The panel is coated on one side only, and the film you are about to peel off is the only thing that says which side.

Do not peel the film until the panel is fastened

The single instruction that matters most, and the reasoning is worth carrying because it is what makes it stick. The UV coating is on one side only, manufacturers mark the coated face on the protective film with printing or a colored side, and one guide states it directly: the film is your only guide, and once it is off you are guessing, with a fifty-fifty guess not being good enough. The failure is delayed and total. The unprotected face begins to yellow and haze in as little as one or two seasons, becomes brittle enough to crack from hail or a thrown ball, and loses the light transmission the material was chosen for. And incorrect orientation is noted as voiding warranties, so there is no recovery on that side either.

Mark the frame before you peel, not the panel

A small habit that solves the practical problem the rule creates, because you will be handling panels, turning them over, setting them down and picking them up again across a long build. Once you have read the film and know which face goes out, put a pencil mark somewhere that survives the process: on the flute ends, or on the frame member the panel is about to sit in, or on the inside face if the film there is coming off anyway. What you are protecting against is not ignorance but the middle of a fifty-hour assembly, where a panel gets moved three times and nobody can remember whether this is the one that was already checked.

Store them out of the sun, or the film may never come off

A compounding failure worth knowing before the delivery arrives. Guidance is to store panels in a cool dry location away from the sun, with one builder warning that if you store them in the sun you may never get the protective film off. Put that next to the first rule and the bind is obvious: the film has to survive until installation, and it also has to come off afterward, and leaving the stack leaning against a sunny wall for three weeks breaks both requirements at once. A shaded garage or under a tarp is enough. Cool and dark is what the suppliers ask for.

If the film is already gone, there are two tells and both are brand specific

For a secondhand greenhouse or unmarked sheets found in a garage, the honest answer is that there is no general test, but somebody who has sold and installed these since 1993 offers two manufacturer-specific ones. One maker puts an invisible tint on the UV protected side which shows up only under a black light. Another prints their name along the edge of the sheet, and if you can read the name the correct way up, the UV side faces the sun, though this only works on a full sheet that has not been trimmed at the sides. The same source is candid that other suggestions in the discussion may not work at all. A black light is a couple of dollars and worth trying before writing off a stack of panels.

The holes are deliberately too big and the screws deliberately not tight

Counterintuitive and the source of a lot of cracked panels. Polycarbonate has a high coefficient of thermal expansion, so pilot holes through the sheet must be oversized, with one source giving three eighths of an inch for a standard quarter inch fastener and another giving holes an eighth of an inch larger than the fastener, specifically so the sheet can move freely without buckling or cracking around the screw shank. Tight holes cause stress cracking. And the screw itself is not driven home: over-tightening damages the internal structure and reduces the insulation value, which is why the fasteners come with neoprene or rubber washers. In a proper bar-cap system, most screws do not pass through the panel at all, holding the cap instead.

Flutes vertical, and the failure is algae inside the wall

Two reasons and one memorable consequence. Panels installed with the channels running vertically drain condensation properly and also maximize strength, and on a roof the equivalent is to run the flutes along the slope so moisture moves downward. Get it wrong and horizontal channels trap moisture, which promotes algae and reduces clarity. That is the version worth picturing: algae growing inside the wall of the greenhouse, in a sealed channel a few millimeters wide, where it cannot be reached or cleaned and will only get worse. It is not a repair, it is a panel replacement, and it was decided at the moment somebody chose which way to face the sheet.

The top and bottom tapes are different products doing opposite jobs

Easy to get wrong because they look similar and often arrive together. Guidance is specific: seal the top of the channels to keep out moisture and dust, and seal the bottom with a tape that allows condensation to drain while keeping bugs out. So solid at the top, vented at the bottom. Fit solid tape at both ends and the flutes become sealed tubes that collect condensation with no way out, which produces exactly the clouding and algae the vertical orientation was supposed to prevent. Fit vented tape at the top and you have let dust and water in from above. Two tapes, two ends, and no visual difference once they are on.

Cut carefully, clear the flutes, and size for your snow

Three practical items. On cutting, use fine-tooth blades designed for plastics, support the panel fully, and blow the debris out of the flutes afterward, because sawdust and plastic swarf left inside a sealed channel is permanent and visible. On thickness, four to six millimeters is described as standard for backyard greenhouses with eight and ten millimeters recommended for larger structures or harsher climates, and manufacturers publish load tables giving maximum spans and spacing rather than leaving it to judgment. In heavy snow regions the levers are thicker panels, shorter spans, or added purlins. And one reassurance, since people ask: the UV coating protects the sheet from degradation and does not block the light plants need for photosynthesis.