Rafter Spacing Guide for Polycarbonate Roofing (Complete UK Guide)
Rafter Spacing Guide for Polycarbonate Roofing
Correct rafter spacing is one of the most important parts of planning a polycarbonate roof. The position of the rafters determines the width of the polycarbonate sheets required and ensures the glazing bars are correctly supported.
Whether you're building a pergola, lean-to, veranda, canopy or carport, it is much easier to plan the rafter centres around your chosen polycarbonate sheet sizes before constructing the roof.
In this guide, we'll explain how rafter centres are measured, how they relate to sheet width and why you need to allow for the glazing system and natural expansion of the polycarbonate.
Choosing timber sizes for your polycarbonate roof
What Is Rafter Spacing?
Rafter spacing is normally measured from the centre of one rafter to the centre of the next rafter.
Polycarbonate roofing sheets sit between glazing bars, with the glazing bars positioned over and fixed to the supporting rafters.
This means the sheet width and rafter centres are closely related — but they are not necessarily the same measurement.
Is Rafter Spacing the Same as Polycarbonate Sheet Width?
No.
The polycarbonate sheet sits between the glazing bars, so the rafter centre measurement needs to account for the roofing system and the clearance required for the sheet to be fitted correctly.
For example, with the PolycarbonateX glazing system:
980mm wide sheet → approximately 1000mm rafter centres
This provides the additional space required for the sheet to sit correctly within the glazing system while allowing for natural thermal movement.
This distinction is particularly important when constructing a new roof. Setting rafters at exactly the same measurement as the sheet width can leave insufficient room for correct installation.
Why Is Correct Rafter Spacing Important?
Correct rafter spacing helps ensure:
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The sheets are properly supported
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Glazing bars sit correctly over the rafters
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Sheets have the required fitting and expansion clearance
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The roof remains secure
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Loads are transferred correctly to the supporting structure
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The finished roof has a consistent appearance
Incorrectly positioned rafters can make the sheets difficult or impossible to fit correctly and may require sheets to be cut down or the supporting structure to be altered.
Which Polycarbonate Thickness Should I Use?
PolycarbonateX supplies 10mm, 16mm and 25mm multiwall polycarbonate sheets for general roofing applications.
The appropriate thickness depends on the roof design, required performance and supporting structure.
10mm Polycarbonate
10mm multiwall polycarbonate is a lightweight and economical roofing option suitable for applications including pergolas, canopies and lean-to roofs where the supporting structure provides appropriate support.
16mm Polycarbonate
16mm multiwall polycarbonate provides increased rigidity and improved thermal performance and is a popular all-round option for pergolas, verandas, lean-tos and carports.
25mm Polycarbonate
25mm multiwall polycarbonate provides greater thermal performance and rigidity and can be used for more substantial roofing projects where these characteristics are required.
Sheet thickness alone should not be used to determine rafter spacing. The sheet specification, width, length, supporting structure, expected loading and glazing system all need to be considered.
Should I Choose My Sheets or Rafter Centres First?
If you're building a new roof and the rafters haven't yet been installed, it is usually better to choose the sheet size and roofing system first.
You can then position the rafters to suit the required sheet layout.
This can help you:
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Use standard sheet sizes efficiently
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Reduce unnecessary cutting and waste
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Keep rafter centres consistent
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Position glazing bars correctly
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Create a more uniform finished roof
If the rafters are already installed, measure the existing centre-to-centre spacing accurately before ordering your sheets.
How to Measure Existing Rafter Centres
Measure from the centre of one rafter to the centre of the next.
Do not simply measure the open gap between the rafters.
For example, if your rafters are 50mm wide, measuring only the clear space between them will give a different measurement from the true centre-to-centre spacing.
Measure several positions across the roof rather than assuming every rafter has been installed at exactly the same spacing.
Older or existing structures can vary slightly across their width.
What If My Existing Rafter Centres Don't Match a Standard Sheet Size?
This is where cut-to-size polycarbonate sheets can be particularly useful.
Instead of rebuilding the supporting structure around a standard sheet width, sheets can be cut to dimensions suitable for the existing roof, provided the finished layout remains compatible with the roofing system and structural requirements.
PolycarbonateX offers 10mm, 16mm and 25mm multiwall sheets cut to your required dimensions.
Rafter Spacing for Pergolas
If you're constructing a new pergola, decide on the polycarbonate sheet layout before permanently positioning the rafters.
For example, where a 980mm wide sheet is being used with the appropriate PolycarbonateX glazing system, the corresponding rafter centres would be approximately 1000mm.
