How to Join Polycarbonate Sheets Using H Sections
Polycarbonate H Sections provide a simple and effective way of joining two polycarbonate sheets end-to-end, allowing you to extend the roof projection beyond the maximum available sheet length. They are commonly used when a single sheet is not long enough to span the full length of the roof.
This guide is written specifically for PolycarbonateX clear H Section joining profiles. Unlike glazing bars, H Sections are not structural roofing components. Their purpose is to join two sheets together, and they must always be fully supported by a timber cross member directly beneath the joint.
In this guide, we'll explain how PolycarbonateX H Sections work, how to install them correctly and the common mistakes to avoid, helping you create a strong, neat and weather-resistant sheet joint.
Before You Begin
Before installing a PolycarbonateX H Section, make sure both polycarbonate sheets have already been cut to their final length and prepared correctly. The sheet ends should be clean, square and free from burrs or plastic swarf to ensure they slide smoothly into the joining profile.
This guide is written specifically for PolycarbonateX clear H Sections used to join two multiwall polycarbonate sheets end-to-end. The H Section is designed to extend the overall roof projection when a single sheet is not long enough to span the full distance.
Unlike a glazing bar, an H Section does not provide structural support. The joint must always be positioned directly above a timber cross member, which supports both sheet ends and the H Section itself. Installing an H Section without adequate support beneath the joint can lead to movement, poor performance and damage to the roofing system.
Before fitting the H Section, check that both sheets are the same thickness as the profile you've selected and confirm that the supporting timber cross member is correctly positioned beneath the joint. Taking a few minutes to prepare the roof properly will make the installation much easier and help ensure a strong, reliable joint.
Understanding How an H Section Works
Before installing an H Section, it's helpful to understand exactly how it works and where it should be used.
The PolycarbonateX H Section is named after its cross-sectional shape, which is the same as a capital letter "H". The profile contains two equal channels, allowing the end of one polycarbonate sheet to slide into one side and the end of a second sheet to slide into the opposite side.
Its purpose is to create a neat, accurate joint between two sheets when the roof projection is longer than the maximum available sheet length. Rather than replacing the entire roof with longer sheets, an H Section allows two sheets to be joined together to achieve the required length.
Although the H Section securely joins the two sheets, it is not designed to carry the weight of the roof. Both sheet ends and the H Section itself must always be fully supported by a timber cross member positioned directly beneath the joint.
Without this support, the joint may flex under load, affecting the performance and lifespan of the roofing system.
When correctly installed on a suitable timber support, the H Section creates a clean, professional joint that allows you to extend the roof while maintaining the appearance and integrity of the roofing system.
Step 1 – Position the Timber Cross Member
Before fitting the H Section, make sure a timber cross member has been installed directly beneath the point where the two polycarbonate sheets will meet. This cross member provides the structural support for both sheet ends and the H Section itself.
The cross member should run across the roof between the supporting rafters, creating a solid fixing point beneath the joint. It should be securely fixed, level and positioned exactly where the two sheets will meet.
Take time to check the position of the cross member before lifting the sheets onto the roof. Once the H Section has been installed, moving the support will be much more difficult.
It's important to remember that the H Section is not designed to span an unsupported gap. Its purpose is simply to join the two sheets together, while the timber cross member carries the load beneath the joint.
Correct positioning of the timber support is the foundation of a strong, reliable H Section installation and should always be completed before fitting either sheet.
Step 2 – Install the H Section
With the timber cross member correctly positioned, you're ready to install the PolycarbonateX H Section and join the two roofing sheets.
Begin by sliding the first polycarbonate sheet into one side of the H Section. The sheet should enter the profile evenly along its full width without being forced into position. Support the sheet as you work to prevent unnecessary stress on the joint.
Next, position the second sheet and slide it into the opposite side of the H Section until both sheet ends are equally inserted into the profile. The joint should be centred directly above the timber cross member, ensuring both sheets are fully supported beneath the H Section.
Check that the H Section sits straight across the roof and that both sheets remain correctly aligned. The sheet surfaces should sit flush with one another, creating a neat, continuous roof line without steps or gaps at the joint.
As with every polycarbonate roofing installation, remember that the sheets will naturally expand and contract as temperatures change. The H Section is designed to accommodate this movement, so the sheets should never be forced tightly into the profile.
Once the H Section has been correctly positioned and both sheets are fully supported by the timber cross member below, the joint is complete and the remaining roofing components can be installed in the normal way.
Check the Finished Joint
Before completing the roof, take a few moments to inspect the H Section and both adjoining sheets. A correctly installed joint should be straight, fully supported and blend neatly into the rest of the roofing system.
Check that both polycarbonate sheets are inserted equally into the H Section and that the profile sits directly above the timber cross member. The joint should feel solid, with no movement or unsupported sections beneath it.
Stand back and look along the roof from both the front and the side. The two sheets should align evenly, creating a smooth, continuous roof surface without noticeable steps or gaps where they meet.
Finally, confirm that the surrounding glazing bars, roofing sheets and fixings remain correctly aligned after the H Section has been installed. Making any small adjustments now will ensure the completed roof looks professional and performs as intended for many years.
Common H Section Installation Mistakes
Although PolycarbonateX H Sections are straightforward to install, using them incorrectly can affect both the appearance and performance of your roof. Here are the most common mistakes we see and how to avoid them.
Using an H Section as a Structural Glazing Bar
An H Section is a joining profile only. It is not designed to support the roof structure and should never be used as a replacement for a glazing bar.
Forgetting the Timber Cross Member
Every H Section must be fully supported by a timber cross member positioned directly beneath the joint. Without this support, the joint can flex under load and reduce the overall strength of the roof.
Using the Wrong H Section
H Sections are manufactured for specific polycarbonate sheet thicknesses. Always check that the profile matches your sheet thickness before installation to ensure a secure fit.
Unequal Sheet Insertion
Both sheet ends should be inserted evenly into the H Section. If one sheet is inserted further than the other, the joint can appear uneven and may not perform as intended.
Forcing the Sheets Into the Profile
Polycarbonate sheets should slide smoothly into the H Section without excessive force. Never force the sheets tightly into the profile, as they need room to expand and contract naturally as temperatures change.
Taking a little extra time to position the timber support correctly and align both sheets carefully will result in a much stronger, neater and longer-lasting installation.
Frequently asked questions
What is an H Section used for?
An H Section is used to join two polycarbonate sheets end-to-end, allowing the roof projection to be extended beyond the maximum available sheet length.
Is an H Section the same as a glazing bar?
No. A glazing bar is a structural roofing component that supports the sheets, whereas an H Section is simply a joining profile. An H Section must always be supported by a timber cross member beneath the joint.
Does an H Section support the roof?
No. The structural support is provided by the timber cross member below the joint. The H Section simply joins the two sheet ends together.
Can I install an H Section without a timber support beneath it?
No. Every H Section should be installed directly above a timber cross member. Installing it without adequate support can reduce the strength and performance of the roofing system.
Do both sheets need to be inserted the same distance into the H Section?
Yes. Both sheets should be inserted equally into the profile to create a balanced, secure and professional-looking joint.
Can I use an H Section with any sheet thickness?
No. H Sections are designed for specific polycarbonate sheet thicknesses. Always select the correct profile to match your sheets.
Can I cut an H Section to length?
Yes. If required, the profile can be cut to suit the width of your roof using suitable cutting tools. Ensure the cut is clean and square before installation.
Can I replace an H Section without replacing the whole roof?
In many cases, yes. If the surrounding roofing system is in good condition, an individual H Section can often be replaced without rebuilding the entire roof.
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.