Clear Polycarbonate

Clear sheets run from 4mm to 35mm: 4mm and 6mm twinwall for greenhouses, 10mm twinwall for light covers, 16mm for lean-tos and canopies, and 25mm, 32mm and 35mm multiwall for conservatory roofs. Thicker sheets insulate better but pass less light. Sizes from 10mm upwards are cut to your measurements from stock up to 2100mm by 7000mm.

Between about 82% and 60%, depending on thickness. A 4mm twinwall sheet passes about 82%, 10mm about 79% and 16mm about 76%. From 25mm upwards the figure settles at about 62% to 63%, because the extra walls scatter more light. Even at 35mm a clear roof feels bright, but expect some glare on a south-facing aspect.

Yes, at different thicknesses. For a greenhouse choose 4mm or 6mm twinwall, which lets in the most light for plants. For a conservatory you sit in year round choose 25mm or thicker. Clear is the brightest option, but a south-facing conservatory can overheat in summer, so consider bronze, opal or heat guard there instead.

Polycarbonate has around 200 times the impact strength of glass, so a dropped tool or a hailstone will not shatter it. It is also much lighter, which means a simpler frame and easier handling on the roof. Glass still suits a roof where you want a fully transparent, glossy finish, because multiwall scatters some of the light passing through.

It is built in. The UV protection is co-extruded into one face of the sheet during manufacture, not a coating you apply later, and it blocks 99.9% of UV. The printed film marks that face, which must go towards the sun. Fitted correctly, the sheet carries a ten-year warranty against loss of light transmission.