How Much Should Polycarbonate Roofing Overhang?
How Much Should Polycarbonate Roofing Overhang? | Guide
Getting the overhang right is an important part of installing a polycarbonate roof. Too little overhang can allow rainwater to run behind or miss the gutter, while excessive overhang leaves more of the sheet unsupported.
So, how much should polycarbonate roofing overhang?
For most domestic polycarbonate roofs, the sheets should typically extend around 50mm beyond the front supporting beam or rafter end towards the gutter. The exact amount can vary depending on the roof design, gutter position and glazing system being used.
In this guide, we'll explain what a polycarbonate roof overhang is, why it matters and what you should consider when measuring your sheets.
What Is a Polycarbonate Roof Overhang?
The overhang is the section of polycarbonate sheet that extends beyond the final supporting structure at the lower end of the roof.
On a typical pergola, lean-to, canopy or carport, the sheets run down the roof slope towards the gutter.
Rather than finishing directly above the front supporting beam, the sheets extend slightly beyond it.
This allows rainwater travelling down the sheets to discharge cleanly towards the gutter rather than running back towards the structure.
How Much Should Polycarbonate Sheets Overhang?
A useful starting point for many domestic installations is approximately:
50mm beyond the front support towards the gutter.
However, this should not be treated as a universal measurement for every roof.
The correct overhang depends on:
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The position of the gutter
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The design of the roof structure
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The glazing system being used
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The position of the front supporting beam
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Any end closure fitted to the sheet
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The manufacturer's installation requirements
The objective is to provide enough projection for water to drain correctly without leaving an unnecessarily large section of unsupported sheet.
Why Does the Overhang Matter?
Polycarbonate roofing is installed at a pitch so rainwater flows down the surface towards the lower edge of the roof.
If the sheet finishes too far back, water may not discharge correctly into the gutter.
An appropriate overhang helps:
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Direct rainwater towards the gutter
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Keep water away from the timber structure
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Reduce the chance of water running behind the gutter
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Create a neat finished roof edge
Overhang should therefore be considered before ordering or cutting your sheets, rather than being treated as an afterthought during installation.
For more information about planning the complete roof, see our guide to measuring a roof for polycarbonate sheets.
What Happens If the Overhang Is Too Short?
An insufficient overhang can cause problems with rainwater drainage.
Depending on the position of the gutter, water may:
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Miss the gutter
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Run behind it
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Drip onto the supporting timber
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Cause persistent wetting around the front of the structure
If you're experiencing water around an existing installation, our guide to common causes of a leaking polycarbonate roof explains other areas worth checking.
Can Polycarbonate Overhang Too Far?
Yes. More overhang is not necessarily better.
Polycarbonate roofing performs as part of a supported roofing system. Extending a large amount of sheet beyond the final support creates a larger unsupported section.
An excessive overhang may also:
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Make the finished roof look untidy
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Make gutter positioning more difficult
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Leave more sheet exposed beyond the structure
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Increase movement at the lower edge
The aim is therefore to create enough overhang for effective drainage without unnecessarily extending the unsupported sheet.
Where Do You Measure the Overhang From?
For a typical lean-to or pergola roof, sheet length should be measured along the roof slope, not horizontally across the ground.
Measure from the required position at the upper end of the roof down to the front supporting structure, then include the required projection beyond that support.
For example, if the required sheet length to the front support is 3,000mm and the design requires a 50mm overhang, the finished sheet length would be approximately:
3,000mm + 50mm = 3,050mm
Always check the complete installation and glazing system before ordering because other allowances may also need to be considered.
Our polycarbonate sheet sizing guide explains sheet width and length in more detail.
Don't Forget the Roof Pitch
Overhang and roof pitch work together.
The roof needs sufficient pitch to move rainwater towards its lower edge. An overhang cannot compensate for a roof that has been installed too flat.
For PolycarbonateX roofing projects, we recommend planning the correct pitch before calculating the final sheet dimensions.
This is particularly important on lean-to roofs, pergolas and canopies where the sheets may be several metres long.
Should the Polycarbonate Sheet Extend Into the Gutter?
