Toughened glass (4mm)

Toughened glass is approx 700% stronger than horticultural glass, so it is far harder to break. When it does break, it disintegrates into hundreds of tiny pieces, so it is far safer than horticultural glass ( which can form dangerous daggers of glass when broken)

However, the main reason that the toughened glass is now so popular is that it is supplied in large sheets- This means that there are no or few overlaps between the panes (with normal horticultural glass, you get a 10mm overlap between every 2ft pane, these fill up with dirt and algae and become unsightly) With large pane toughened glass, you get a greenhouse that looks far neater, is easier to keep clean, and of course is safer.