How to Fix a Leaking Polycarbonate Roof

A leaking polycarbonate roof does not always mean the roofing sheets have failed. In fact, most leaks are caused by installation details, ageing components or blocked drainage rather than damage to the polycarbonate itself.

Before attempting any repair, it's important to identify exactly where the water is entering the roof. Water can often travel along glazing bars, rafters or flashings before becoming visible inside, meaning the point where the drip appears is not always the source of the leak.

Common causes include incorrectly installed glazing bars, worn or damaged flashings, loose fixing buttons, blocked gutters, missing sealing tapes or insufficient roof pitch preventing rainwater from draining correctly.

By carrying out a systematic inspection, most leaks can be traced to their true source and repaired without replacing the entire roofing system. This guide will help you identify the most common leak locations and explain the appropriate solution for each one.

Glazing bars are one of the first areas to inspect when investigating a leaking polycarbonate roof. They support the sheets, create the weatherproof seal between panels and are exposed to rainwater throughout the year.

Check that every glazing bar cap is fully clipped into position and that there are no visible gaps along its length. If a cap has lifted or become displaced, rainwater may be able to enter beneath it.

Inspect the rubber gaskets or seals, where fitted, for signs of damage, deterioration or movement. Leaves and debris trapped beneath the glazing bars can also prevent the sheets from seating correctly, allowing water to bypass the seal.

Look along the full length of each glazing bar to ensure it remains straight and securely fixed to the roof structure below. If you discover a loose or damaged glazing bar, it should be repaired or replaced before investigating more complex causes.

In many cases, a careful inspection of the glazing bars identifies the source of a roof leak before any components need to be removed.

If the glazing bars appear to be in good condition, the next area to inspect is the wall flashing. This is one of the most common locations for water to enter a lean-to polycarbonate roof.

Carefully examine the full length of the flashing where it meets the wall. Look for visible gaps, loose fixings, cracked sealant or sections that have lifted away from the wall. Even a small opening can allow rainwater to travel behind the flashing before appearing inside the roof.

Check that the flashing sits tightly against both the wall and the roof, with no signs of movement or distortion. If the flashing has become loose over time, it should be securely refixed before replacing any sealant.

Also inspect the wall itself. Cracks in brickwork, damaged render or deteriorated mortar joints above the flashing can sometimes allow water to travel behind the flashing, creating the impression that the roof itself is leaking.

Where defects are found, repair or replace the flashing as necessary before continuing to inspect the remaining roof components.

If no issues are found with the wall flashing, the next area to inspect is the side flashings. These protect the exposed edges of the roof where the outer polycarbonate sheets meet the supporting structure and help prevent wind-driven rain from entering beneath the roofing system.

Check that each side flashing sits securely against both the roof and the supporting structure with no visible gaps or signs of movement. Loose fixings, damaged flashings or incorrect installation can allow water to track underneath before becoming visible inside the roof.

Pay particular attention to the upper and lower ends of the flashing, where small openings are most likely to occur. Also inspect the surrounding roof edge for leaves or debris that could prevent rainwater draining away freely.

If the flashing has become bent, damaged or detached, it should be repaired or replaced before continuing with the inspection. A correctly fitted side flashing should direct rainwater safely away from the roof edge while protecting the outer sheets from wind-driven rain.

If your polycarbonate roof has been installed using fixing buttons rather than a glazing bar system, each fixing should be inspected carefully. Even a single loose or damaged fixing can allow water to enter around the screw during heavy rainfall.

Check that every fixing button sits flat against the surface of the polycarbonate sheet and that the rubber sealing washer remains in good condition. The washer should be compressed just enough to create a weather-resistant seal without being crushed.

Look for cracked plastic button caps, perished rubber washers or fixings that have worked loose over time. Also inspect the polycarbonate around each fixing hole for signs of stress or cracking, which can occur if the fixings have been overtightened.

Remember that fixing holes should be slightly oversized to allow the sheets to expand and contract naturally as temperatures change. If a fixing has been overtightened, loosening and correctly refitting it may resolve the issue without replacing any components.

Once every fixing has been checked, continue inspecting the remaining roof components before deciding where the leak is originating.

The end closures and sealing tapes play a vital role in protecting multiwall polycarbonate sheets from moisture, dirt and insects. If these components are missing, damaged or incorrectly installed, water can enter the sheet flutes or track into other parts of the roofing system.

Begin by inspecting the upper end of each sheet. The foil tape should completely seal the open flutes, preventing water, dust and insects from entering from above. Check that the tape remains firmly bonded across the full width of the sheet with no lifting edges or visible damage.

Next, inspect the lower end of the sheets. The breather tape should be fitted neatly across the entire width of the open flutes and protected by the U-shaped end closure. This allows any condensation inside the sheet to drain away naturally while still preventing insects and debris from entering.

Look for signs of lifting tape, damaged end closures or gaps where water could enter the sheet. If any sealing components have deteriorated over time, replacing them is often a straightforward way to restore the roof's weather resistance and improve long-term performance.

A leak isn't always caused by the roofing system itself. In many cases, overflowing gutters or blocked drainage can allow water to back up beneath the roof edge, making it appear as though the polycarbonate roof is leaking.

Inspect the entire length of the gutter and remove any leaves, moss or debris that may restrict the flow of rainwater. Check that downpipes are free from blockages and that water can drain away quickly during heavy rainfall.

Also examine the lower edge of the roof to ensure rainwater is flowing cleanly from the polycarbonate sheets into the gutter without overshooting or collecting behind the fascia.

If water is unable to drain efficiently, it may overflow during periods of heavy rain and find its way into areas that would normally remain dry. Regular gutter maintenance is therefore an important part of preventing unnecessary roof leaks.

Once the gutters have been cleared, test the system using a garden hose to confirm that rainwater flows freely from one end of the roof to the other without overflowing.

If you've inspected the glazing bars, flashings, fixing buttons, sealing tapes and gutters but the roof is still leaking, the final step is to assess the overall roof design and carry out one last inspection.

A polycarbonate roof should always be installed with sufficient fall to allow rainwater to drain quickly towards the gutter. If the roof pitch is too shallow, water may remain on the surface for longer than intended, increasing the likelihood of water tracking beneath flashings or roofing components during heavy rainfall.

Walk around the roof and check that every component remains securely fixed and correctly aligned. Pay particular attention to the wall flashing, side flashings, glazing bars, fixing buttons, end closures and guttering to ensure nothing has moved or deteriorated over time.

Where possible, test the roof using a gentle flow of clean water from a standard garden hose. Start at the lowest point and gradually work upwards while observing where water enters the roof. This controlled method is often the easiest way to identify the true source of a leak.

Taking a systematic approach to inspection will usually identify the cause of the problem without unnecessary replacement of roofing sheets or accessories. Once the underlying issue has been corrected, your polycarbonate roof should continue to provide many years of reliable weather protection.