A farm building in County Down needed a covered walkway between two sheds: 34 m long, 2.4 m wide, on purlins at 800 mm. Solid 8 mm polycarbonate would have been correct and also three times the cost. Corrugated sheet at 1 mm, 76/18 profile, did the job and the whole roof went on in a day.
The interesting part is not the material choice. It is that corrugated polycarbonate sheet has a span limit that is not obvious from the data sheet, and a lapping rule that decides whether the roof leaks in the first winter.
Corrugated polycarbonate sheet profiles and spans
- 0.8 mm, 76/18: light covers up to 600 mm purlin spacing
- 1.0 mm, 76/18: roofs to 900 mm spacing, the common default
- 1.5 mm, 76/18: 1200 mm spacing, or exposed sites
- 10/3 miniature profile: short spans and screens, not primary roofing
Span tables assume a warm climate and a closed roof. Wind uplift, and a roof open at the eaves, both reduce the safe spacing. A simple rule that holds up in practice: if the sheet deflects enough to pond water after rain, the spacing is too wide, regardless of what the table says.
Fasteners go in the crest, never the valley, and always with a sealing washer. A screw through a valley leaves a hole where water sits, and the leak shows up as a stain two metres downhill rather than at the fixing.
Lap detail and the thermal movement nobody allows for
Polycarbonate expands about 0.065 mm per metre per degree C. On a 34 m walkway that is roughly 100 mm of movement across a 45 C swing, which is why a roof should be fixed at one end and allowed to slide at the other. Fasteners in slotted holes at the far end, not tight round holes, and the sheets will not buckle at mid-span.
Side laps of one full corrugation give a weather seal without sealant. Adding a bead of neutral-cure silicone in the lap is fine on a low-pitch roof, but on a steep one it traps water behind the bead and the lap rusts its fasteners.
Condensation is the other complaint. Single-skin corrugated sheet drips on the underside in cold weather. If the space below is occupied, a multiwall product is a better answer, and our greenhouse roof panel guide explains the U-values of the twinwall alternatives.
For roofs where fire performance is specified, note that standard polycarbonate is not a fire-rated assembly. The PC UL94 flame rating guide sets out what the ratings actually cover.
Send the roof area, purlin spacing and wind load to oversales@rpplexiglass.com for a corrugated sheet take-off.
Frequently Asked Questions
What span can corrugated polycarbonate cover?
A single-skin 1 mm 76/18 profile spans roughly 600 to 900 mm between purlins for a normal roof load, depending on wind and snow. Beyond that the sheet flutters and the fasteners work loose.
Which way does corrugated polycarbonate overlap?
Side laps follow the profile: one full corrugation, lapping away from prevailing weather. End laps are at least 150 mm on a roof, with the upper sheet over the lower so water runs off rather than into the joint.
Do I need anti-dust tape on a polycarbonate roof?
On multiwall, yes: the open ends must be taped or the flutes collect dust and insects, which then show through the panel. On single-skin corrugated sheet there is no cavity, so taping is only for cut edges that will be seen.