Diagnosing Cracks in Multiwall Polycarbonate Veranda Roofs
Polycarbonate is prized for its high impact resistance, but it is not indestructible. Over time, factors such as extreme thermal cycling, UV degradation, or improper installation can lead to fracturing. Unlike glass, which tends to shatter completely when compromised, polycarbonate often develops 'stress cracks' or 'crazing'. Identifying the root cause of these fractures is essential to prevent water ingress and to decide whether a single sheet or the entire roof needs updating.
Thermal Expansion and Contraction Fatigue
Polycarbonate expands and contracts significantly more than the aluminium frame it sits in. If the panels are fixed too tightly or if the expansion gaps are insufficient, the material will buckle and eventually crack around the fixing points. This usually manifests as small, radiating cracks at the edges of the sheet where it is held by the glazing bars.
Correct installation requires 'floating' fixings that allow for at least 3mm of movement per metre.
UV Degradation and 'Crazing'
While most modern sheets are UV-protected, this coating is usually only on one side. If a panel was installed upside down, or if the coating has been eroded by harsh chemicals, the polycarbonate becomes brittle. This leads to 'crazing'—a network of fine surface cracks that make the panel look cloudy and significantly reduce its structural strength.
- Check for yellowing of the material
- Identify if cracks are on the surface or through the flutes
- Look for a brittle, 'crisp' texture when touched
- Verify the UV-protected side is facing the sun
Impact Damage and Punctures
Heavy hail or falling branches can cause localized fractures. In multiwall polycarbonate, an impact might only crack the top layer, leaving the lower layers intact. While this may not cause immediate leaks, it allows water to enter the internal flutes, leading to algae growth and unsightly dark streaks that are impossible to clean out.
- 1Inspect the top skin for punctures or dents
- 2Check for water accumulation inside the channels
- 3Seal any minor punctures with neutral-cure silicone
- 4Plan for replacement if the internal structure is crushed
Fixing-Point Failures
Often, the crack doesn't start in the middle of the sheet but at a screw hole. If the hole was not pre-drilled larger than the screw shank, the sheet has no room to move. The resulting stress eventually snaps the plastic. These cracks are particularly problematic as they often occur directly over the support beams, where water can then seep into the structure's framework.
Common questions
Can I use tape to fix a crack in polycarbonate?
Temporary 'flashing tape' can stop leaks, but it is not a permanent solution. The tape will eventually fail due to the thermal movement of the sheet, and the adhesive can leave permanent stains.
Why is there water inside the flutes if there's no crack?
This is often condensation rather than a leak. However, if the end-caps or breather tape have failed, water can be sucked into the channels via capillary action.
How long does it take to replace a single panel?
A standard 3-metre sheet can usually be swapped in under two hours, provided the glazing bars are in good condition and the seals don't need full replacement.
Is a cracked panel dangerous?
While polycarbonate rarely falls out in large shards, a cracked panel has lost its structural rating for wind and snow. It should be replaced to ensure the safety of anyone underneath.
In short
Fractures in polycarbonate are usually caused by thermal stress or UV aging, requiring professional panel replacement to restore the roof's integrity.
What to do next
Is your polycarbonate roof showing signs of age or impact damage? We can source and fit exact-match replacements.
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