Insufficient roof pitch or poor drainage

A shallow roof drains slowly and allows water to remain around overlaps, fixings and wall connections. Wind-driven rain can then be pushed into the joints. Local low points, blocked gutters and insufficient outlets may also cause leakage even when the sheet itself is not damaged.

Roof pitch should be selected according to the panel profile, roof length, local rainfall, prevailing wind and drainage design rather than applying one universal value.

Incorrect overlaps and flashing details

Corrugated and profiled polycarbonate sheets must be installed in the drainage direction, with side laps arranged for the prevailing wind. Insufficient lap width, reversed end laps or incomplete wall flashing can create a direct path for water.

Fixings and washers do not form a stable seal

Loose screws prevent the washer from sealing evenly. Overtightened screws can crush the gasket and deform the sheet. Angled screws create uneven pressure and leave one side of the washer inadequately sealed. The correct fixing condition is firm and even, without visible sheet distortion and without preventing thermal movement.

Thermal expansion pulls on joints and fixing holes

Polycarbonate expands as its temperature rises and contracts as it cools. If the sheet is locked tightly by screws and profiles, the movement is converted into internal stress. The roof may first warp or click and later develop damaged seals, enlarged holes or cracks.

As a practical reference, allow approximately 3 mm of thermal movement per metre. The final allowance must be based on sheet length, local temperature range, profile design and the supplier’s installation drawing.

Expansion can be accommodated by oversized holes, sliding connections, end clearance and profiles designed to permit movement. Simply loosening screws is not a complete solution, and the sheet should not be left free to vibrate inside the frame.

Condensation is mistaken for leakage

Warm, humid indoor air can condense on a cooler sheet surface. Condensation is usually distributed over a wider area and may appear during clear weather or early morning. Multiwall sheets may also trap moisture when the open ends are sealed incorrectly. Ventilation, humidity control, insulation and correct end closure should be checked before applying more sealant.

Fixing holes are too small

When the hole is nearly the same diameter as the screw, thermal movement is blocked and stress concentrates at the hole wall. Repeated heating and cooling may first cause whitening and microcracks, followed by radial cracking. As a common reference, the hole may be 2–4 mm larger than the screw, but long sheets and large temperature ranges require a project-specific calculation.

Stress concentration develops at drilled and cut edges

Hole edges, notches, sharp corners and scratches are stress-concentration zones. Under ultraviolet exposure, temperature cycling or mechanical loading, a small defect may develop into a microcrack. Continued crack growth can cause warping, hole tear-out or local fracture.

Drilled edges should be smooth and free of burrs. Fixing points should not be positioned too close to a cut edge. As a practical reference, the distance from the hole centre to the sheet edge should be at least 2.5 times the hole diameter.

Screws are overtightened or installed at an angle

Overtightening creates a local depression and places the hole edge under continuous stress. Angled screws apply lateral force. When cracks are concentrated around fixings, inspect hole size, edge finish, screw angle and washer compression before concluding that the raw material is defective.

Support spacing is excessive or the sheet is too thin

Roof capacity cannot be described by one kilograms-per-square-metre value without structural conditions. Thickness, profile, support span, fixing method, load distribution and local wind or snow loads must be considered together. Excessive spans increase deflection and repeated fatigue loading at the fixings and overlaps.

The sheet is forced below its minimum bending radius

Polycarbonate can be cold curved, but each thickness and structure has a minimum permitted radius. Forcing the panel into a tighter curve creates permanent stress. Cracking may appear months later after combined exposure to sunlight, temperature cycles and wind loads.

The UV-protected side faces inward

A one-side UV-protected sheet must be installed with the protected face toward the sun. Incorrect orientation can accelerate yellowing, surface degradation, loss of toughness and embrittlement. Confirm the UV side from the protective film before removing it.

Strong acids, strong alkalis or incompatible cleaners contact the sheet

Polycarbonate is sensitive to extreme pH conditions and certain chemicals. Avoid liquids with a pH below 3 or above 11, particularly around stressed areas such as holes, bends and cut edges, where chemical exposure can accelerate environmental stress cracking.

Direct rain impact

Transparent roofs often have no suspended ceiling, insulation or sound-absorbing layer below them. Rain impact is therefore transmitted directly into the occupied space. Noise is often more noticeable with thin sheets, large spans, large roof areas and empty spaces that create resonance.

Thermal friction and sudden sliding

As the sheet expands during the day and contracts at night, it moves along the glazing bars or supporting frame. When friction prevents smooth movement, stress may build up and then release suddenly, producing clicking or popping sounds. Proper clearance, sliding connections and compatible isolation tape can reduce this type of noise.

Wind-induced vibration

Too few fixings, loose edges, excessive support spans or long overhangs can allow the sheet and connectors to vibrate. Tightening every screw is not always the correct repair because excessive restraint may reduce vibration temporarily while increasing the risk of thermal cracking.

Leak inspection sequence

1. Separate rain penetration from indoor condensation.
2. Test the roof in sections from the highest point instead of wetting the entire roof at once.
3. Inspect wall flashing, ridges, end laps, side laps and gutters.
4. Check whether screws are angled and whether washers are cracked or excessively compressed.
5. Look for warping caused by restricted thermal expansion, which may pull joints apart.
6. Remember that water can travel along sheets, purlins and profiles, so the indoor drip point may not be the actual entry point.

Crack-location diagnosis