How to Fit Guttering to a Polycarbonate Roof
A correctly installed gutter is an important part of any polycarbonate roof. Whether you're building a pergola, lean-to, canopy or carport, the gutter needs to collect rainwater as it runs off the polycarbonate sheets and direct it safely away from the structure.
The key is to plan the roof pitch, sheet overhang and gutter position together. If the gutter is positioned too far forward, too far back or at the wrong height, rainwater may overshoot it or run behind it.
In this guide, we'll explain how to plan and fit guttering to a polycarbonate roof, where the gutter should sit and the common mistakes to avoid.
Do You Need Guttering on a Polycarbonate Roof?
Not every small polycarbonate structure will necessarily have guttering, but for most pergolas, lean-to roofs, canopies and carports, fitting a gutter is a sensible part of the installation.
Without controlled drainage, rainwater simply runs from the lower edge of the sheets.
This can result in water:
- Splashing onto patios and walls
- Collecting around posts or foundations
- Running onto neighbouring areas
- Creating persistent wet areas beneath the roof edge
- Splashing back onto timber or other parts of the structure
A properly positioned gutter collects this water and directs it towards a suitable downpipe and drainage point.
Where Should the Gutter Sit on a Polycarbonate Roof?
The gutter should sit beneath the lower edge of the polycarbonate roofing so that water flowing down the sheets naturally falls into it.
It shouldn't be positioned so far underneath the roof that water can run behind it, but it also shouldn't sit so far forward that slower-moving water misses the gutter.
The correct position depends on several factors, including:
- The amount of sheet overhang
- Roof pitch
- Gutter size and profile
- Position of the front supporting beam
- How the gutter brackets are being attached
The important point is to plan the gutter position before finalising your sheet lengths.
Our guide to polycarbonate roof overhang explains this relationship in more detail.
How Far Should Polycarbonate Overhang the Gutter?
For many domestic polycarbonate roofs, approximately 50mm of sheet projection beyond the front support is a useful starting point.
This doesn't mean the polycarbonate needs to extend 50mm into the gutter itself.
The objective is simply for the lower edge of the roof to be positioned so rainwater leaves the sheet and enters the gutter cleanly.
The exact amount will depend on the roof structure and gutter arrangement, so check the complete installation before cutting or ordering your sheets.
Make Sure the Roof Has Enough Pitch
Before thinking about the gutter itself, make sure the polycarbonate roof has been designed with sufficient pitch.
The roof pitch is what allows gravity to move rainwater down the sheets towards the gutter.
A gutter cannot compensate for a roof that has been installed too flat.
Insufficient pitch can contribute to standing water, poor drainage and leaks around joints or fixings.
Read our guide to polycarbonate roof pitch when planning your structure.
What Guttering Can Be Used With a Polycarbonate Roof?
Polycarbonate roofing doesn't normally require a special type of gutter simply because the roof covering is polycarbonate.
The appropriate gutter will depend on the size and design of the structure, the amount of water the roof needs to handle and how the gutter can be securely supported.
Common domestic guttering systems include:
- Half-round guttering
- Square-line guttering
- Deep-flow guttering
For a relatively small pergola or lean-to, standard domestic guttering may be suitable. Larger roof areas may require greater drainage capacity.
The gutter itself should be attached to a suitable structural element, rather than relying on the polycarbonate sheet for support.
How to Fit Guttering to a Polycarbonate Roof
The exact installation will depend on your structure and guttering system, but the basic process is as follows.
1. Plan the Gutter Before Cutting Your Sheets
Work out where the front supporting beam, lower edge of the sheets and gutter will sit.
This allows you to determine the correct finished sheet length and overhang.
If you're still calculating your roof dimensions, read our guide to measuring a roof for polycarbonate sheets.
2. Decide Where the Downpipe Will Go
Before installing the gutter, decide where you want the collected water to discharge.
The downpipe will usually be positioned towards one end of the gutter run, although the most appropriate position depends on the structure and available drainage.
Make sure the water will discharge to a suitable drainage point rather than simply creating a new water problem at ground level.
3. Establish the Gutter Fall
Guttering should have a slight fall towards the outlet so water naturally travels towards the downpipe.
Follow the instructions supplied with your chosen guttering system when setting the required fall and bracket spacing.
A string line can be useful for establishing a consistent fall along the front of the structure before fitting all the brackets.
4. Fix the Gutter Brackets
Gutter brackets need to be fixed securely to an appropriate structural component.
Depending on the roof design, this may involve a fascia board or suitable front timber.
Do not attach the gutter to the polycarbonate sheets themselves.
Position the brackets according to the gutter manufacturer's instructions, making sure the gutter will remain adequately supported when carrying rainwater.
5. Install the Gutter
Once the brackets are correctly positioned, install the gutter sections, connectors, stop ends and outlet according to the manufacturer's instructions.
Check that joints are properly engaged and sealed as required by the guttering system.
6. Fit the Downpipe
Connect the outlet to the downpipe and direct the water towards an appropriate drainage point.
Make sure the downpipe is securely supported and does not discharge somewhere that could cause waterlogging or damage around the structure.
