roof layout.

Remember:

• Sheet width and rafter centres aren't necessarily the same measurement

• Measure existing rafters centre to centre

• Sheet length follows the roof slope

• Don't choose dimensions without allowing for the glazing system and thermal movement

• 10mm, 16mm and 25mm are the PolycarbonateX options for general roofing applications

• Standard sheets can be economical for new structures

• Cut-to-size sheets are particularly useful for existing or bespoke structures

• Plan sheets, rafters and glazing bars together rather than independently

If you're designing a new roof, choosing your sheet layout before fixing the rafters can save considerable time later. If the structure already exists, measure it carefully before ordering.