Safety of your Roof Glazing

When looking at specifying roof glazing, either in the form of overhead roof lights or larger structural glass roofs, the safety of these glass installations is paramount. Glass roofs are required to perform similar functions to the building elements they are installed within as they place demands on the fabric of the building. These demands range from the structural loads forced on the glass unit to how well insulating it is (including sound and heat insulation).

NB Toughened and toughened laminated security glass should always be specified for any overhead roof glazing. Not all glazing companies will specify this as standard as it is not yet a legal requirement, however, IQ Glass strongly advise that any roof glazing installed on your project satisfy the requirements for “non-fragile” roof installations and use security glass.

All roof glazing installations are designed to provide natural light, and sometimes ventilation, into the interior of a building. The glass specification that is used, is manufactured with a loading allowance for maintenance and not regular foot traffic. If roof glazing is desired to be a walk-on-rooflight then the glass specification needs to be manufactured to meet the load demand to ensure the glass can withhold the predicted weight.

3D Structural Glass Roof
Strip rooflight

Safety for Occupants

Glass is heavy. If singular toughened glass is specified in your roof and the glass breaks it will fall onto the occupants below and has the ability to cause extensive damage to the space and people below.

Depending on the intended purpose of the roof, “non-fragile” rooflights are installed to prevent people or objects from falling through the glass if an accident were to occur on the roof. Non-fragile glass refers to a glass unit that’s designed to be accessible but doesn’t support frequent footfall. Just because a glass roof is classed as “non-fragile” doesn’t mean that it is strong enough to be used as walk-on glass, this requires an enhanced glass specification. All roof glazing from IQ Glass uses a toughened glass outer pane and a toughened laminated inner. This means that should the glass roof ever break the broken glass will be held in place by the interlayer, therefore not fall onto the occupants below.

At the design stage of a project, it’s important to consider and discuss with IQ Glass what the roof access requirements are so that the glass can be carefully specified. Always specify security, toughened laminated glass for your roof glazing and ensure the roof structure is non-fragile. Most rooflights tend to be designed as non-fragile systems and meet the same non-fragility as the surrounding roof structure but it’s best to get advice from a glazing contractor to confirm this.

What is laminated security glass?

When specifying roof glazing, it’s important to understand that how the glass is manufactured is the most critical part to achieve the desired performance of the product. As mentioned before, IQ Glass use toughened laminated safety glass for their roof glazing. Laminated glass is when two or more sheets of glass are bonded together with a strengthening interlayer. This interlayer is specially designed to hold the glass together if any or all of the glass should break. It’s therefore considered to be a safety glass as the broken glass is held together and won’t fall from overhead into the room below.

Laminated glass is also typically used where security is a priority as when the glass is smashed it doesn’t fall through, which delays and even prevents burglars from managing to get through the glass before emergency services arrive.

glass lantern rooflight
Glass roof with glass beams

How is the strength of roof glazing tested?

Both hard and soft body impact testing is carried out to test which Class a glass unit falls into; these classes go from Class 0 through to Class 3. Class 0 denotes glass roofs that are designed for unrestricted access by the buildings occupants, as the Class number increases more restrictions are put in place, especially Class 3 that is deemed a fragile roof where additional safety measures need to be put in place.

So, what is a hard body and soft body test? These are drop tests using a sandbag for the soft body testing or a steel ball for the hard body testing. During the “hard body” test, a steel ball of 4.11kg is dropped from a height of 1.2m to generate the force equivalent to a person stumbling on to the glass rooflight while carrying something (creating an impact energy of 448J). The “soft body” test is designed to simulate somebody falling onto the rooflight, so a 45kg sandbag is dropped from 1.2m high to create an impact energy of 530J.

To further test the strength of the glass, any glass that isn’t broken is then shattered and a sandbag weighing 180kg is placed on the glass to represent an injured person and somebody who has come to help them. With this weight on the glass, a timer is set for 30 minutes to see if the broken glass can stay in place for this long. The whole test procedure is then repeated 2 more times to ensure the results are consistent.

