In sloping patent glazing situations, it is a requirement to install glass which will either tend to stay in place if it is cracked (Wired and Laminated Safety Glass come into this category) or to fracture into relatively harmless pieces (Toughened Safety Glass comes into this category) which are less likely to cause serious injury, if they fall, than sharp shards of annealed glass.
BS 5516: 1994 Code of practice for vertical and sloping patent glazing is the only regulation or standard which gives any recommendations about the type of glass to use in roofs.
For single glazing, either wired, laminated or toughened glass is recommended by BS 5516, while double glazed units should have one of the three types as the lower pane.
With regard to toughened glass, BS 5516 suggests that, if the lower pane of a double glazed unit is toughened, then the upper pane should also be one of the three recommended types of glass.
Recommendations for the possible types of glass suitable for use in sloping Patent Glazing
Glazing at a height less than 5m above floor level
(this covers single storey buildings – conservatories etc.)
Single glazing: Toughened Safety Glass, Laminated Safety Glass, or Wired glass.
Double Glazed units: The lower pane should be one of the above types of glass. If the lower pane is Toughened Safety Glass, then the upper pane should also be one of the above types of glass.
Glazing at a height over 5m but less than 13m above floor level
Single glazing: Ideally Laminated Safety Glass or Wired glass. Toughened Safety Glass may be considered with the following restrictions; not more than 6mm thick and not more than 3m² in area.
Double Glazed units: Ideally the lower pane should be Laminated Safety Glass or Wired glass. Toughened Safety Glass may be considered with the following restrictions; not more than 6mm thick and not more than 3m² in area. If the lower pane is Toughened Safety Glass, then the upper pane should also be one of the above types of glass.
Glazing at a height over 13m above floor level
Single glazing: Laminated Safety Glass, or Wired glass. Double Glazed units: The lower panes should be Double Glazed units: The lower panes should be Laminated Safety Glass or Wired glass, Laminated Safety Glass or Wired glass.
Recommendations for the thickness of glass used in sloping Patent Glazing
Glass in sloping patent glazing throughout its long history has generally been installed at a thickness of 6mm. This is still by far the most common thickness used on all patent glazing projects and patent glazing's excellent safety record shows that a 6mm glass is both safe and practical.
Due to the relatively narrow widths of glass used in patent glazing, there is generally no need to use a glass thicker than 6mm to withstand all the usual loadings that the glass is likely to be subjected to.
Patent Glazing is predominantly situated in a sloping situation, therefore in the event of a glass breakage and depending on the location, consideration may need to be given to the protection of the public below from falling glass.
As stated above, Toughened glass should not be used above 6mm thick in patent glazing if the height is between 5 metres and 13 metres above floor level. When broken, Toughened glass at 6mm thick fractures into small pieces (called dice). As these particles do not have the sharp edges and dagger points of broken annealed glass, it is regarded as a safety glass. While these dice may cause minor cuts, it is very difficult to cause a severe injury with them, provided the fragments are small enough. Toughened glasses thicker than 6mm however, if installed above 5 metres from floor level may cause harm to the public below when broken due to the increased weight of the glass which is falling to the ground.
Toughened glass of any thickness should not be used in sloping glazing if the height is over 13 metres from floor level because the broken glass pieces (dice), falling from such a height, have a more likely chance of causing injury to people below than from a lower height.