The right polycarbonate sheet size depends on your framework, glazing system and how the roof edges will be finished. Checking these details before ordering helps avoid poorly fitting sheets, unnecessary cutting and panels that bow between supports.

At The Polycarbonate Store, we can help you work out suitable dimensions for your multiwall polycarbonate roofing sheets, whether you are replacing an existing roof or building a new structure.

Choosing the right sheet thickness

Sheet thickness and support spacing need to be considered together. Our general recommended maximum spans are 700mm for 10mm twinwall, 800mm for 16mm multiwall, 900mm for 25mm multiwall and 1000mm for 32mm or 35mm multiwall.

These are planning recommendations for twinwall and multiwall polycarbonate. The final support arrangement must suit the exact sheet specification, glazing system and expected wind and snow loads. They should not be applied to solid polycarbonate or corrugated roofing.

Calculating your sheet widths

The distance between rafter centres is not automatically the sheet width you should order. Allowances are needed for the glazing profiles and the clearance required within the system.

For the arrangements shown in our updated guide, a sheet between two glazing bars has a total 20mm deduction from the rafter centre measurement. Where aluminium F-sections finish the outer edges, the end sheets have a 10mm deduction and the intermediate sheets have a 20mm deduction.

For example, at 600mm rafter centres, this gives 580mm intermediate sheets and 590mm end sheets where the specified F-section arrangement is used. These allowances relate to the system illustrated; confirm compatibility before applying them to another profile.

See our polycarbonate sheet size and rafter spacing recommendations for the calculation methods and worked example.

Measuring the length down the roof

Measure the required sheet length along the roof slope. A lean-to roof’s sloping length is longer than its horizontal projection, so using the projection alone can leave your sheets too short.

The finished length must also account for the wall connection, eaves detail and thermal movement allowance. Polycarbonate expands and contracts in both length and width, so sheets should never be forced tightly against the surrounding profiles.

Plan the finishing components too

Choose your glazing bars, F-sections and sheet end closures alongside the panels. Their dimensions and fitting arrangements affect how the roof comes together.

For multiwall roofing, the internal channels should run down the slope. Suitable end tapes and closures help protect the open channels while maintaining the required drainage. Our sheet closure installation guide explains this detail.

Before ordering bespoke sheets, contact The Polycarbonate Store if you would like help checking your measurements. Make clear whether you are supplying finished cutting sizes or asking us to calculate them from your framework dimensions.

FAQs

Should I order sheets to match my rafter centres?

Usually, no. The finished sheet width must account for the glazing system and outer edge details. Use the calculation for your chosen arrangement rather than ordering directly from the rafter centre measurement.

Will you automatically deduct an allowance for glazing bars?

No. If you supply specific sheet dimensions, we work to those requested sizes. If you need us to calculate the dimensions, tell us when requesting your quotation and provide the framework and glazing system details.

Can thicker polycarbonate span further?

Some thicker multiwall sheets permit greater support spacing, but the internal sheet structure, loading and fixing arrangement also matter. Check the manufacturer’s guidance for the exact product before changing your framework.

Can I curve multiwall polycarbonate?

Some sheets can be cold curved within the manufacturer’s permitted bending radius and direction. The limit depends on the sheet thickness and internal structure. Bending a sheet too tightly can cause stress and premature failure, so check suitability before ordering.