6mm polycarbonate sheets are where polycarbonate becomes robust enough for exterior glazing purposes.

The primary limiting function of polycarbonate is not its durability, which is impressive, but its weight, or lack thereof. Polycarbonate is so light that panels are vulnerable to the wind if they cover too much surface area relative to their width. So while 6mm polycarbonate should not be used for large structures you don’t need to worry about rain volume. This means that for instance, very small greenhouses will work fine with 6mm thick polycarbonate. 

6mm sheets are also perfect for protecting small outdoor installations. Fish ponds can be protected this way, as can a piece of outdoor exercise equipment or a table that adjoins a wall. The untapped potential of polycarbonate opens up all sorts of options for the imaginative homeowner. As long as glazing beams can hold it up, it can be placed somewhere.