25mm Multiwall Polycarbonate

We recommend a maximum sheet width of 900mm for 25mm multiwall without extra support. The manufacturer allows wider spans at light loads, but 900mm is the safe figure for a DIY roof on standard glazing bars. That is a sheet figure, not a structural one, so check your wind and snow exposure, or call the technical team.

25mm is the usual starting point for a conservatory you sit in year round. It loses about a third less heat than 16mm and passes about 62% of daylight in clear. 35mm loses about a sixth less heat again and is the current standard for new roofs. If winter comfort matters more than cost, choose 35mm.

Just the top face. The UV layer is co-extruded into one side of the sheet during manufacture, and the printed protective film marks it. That side must face the sun. Fitted upside down, we have seen sheets yellow and crack within a couple of years. Fitted correctly, the sheet is warranted for ten years against loss of light transmission.

Look for the protective film with printing on it: that is the UV-protected face, and it goes towards the sun. Leave the film on until the sheet is in place, then peel it off. The flutes, the hollow channels inside the sheet, must run down the slope so condensation drains out at the bottom edge, never across the roof.

The largest 25mm sheet is 2100mm wide by 7000mm long. We cut from that stock size to your measurements, to a tolerance of plus or minus 5mm. Cut sheets are bespoke and cannot be returned, so measure your rafter centres and roof length twice. We can pre-tape the cut ends on request, aluminium tape at the top and breather tape at the bottom.

What are pre-taped edges on multiwall polycarbonate sheets?

Pre-taped edges mean that protective tapes have already been applied to the open ends of your multiwall polycarbonate sheets before delivery, saving you time during installation.

At The Polycarbonate Store, our complimentary pre-taping service is available on 10mm, 16mm, 25mm, 32mm and 35mm twinwall and multiwall polycarbonate sheets.

The two open ends of the sheet's hollow channels (flutes) are taped differently:

• Top edge – Aluminium sealing tape: Solid silver tape seals the upper end of the flutes, helping prevent water, dirt and debris from entering.
• Bottom edge – Anti-dust breather tape: Helps prevent dust and insects from entering while allowing ventilation and moisture to escape, reducing the risk of condensation build-up.

For a standard sloping roof, the aluminium-taped edge must be positioned at the top, with the breather-taped edge at the bottom, towards the gutter. The tapes should remain in place during installation.

Important: Pre-taping only covers the two open flute ends, not all four sides of the sheet. It does not replace the need for PVC sheet end closures, which should be fitted over the taped edges to provide additional protection and a neat finish. Pre-taping also does not completely prevent condensation.

During preparation, approximately 10mm of protective film is removed from each taped end. This may leave a visible preparation line, which is normally concealed by the roofing accessories. If you need to trim a pre-taped sheet, the newly exposed flutes must be cleaned and re-taped correctly before installation.

For curved roofing or other applications, the required tape arrangement may differ. Please contact us before ordering if you are unsure.

For further information, read our full guide: What Do Pre-Taped Edges Mean on Multiwall Polycarbonate Sheets?