Keeping the rafter centres consistent makes installation easier and gives the completed roof a uniform appearance.
The appropriate sheet thickness and support arrangement should still be selected for the particular structure.
Rafter Spacing for Lean-To Roofs
The same principle applies to lean-to roofs.
First establish:
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Overall roof width
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Roof projection
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Sheet thickness
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Sheet widths
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Glazing-bar positions
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Outside-edge detail
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Required expansion clearance
The rafter positions can then be planned around the sheet layout.
For a roof against a wall, you'll also need to consider the roof pitch and the method used to flash the roof against the building.
Rafter Spacing for Carports and Larger Roofs
Larger roofs require particular attention to structural support.
Don't simply increase the distance between rafters because a thicker polycarbonate sheet is being used.
Wind loading, snow loading, sheet dimensions, intermediate support, the glazing system and the supporting structure can all affect the required design.
For larger or particularly exposed structures, the structural design should be checked appropriately rather than relying on a generic rafter-spacing figure.
Glazing Bars and Rafter Alignment
The glazing bars joining adjacent sheets need to be properly supported.
With a rafter-supported system, the glazing bars are positioned over the rafters and fixed into the supporting structure.
Accurate rafter positioning is therefore important because it determines where each sheet joint will fall.
A poorly aligned rafter can result in:
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Incorrect sheet fit
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Misaligned glazing bars
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Insufficient expansion clearance
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Difficult installation
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An uneven finished roof
Remember to Allow for Thermal Expansion
Polycarbonate naturally expands and contracts as temperatures change.
The sheets should not be fitted tightly between fixed points.
The glazing system must allow the sheet to move without placing unnecessary stress on the polycarbonate or fixings.
The required allowance becomes particularly important with longer sheets because thermal movement increases with sheet length.
See our Polycarbonate Expansion and Contraction Guide for more information before installing your roof.
Roof Pitch Also Matters
Rafter spacing isn't the only structural measurement to consider.
Polycarbonate roofing also requires sufficient pitch to allow rainwater to drain effectively.
As a general rule, PolycarbonateX recommends a minimum roof pitch of 5°, subject to the requirements of the particular structure and roofing system.
The internal flutes of multiwall sheets should run down the roof slope, allowing moisture within the channels to drain towards the lower end.
Common Rafter Spacing Mistakes
Setting rafter centres equal to the sheet width
The sheet needs to fit correctly within the glazing system and have appropriate clearance for thermal movement.
Building the roof before choosing the sheets
This can leave you with awkward sheet dimensions or unnecessary cutting.
Where possible, plan the sheet layout first.
Measuring the gap between rafters
Rafter spacing is normally measured centre to centre, not from the inside edge of one rafter to the next.
Assuming every existing rafter is equally spaced
Measure each section of an existing roof before ordering.
Increasing the span simply because the sheet is thicker
A thicker sheet does not automatically mean that the supporting rafters can simply be moved further apart. The complete roof design needs to be considered.
Forgetting about the outside edges
When planning the overall roof width, remember to account for how the two outside sheet edges will be supported and finished.
Using Cut-to-Size Sheets With Existing Rafters
If you're replacing an existing roof or your rafters are already installed, cut-to-size sheets can make it much easier to achieve the required layout.
PolycarbonateX can cut 10mm, 16mm and 25mm multiwall polycarbonate to your required dimensions, within the available manufacturing limits.
This can be particularly useful where the existing rafter centres don't correspond conveniently with standard sheet sizes.
Planning a New Polycarbonate Roof?
If you're still designing the roof, don't choose rafter spacing in isolation.
Plan the:
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Overall roof dimensions
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Sheet thickness
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Sheet width and length
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Rafter positions
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Glazing bars
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Outside-edge detail
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Roof pitch
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Flashings and end closures
The PolycarbonateX Roof Kit Calculator can help determine the sheets and accessories required for your roof dimensions.
Installing polycarbonate roofing on a metal frame?
Conclusion
Correct rafter spacing is essential for a properly installed polycarbonate roof, but there isn't one universal rafter spacing that applies simply because a sheet is 10mm, 16mm or 25mm thick.
The important principle is to plan the polycarbonate sheet layout, glazing system and supporting structure together.
For a new structure, choosing your sheet layout before positioning the rafters can make installation considerably easier. For an existing structure, accurately measure the rafter centres before ordering and use cut-to-size sheets where necessary.
And remember: rafter centres and sheet width aren't necessarily the same measurement. With a typical PolycarbonateX arrangement, for example, a 980mm sheet corresponds to approximately 1000mm rafter centres.
