Today’s specifier has a wide range of excellent glazing material options to consider when specifying rooflights. However, glass has unique properties and is often selected for flat glazing applications, in domestic, retail and commercial projects. Glass manufacturers have developed products to satisfy the most demanding requirements, from sophisticated solar control options, to self-cleaning and other properties. Durability and impact resistance are other key areas in which manufacturers offer a range of alternatives to suit different applications and budgets. Understanding the properties of these diferent types of glass is important when considering the safety of those above the glass (often maintenance staff), and those below it (general building users).
Types of glass
Annealed and Heat Strengthened Glasses
Annealed glass is not classified as safety glazing when used as a monolithic pane, so its rarely used in rooflighting applications as it forms large shards when broken, which be extremely dangerous. This also applies to Heat Strengthened Glass which while stronger and more resistant to thermal heat stress than annealed glass, is also not classified as safety glazing.
Toughened Glass and Heat Soaked Toughened Glass
Toughened Glass is similar to heat strengthened glass, but when broken, it forms small, slightly rounded pieces of glass, known as ‘dice’, which are less likely to cause injury. Toughened glass is classified as a ‘safety glass’. However on rare occasions, these dice can clump together and fall as larger sections of glass. Another point to be aware of is that toughened glass can spontaneously shatter due to the presence of nickel sulphide (NiS) inclusions. This risk is significantly reduced, but not eliminated, by specifying heat soaked toughened glass.
Laminated panes are produced by bonding a film, or interlayer between two or more plies of glass (typically annealed /heat strengthened or toughened glass). If fractured, the interlayer holds the broken shards of glass, preventing injury to those immediately below the rooflight. The interlayer, depending on thickness, material and pane size, may also have the ability to hold a load lying on the glass, preventing falls from height through the rooflight.
There are two basic form of interlayer – PVB (polyvinyl butyral) or ionomer. PVB interlayers are the most commonly used in glass for the majority of domestic and architectural applications. However, ionomer interlayers offer five times the shear strength and up to 100 times the rigidity of conventional PVB interlayers. This makes it an excellent candidate for demanding architectural and structural applications. Along with its high strength, ionomer interlayers are also less susceptible to moisture penetration or hazing over time.
In addition to the basic categories of glass listed above, there are a number of specialised materials available, to address issues such as thermal, light and acoustic performance.
Safety for building users
To protect those below rooflights, the first choice for the inner pane of a rooflight should usually be a laminated glass, as this generally minimises the risk of any glass falling into the room beneath and consequent injury, if the inner pane should break.
British Standards define that inner panes must always be laminated wherever rooflights are more than 5 metres above floor level (increased to 13 metres in limited circumstances) or are located over water (eg swimming pools). British Standards also set out other circumstances and locations when a toughened inner pane can be used, subject to satisfactory risk assessment.
However, NARM recommends that the inner pane should always be laminated glass, unless a stringent risk assessment has been undertaken which shows that use of a toughened glass inner pane does not give any additional risk to those below rooflights. If after a risk assessment a monolithic toughened inner pane is being considered, it should always be heat soaked toughened glass. Guidance on how to go about carrying out a glass risk assessment can be found in a document published by CIRIA (Construction Industry Research and Information Association); document C632 ‘Guidance on Glazing at Height’.
Note that a laminated inner pane does not necessarily mean a rooflight can also be classified as non-fragile (which may be required to protect those above the rooflight). If required, non-fragility should be specified separately.
Where an annealed inner laminated pane is proposed, careful thought should be given to the risk of Thermal Heat Stress failure which could affect the durability of a laminated inner pane. To avoid this risk, and should additional strength be required to the inner pane, a heat strengthened or toughened laminate can be specified (with care taken to ensure this does not affect non-fragility classification).
Safety for personnel accessing the roof
In addition to using a laminated inner pane to protect building users, rooflights can also be specified and designed to be non-fragile*. This is intended to ensure that anyone accidentally walking or falling onto the rooflight will not fall through it, even if the glass is broken or the rooflight damaged. CDM Regulations require that a risk assessment should be carried out for every construction project, and this should define the measures required to prevent significant risk of anyone falling through a rooflight.
If the risk assessment confirms that satisfactory measures have been taken to ensure there is no risk of anyone falling through a fragile rooflight, then fragile rooflights can be specified, but otherwise rooflights should always be specified to be non-fragile.
Non-fragility for roofing products is generally defined by the “Red Book” ACR[M]001 issued by the
Advisory Committee for Roof Safety, but for glass rooflights that document cross refers to documents issued by The Centre for Window and Cladding Technology: CWCT Technical Notes 66 and 67 “Safety and Fragility of Glazed Roofing: guidance on specification” and “testing and assessment” respectively, and CWCT Technical Note 92 “Simplified Method for Assessing Glazing in Class 2 roofs” which includes a simplified test method and a “deemed to satisfy” solution for rooflight applications in a Class 2 roof (one not intended for any foot traffic).
These CWCT documents should be used to define non-fragility of glass rooflights. They are based on the ACR[M]001 soft body impact test, together with an additional hard body impact test specifically relevant to glass, and a static load test to ensure that a broken IGU can retain an injured person who has fallen onto the glass. Pass criteria include ensuring the soft body does not fall through a broken pane of glass, and that no significant shards of glass drop.
It’s important to note that any non-laminated pane of glass is incapable of retaining a body once broken, and any rooflight with a toughened inner pane must therefore always be considered ‘Fragile’. It should also be noted that not all laminated glass combinations will achieve a ‘Non-Fragile’ classification, and many rooflights with laminated inner panes may be fragile: an enhanced specification of laminated glass, and how it is secured in the framing system, may be necessary to achieve non-fragility. Rooflights should either be tested to CWCT TN 92 (or to TN 66 and TN67), or comply with the deemed to satisfy standards defined in CWT TN92 to be regarded as non-fragile.
Deemed to satisfy standards specify a minimum inner pane of 9.5mm or 11.5mm laminated glass (depending on size) comprising 2 plies of annealed or heat strengthened glass with a 1.5mm pvb or ionomer interlayer, continuously supported in a rebated frame with an edge cover of at least 15mm on all four sides.
Further information and NARM Technical Documents on safety in relation to rooflight specification, plus documents from the Advisory Committee for Roofsafety, can be accessed free of charge on this website, at: https://www.narm.org.uk/downloads/safety-on-roofs/