Polycarbonate Thermal Cracking: Why Aluminum Extrusions Must Let Panels Move

Thermal Cracking Starts When the Frame Stops Moving

Cracks in polycarbonate rarely begin with a bad sheet. They begin when an aluminum frame behaves like a vise. Polycarbonate is expected to grow and shrink with every temperature swing, and when the extrusion clamps that movement away, stress builds at the weakest point: the edge, the fastener hole, or the corner radius.

On failed greenhouse roofs and patio covers, the damage pattern is remarkably repeatable. The panel may look perfect after installation, then one hot week later a hairline appears at a screw, the edge goes white, or the panel bows hard enough to pop a cap. That is not random material failure. It is a restraint failure.

The correct design goal is simple: the extrusion must hold the sheet, seal the joint, and still let the panel move. The best expansion-friendly extrusion profiles are designed around that idea.

Thermal Expansion Is Not Small

Polycarbonate expands about six times more than steel and about eight times more than glass. That difference becomes large fast. A 20-foot panel can want nearly 3 inches of movement across a 100°F swing. A 6-foot sheet may only move a quarter inch on each edge, but that is enough to crack a tight channel.

Darker sheets absorb more solar energy, so bronze or tinted panels need more allowance than clear ones. A sunny roof can get much hotter than the air temperature, which is why a panel that looked fine in a cool morning dry-fit can fail after its first full summer.

What the Extrusion Has to Accomplish

Proper aluminum extrusion does four jobs at once:

Resists uplift and side loads.
Keeps rain and dust out.
Leaves controlled clearance for thermal growth.
Lets gaskets or caps absorb small cyclic changes without locking the panel.

A rigid clamp is the wrong mental model. The extrusion should behave more like a guided track. The panel should be held, but not pinched.

In base-and-cap systems, the base anchors to the structure and the cap compresses the gasket. In H-channels, the fit has to be generous enough to accept movement but tight enough to keep water from blowing through. If the profile width is only matched to the nominal sheet thickness and ignores expansion space, the joint is too tight for real-world service. That is why a movement-ready channel system matters more than an attractive profile drawing.

The Details That Prevent Stress Cracks

The movement allowance has to be built into the small stuff, not just the profile shape.

Drill oversized holes so screws do not lock the sheet in place.
Keep screw holes away from the edge where stress concentrates.
Use EPDM or neoprene washers and gaskets so the panel can cycle without abrasion.
Leave calculated edge gaps instead of forcing the sheet to fill the channel wall-to-wall.
Tighten fasteners enough to seal, not enough to distort the sheet.
Use sealants that are chemically compatible with polycarbonate.

The sealant cannot rescue a joint that is mechanically over-constrained. If the panel is already trapped, the first heat spike turns every fastener and every edge into a stress concentrator.

What Failure Looks Like Before the Crack Arrives

A bad thermal design usually gives warning signs before it splits:

The panel needed force to fit into the channel.
Screw heads are dimpling the sheet.
The cap had to be pried down hard to seat.
The panel edge is touching both walls of the extrusion.
The sheet is already bowed on the first warm afternoon.

Those symptoms matter because they show the system is storing stress instead of releasing it. Once the seasons change, the stored stress shows up as whitening, crazing, or a clean fracture line.

Why the Best Systems Seem Slightly Loose

A good installer often gets nervous the first time a polycarbonate panel drops into a properly sized extrusion. It does not feel brutally tight. It may even feel a little loose during dry-fit. That is usually a sign the design is correct.

The system is supposed to be secure after the cap is seated and the gasket is compressed. It is not supposed to be rigid before the sun has had a chance to heat the panel. The best installations leave room for the panel to move quietly so the movement never turns into visible damage.

The Rule That Prevents Most Cracks

If the panel cannot slide quietly as temperatures change, the extrusion is wrong, not the polycarbonate.

That one rule explains most of the cracking seen in greenhouses, skylights, carports, and patio covers. The sheet did not fail because it was brittle. It failed because the frame prevented normal thermal movement. Build the joint to guide expansion instead of fighting it, and the panel stays clear, watertight, and intact through summer heat and winter cold.