Throughout all these tests, minimal glass must fall onto the ground and nothing must pass through the glass. If small fragments of glass fall from the tested glass, then they need to be collected and weighed to calculate how much of a risk it will cause to the people potentially below the “glass roof”.

When observing the test results of glass, it’s important to take into account that the results are only accurate for the size and glass specification used during the test and also the frame support system used. This is especially something to consider when using bespoke rooflights on a project.

Maintenance Loading

All of IQ's structural glass roofs and rooflights are also designed and engineered for a standard maintenance load and are classified as a Class 1 non-fragile roof structure. The safety concerns about structural glass roofs often leave architects with the question “how will I clean a glass roof?”.

The answer is that the maintenance load allowance is put in place just for this reason. The strength of the glass specification used can take the load of one human in order for them to clean and carry out general maintenance on the glass roof.

Glass Roof
glass floor at elm park drive
Automatic rooflight

Do glass roofs have wind load requirements?

Yes. It’s not only loads imposed by people that need to be considered, it’s also important to ensure that the glass is suitable for the loads created by the elements in the project location. All year round, a roof is exposed to wind loading, so therefore the glass specification needs to be tested using the method outlined in BS EN 12211 which measures wind resistance, frame deflection and pressure resistance.

Site location is one of the most important factors when determining the wind load requirements, as some areas are more exposed to harsh weather conditions that others and this needs to be carefully considered.

Snow loading is another important loading aspect to take into account, but it is hard to predict how much snow loading will be required. Snow is more of a seasonal occurrence compared to wind, but snow can linger and create substantial weight. A glass specification is tested for “long duration loading” to check whether it is suitable for snow loading. It’s measured by testing whether there is any deflection in the centre of the glass and whether this poses any risks

Can I walk on a glass roof?

Only roof glazing that has been specifically designed to be used for foot traffic can be considered a “walk-on rooflight”, meaning that it must be designed to floor loadings.

As standard IQ Glass ensure that their rooflights are designed and manufactured with a maintenance loading – but this doesn’t mean it’s suitable for regular foot traffic. This means that if a window cleaner, maintenance worker or builder ever need to, they will be able to walk on IQ’s structural glass roofs for access, cleaning and maintenance.

This maintenance allowance also means that our glass roofs can be used as fire escape access in the event of an emergency, which is extremely important in inner city homes where there may be no other means of escape from upper floor bedrooms.

Walk on glass (also referred to as floor lights or glass floors) has a heavier glass specification compared to structural glass roofs, and the surrounding structure needs to be designed to support the loads created by the weight of the walk on glass. Usually walk on glass is created using three panes of glass: a toughened outer pane with a laminated interlayer. The thickness of the glass is dependent on the location and expected usage/ footfall.

Glass roof terrace
Kings Road Automated Rooflight
Automatic rooflight

What are the anti-slip options for walk on glass?

Anti-slip finishes can be applied to walk on glass to reduce the risk caused by wet and slippery glass surfaces caused by rain and melting snow.

One method to create an anti-slip surface is to sandblast the surface of the outer pane which not only adds slip resistance but also adds privacy to the space below and diffuses harsh light. Sandblasting is the ideal solution if privacy and light obscurity is desired, but if a clearer finish is better for the architectural design then there is the option of a ceramic frit. The frit is screen printed in your choice of pattern onto the surface of the glass and fired to permanently affix the frit in place, resulting in an anti-slip walk on rooflight.

Anti-slip finishes can be applied to walk on glass to reduce the risk caused by wet and slippery glass surfaces caused by rain and melting snow.

One method to create an anti-slip surface is to sandblast the surface of the outer pane which not only adds slip resistance but also adds privacy to the space below and diffuses harsh light. Sandblasting is the ideal solution if privacy and light obscurity is desired, but if a clearer finish is better for the architectural design then there is the option of a ceramic frit. The frit is screen printed in your choice of pattern onto the surface of the glass and fired to permanently affix the frit in place, resulting in an anti-slip walk on rooflight.

When specifying roof glazing you should not just consider the likely scenarios, but the unforeseen, emergency ones too.

If you have any questions and concerns about the specification of glass to your roof structure, please contact the team at IQ Glass.