The lower edge of the sheet should be positioned so water can discharge effectively into the gutter, but it should not extend unnecessarily far across it.
The exact relationship between the sheet and gutter will depend on:
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Gutter size
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Gutter position
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Roof pitch
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Front beam position
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Sheet projection
It is therefore better to plan the sheet, supporting structure and gutter together rather than choosing each measurement independently.
For more information on positioning and drainage, see our guide to fitting guttering to a polycarbonate roof.
Does the End Closure Affect the Overhang?
Multiwall polycarbonate sheets normally have an end closure fitted over the lower taped edge.
The lower end of the sheet should first be correctly prepared using the appropriate breather tape before the end closure is fitted.
When planning your gutter position, remember that the finished lower edge therefore consists of more than just the exposed polycarbonate sheet.
Browse our polycarbonate roofing accessories for compatible tapes, end closures, glazing bars and fixings.
Does Polycarbonate Expand Beyond the Overhang?
Polycarbonate naturally expands and contracts as temperatures change.
This thermal movement must be considered throughout the installation. Sheets should never be fitted so tightly that they cannot move within the roofing system.
Overhang should therefore be planned as part of the overall installation rather than simply cutting every sheet flush with the structure.
If you're installing a complete roof, follow our multiwall polycarbonate roofing installation guide for the full installation process.
How Do I Calculate the Correct Sheet Length?
Start by measuring the roof along the slope from the upper sheet position to the front support.
Then account for the required overhang at the lower end.
For example:
Roof slope measurement: 2,950mm
Required overhang: 50mm
Approximate finished sheet length: 3,000mm
This is particularly useful when ordering cut-to-size multiwall polycarbonate sheets, as the sheets can be supplied to the dimensions required for your project.
If you're unsure how many sheets, glazing bars and accessories you'll need, you can also use our Polycarbonate Roof Calculator to plan your complete roofing system.
Common Polycarbonate Overhang Mistakes
Cutting the Sheets Flush With the Front Support
This may leave insufficient projection for rainwater to discharge correctly towards the gutter.
Adding Too Much Overhang
A large unsupported projection is unnecessary and can make the roof edge more difficult to finish correctly.
Forgetting the Overhang When Measuring
If you order sheets based only on the distance between the upper and lower structural supports, they may be too short once the required projection is considered.
Measuring Horizontally Instead of Along the Roof
Sheet length follows the slope of the roof. Measure along this slope when calculating the required length.
Positioning the Gutter After the Roof Is Finished
Ideally, the gutter position should be considered while planning the structure and sheet overhang so the components work together.
Frequently Asked Questions
How far should polycarbonate roofing overhang?
For many domestic polycarbonate roofs, approximately 50mm beyond the front support is a useful starting point. The final measurement should suit the particular roof structure, gutter position and glazing system.
Does polycarbonate need to overhang into the gutter?
The sheet should project sufficiently for rainwater to discharge cleanly towards the gutter. It does not need to extend a large distance across the gutter.
What happens if my polycarbonate sheets are too short?
If there is insufficient projection at the lower edge, water may not drain correctly into the gutter. Depending on how short the sheets are, replacement may be preferable to trying to modify the finished installation.
Can I cut polycarbonate sheets if the overhang is too long?
Yes. Multiwall polycarbonate can be trimmed using suitable cutting tools and techniques. Read our guide to cutting polycarbonate roofing sheets before making adjustments.
Do I measure polycarbonate sheet length along the slope?
Yes. Measure the required sheet length along the roof slope, rather than using the horizontal projection of the structure.
Plan Your Polycarbonate Roof Before Ordering
Getting the overhang right is only one part of planning a successful polycarbonate roof.
Before ordering, check your roof dimensions, pitch, rafter spacing, sheet thickness and glazing system so that every component works together.
For most domestic installations, around 50mm of overhang beyond the front support is a useful starting point, but the final measurement should always suit the design of your particular roof and gutter.
If you're planning a new pergola, lean-to, carport or canopy, use our Polycarbonate Roof Calculator to work out the sheets, glazing bars and accessories required for your project.
