How to Install Polycarbonate Glazing Bars
Polycarbonate glazing bars are one of the most important components of any roofing system. They support the polycarbonate sheets, keep them securely in position and help create a strong, weatherproof roof that will perform reliably for many years.
This guide is specifically written for rafter-supported glazing bars, where an aluminium base is fixed directly to a timber rafter before a white PVC top cap is fitted to secure the polycarbonate sheets. This is the roofing system supplied by PolycarbonateX for most domestic pergolas, verandas, canopies and lean-to roofs.
We'll explain the installation process step by step, from preparing the roof structure and fixing the aluminium bases to fitting the roofing sheets and clipping the PVC top caps into place. We'll also highlight the most common installation mistakes and share practical tips to help you achieve a professional finish.
Before You Begin
Before installing your glazing bars, take a few minutes to prepare the roof structure and check that all of your components are ready. A well-prepared roof frame will make the installation quicker, easier and help ensure a professional finish.
This guide is written specifically for PolycarbonateX rafter-supported glazing bars, which consist of an aluminium base fixed directly to a timber rafter and a white PVC top cap that secures the polycarbonate sheets in place. Before starting, make sure you have all of the required components, including the glazing bars, polycarbonate sheets, sealing tapes, end closures and suitable fixings.
Check that every timber rafter is straight, securely fixed and evenly spaced. Any twists, bows or movement in the rafters should be corrected before the glazing bars are installed, as the roofing system can only perform as well as the structure supporting it.
Finally, check that the roof has the correct fall to allow rainwater to drain away effectively. A correctly constructed roof frame is the foundation of a long-lasting, weatherproof polycarbonate roof.
Understanding the Parts of a Rafter-Supported Glazing Bar
Before installing the glazing bars, it's helpful to understand how each component works. Although the system is simple, every part has an important role in creating a strong, weatherproof roof.
Aluminium Base
The aluminium base forms the foundation of the glazing bar system. It is fixed directly to the timber rafter and provides the structural support for the polycarbonate sheets.
PVC Top Cap
Once the roofing sheets have been positioned, the white PVC top cap clips securely onto the aluminium base. This locks the sheets in place while allowing them to expand and contract naturally as temperatures change.
Rubber Gaskets
The glazing bar includes rubber sealing gaskets that create a weatherproof seal against the polycarbonate sheets. These help prevent water entering the roofing system while protecting the sheets from unnecessary pressure.
Polycarbonate Roofing Sheet
The roofing sheet sits between two glazing bars and is fully supported along both edges. Correct positioning is important to ensure the roof remains secure and performs correctly over time.
Together, these components create a roofing system that's strong, weatherproof and designed to provide many years of reliable performance when installed correctly.
Step 1 – Fix the Aluminium Base to the Timber Rafters
The first stage of the installation is to fix the aluminium glazing bar bases to the timber rafters. These form the structural foundation of the roofing system, so taking your time at this stage will make the rest of the installation much easier.
Position each aluminium base centrally over the timber rafter, ensuring it runs perfectly straight from the top of the roof to the bottom. Before fixing it permanently, check the alignment against the neighbouring rafters to ensure every glazing bar will run parallel across the roof.
Secure the aluminium base using suitable corrosion-resistant fixings, working from one end of the bar to the other. Tighten each fixing firmly so the glazing bar is held securely against the timber, but avoid over-tightening as this may distort the aluminium profile.
As each glazing bar is installed, use a spirit level or straight edge to confirm it remains perfectly aligned. Even a small amount of movement at this stage can become much more noticeable once the roofing sheets and top caps have been fitted.
Before moving on, double-check that every glazing bar base is securely fixed, evenly spaced and correctly aligned. A few extra minutes spent checking the framework now will help ensure the completed roof looks professional and performs as intended.
Step 2 – Position the Polycarbonate Sheets
With the aluminium glazing bar bases securely fixed, you're ready to position the polycarbonate sheets.
Before lifting each sheet into place, check that the UV protected side is facing upwards and that the foil tape, breather tape and end closures have already been fitted correctly. Preparing the sheets before installation is much easier than trying to do it once they're on the roof.
Carefully lower the first sheet into position so that it sits neatly between two glazing bar bases. The sheet should lie flat and be fully supported along both edges without rocking or twisting.
When positioning the sheet, always allow sufficient clearance for thermal expansion. Polycarbonate naturally expands and contracts as temperatures change, so the sheet should never be forced tightly into the glazing bars. The glazing system is designed to hold the sheet securely while still allowing it to move naturally.
Continue fitting each sheet across the roof, checking that every panel remains straight and evenly spaced. As you work, regularly stand back and inspect the roof from the front to ensure the sheets remain correctly aligned. Small adjustments are much easier to make before the PVC top caps have been fitted.
Once all of the sheets have been positioned correctly, you're ready to secure them by fitting the PVC top caps.
Step 3 – Fit the PVC Top Caps
With all of the polycarbonate sheets correctly positioned, the final stage of the glazing bar installation is to fit the PVC top caps. These lock the sheets securely into the aluminium bases while allowing them to expand and contract naturally as temperatures change.