7. Check the Relationship With the Polycarbonate
Once everything is positioned, check that water leaving the lower edge of the sheets will fall comfortably into the gutter.
The polycarbonate should not need to rest on the gutter.
There should simply be a suitable relationship between the sheet edge, overhang and gutter position to allow effective drainage.
Don't Forget the Lower Edge of Multiwall Polycarbonate
The lower end of a multiwall polycarbonate sheet needs to be finished correctly.
Multiwall sheets contain internal flutes, and the lower end is normally finished using the appropriate tape and sheet end closure.
The end closure helps protect and finish the lower edge while still allowing the roofing system to function correctly.
You can find compatible end closures, tapes, glazing bars and fixings in our polycarbonate roofing accessories.
Should the Gutter Touch the Polycarbonate?
The gutter doesn't need to support the polycarbonate roofing.
Your polycarbonate sheets should be supported by the roof structure and glazing system, while the gutter is separately supported by suitable brackets.
The sheet should be positioned so water naturally discharges from its lower edge into the gutter.
Don't use the gutter as a support for an incorrectly measured sheet.
How Do You Stop Water Running Behind the Gutter?
Water running behind the gutter can occur if the relationship between the sheet edge and gutter hasn't been planned correctly.
Possible causes include:
- Insufficient sheet overhang
- Gutter positioned too far back
- Gutter positioned too low
- Incorrect roof pitch
- Poorly positioned lower sheet edge
- Incorrectly installed guttering
If you're experiencing water ingress elsewhere on the roof, see our guide to common polycarbonate roof leaks.
What If the Polycarbonate Sheets Are Too Long?
If the sheets extend too far beyond the front support, it may be necessary to trim them.
Multiwall polycarbonate can be cut to size, but it is important to use the correct tools and technique to avoid damaging the sheet.
Read our guide to cutting polycarbonate roofing sheets before attempting to shorten installed or uninstalled sheets.
If you're planning a new roof, ordering cut-to-size multiwall polycarbonate sheets can reduce the amount of cutting required during installation.
Common Guttering Mistakes on Polycarbonate Roofs
Positioning the Gutter Too Far Forward
If the gutter is too far away from the lower sheet edge, water may run between the roof and gutter instead of being collected.
Positioning the Gutter Too Far Back
Depending on the roof pitch and rainfall, water can travel beyond a badly positioned gutter.
Forgetting the Sheet Overhang
Sheet length, overhang and gutter position should be calculated together.
Ordering sheets to finish exactly at the front support may leave insufficient projection for effective drainage.
Installing the Gutter Without a Fall
A gutter installed perfectly level may not drain as effectively as one installed with the fall specified by its manufacturer.
Supporting the Gutter From the Polycarbonate
Polycarbonate roofing sheets are not structural supports for guttering. Gutter brackets should be securely attached to the appropriate part of the roof structure.
Ignoring Where the Downpipe Discharges
Collecting water successfully is only half the job.
The downpipe needs to discharge the water somewhere appropriate.
Planning a New Polycarbonate Roof
If you're building a new pergola, lean-to, canopy or carport, it's much easier to plan the gutter before installation than try to adapt the structure afterwards.
Think about:
- Roof dimensions
- Roof pitch
- Rafter spacing
- Sheet thickness
- Sheet length
- Overhang
- Glazing system
- Gutter position
- Downpipe position
Our multiwall polycarbonate roofing installation guide explains the wider installation process.
You can also use our Polycarbonate Roof Calculator to work out the sheets, glazing bars and accessories required for your project.
Frequently Asked Questions
Do you need guttering on a polycarbonate roof?
Guttering is recommended for many pergolas, lean-tos, canopies and carports because it provides controlled drainage for rainwater running from the roof.
How far should polycarbonate overhang into a gutter?
The polycarbonate doesn't need to extend a specific distance into the gutter. For many domestic installations, approximately 50mm beyond the front structural support is a useful starting point, with the gutter positioned to collect water leaving the sheet.
Can I attach guttering directly to polycarbonate sheets?
No. Guttering should be supported by appropriate brackets fixed to a suitable structural element, not to the polycarbonate roofing sheet.
Should a polycarbonate roof slope towards the gutter?
Yes. Polycarbonate roofing should be installed with an appropriate pitch so rainwater flows towards the lower edge and gutter.
Can water run behind the gutter?
Yes. Insufficient overhang or incorrect gutter positioning can allow water to run between the roof edge and gutter.
Where should the downpipe go?
The best position depends on the structure and available drainage. The gutter should fall towards the outlet, with the downpipe directing water to an appropriate drainage point.
Plan the Roof and Gutter Together
The most important thing to remember is that roof pitch, polycarbonate overhang and gutter position work together.
For many domestic polycarbonate roofs, approximately 50mm of sheet projection beyond the front support is a useful starting point, but the final arrangement should suit the design of the roof and the guttering system being installed.
Plan the drainage before ordering your sheets, make sure the gutter is securely supported by the structure, and position it so rainwater leaving the polycarbonate is collected effectively.
If you're planning a complete roof, use our Polycarbonate Roof Calculator to calculate the sheets, glazing bars and accessories required.