Starting at one end of the roof, carefully align the PVC top cap with the aluminium base below. Check that the cap is correctly positioned along its entire length before applying any pressure.
Working steadily from one end of the glazing bar to the other, press the PVC top cap firmly into place until it clips securely onto the aluminium base. Applying even pressure along the length of the cap helps achieve a consistent fit and reduces the risk of damaging the glazing system.
Using a rubber mallet, gently tap the PVC top cap into position, working steadily from one end of the glazing bar to the other. Light, even taps will help the cap engage correctly with the aluminium base without damaging the profile. Never use a metal hammer directly on the PVC top cap, as this can crack or permanently mark the surface.
Once the top cap has been fully fitted, check that it is securely engaged along the entire length of the glazing bar and that the polycarbonate sheets remain correctly aligned beneath it.
Repeat the process for each glazing bar until every sheet has been securely installed.
Final Checks Before Finishing the Roof
Before considering the installation complete, take a few minutes to inspect the entire roofing system. Carrying out a final check now can help identify any small issues before the roof is exposed to the weather.
Walk around the roof and check that every PVC top cap is fully clipped onto the aluminium base along its entire length. There should be no raised sections or gaps between the two components.
Confirm that every polycarbonate sheet is sitting correctly within the glazing bars and remains properly aligned across the roof. The spacing between each sheet should be consistent, with no panels forced tightly into the glazing system.
Check that the foil tape has been fitted to the top of every sheet, the breather tape has been fitted to the bottom and all end closures are fully installed.
Finally, remove the protective film from the polycarbonate sheets. Leaving the film on after installation can make it much more difficult to remove later, particularly after prolonged exposure to sunlight.
Once these checks have been completed, your rafter-supported glazing bar system is ready to provide many years of reliable performance.
Common Glazing Bar Installation Mistakes
Installing rafter-supported glazing bars is a straightforward process, but a few simple mistakes can affect the appearance, weather resistance and long-term performance of your roof. Here are the most common issues we see and how to avoid them.
Installing the Aluminium Base Out of Line
Every glazing bar should run perfectly straight along the centre of its timber rafter. If one aluminium base is installed out of alignment, the roofing sheets and PVC top caps may not fit correctly, making the finished roof look uneven.
Over-Tightening the Fixings
The aluminium base should be firmly secured to the timber rafter, but the fixings should never be over-tightened. Excessive force can distort the aluminium profile and make it more difficult for the PVC top cap to clip into place correctly.
Forgetting to Allow for Expansion
Polycarbonate sheets naturally expand and contract as temperatures change. They should never be forced tightly into the glazing bars. Allowing the correct expansion clearance helps prevent unnecessary stress on the roofing system and ensures it performs correctly throughout the year.
Using a Metal Hammer
PVC top caps should always be fitted using a rubber mallet. A metal hammer can crack, dent or permanently mark the PVC profile, affecting both its appearance and performance.
Fitting the Top Cap Before Checking the Sheet Position
Before clipping the PVC top cap into place, always check that the polycarbonate sheet is sitting correctly within both glazing bars. Once the cap has been fully fitted, making adjustments becomes much more difficult.
Taking a few extra minutes to check your work at each stage of the installation will help you achieve a professional finish and a roofing system that's built to last.
Frequently asked questions
What is a rafter-supported glazing bar?
A rafter-supported glazing bar is a two-part roofing system consisting of an aluminium base fixed to a timber rafter and a PVC top cap that secures the polycarbonate sheets. It's one of the most popular systems for domestic pergolas, verandas and lean-to roofs.
Do I fix the aluminium base before the sheets?
Yes. The aluminium bases should always be securely fixed to the timber rafters before the polycarbonate sheets are positioned.
Can I use a normal hammer to fit the PVC top cap?
No. We recommend using a rubber mallet to gently tap the PVC top cap into place. A metal hammer can damage the PVC profile.
How do I know if the top cap is fully fitted?
The PVC top cap should be clipped securely onto the aluminium base along its entire length with no raised sections or visible gaps.
Should the sheets fit tightly inside the glazing bars?
No. Polycarbonate sheets need room to expand and contract as temperatures change. They should sit securely within the glazing bars without being forced tightly into position.
Can I replace a damaged PVC top cap without replacing the whole roof?
In many cases, yes. Individual glazing bar components can often be replaced without rebuilding the entire roofing system, provided the remaining parts are still in good condition.
How often should glazing bars be inspected?
We recommend visually inspecting your roofing system at least once a year. Check that the PVC top caps remain securely fitted, all fixings are tight and there are no signs of damage or excessive movement.
Are PolycarbonateX glazing bars suitable for all polycarbonate sheets?
PolycarbonateX rafter-supported glazing bars are designed for use with compatible multiwall polycarbonate roofing sheets. Always check that the glazing bar system matches your chosen sheet thickness before installation.
Need More Help?
Still unsure which sheet thickness is right for your project? Our UK team can help you check your requirements, calculate the materials you need or visualise how your finished roof could look.