11 Jan 2021

The Elephant in the Roof


Paul Trace from Stella Rooflight discusses the practicalities involved in specifying large rooflights.


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As a bespoke rooflight manufacturer, we often see grand plans with expansive areas of roof glazing, as architects continue to seek innovative ways in which to exploit natural daylight. As much as bespoke rooflights can certainly help bring these designs to life, there are some important factors that need to be considered when the glazing is turned from drawing to reality.

In recent years, we have noticed a trend towards larger rooflights, with sizes regularly exceeding 2500mm in width, height or both. While Stella can certainly accommodate these, one has to appreciate the practicalities of weight, transportation and cost. It stands to reason that the larger the rooflight, the thicker the glass will need to be, therefore, as rooflight sizes get bigger, their weight can increase exponentially. In context, our double-glazed units comprising 4mm-thick glass weigh 20kgs per square metre, and those using 6mm are 30kgs per square metre. Triple-glazed units are 30kgs and 45kgs, respectively. Add the stainless steel frames and hardwood liners into the equation, and you can start to see how even a modest-sized rooflight can weigh something akin to a small elephant! Indeed, it’s not uncommon for us to produce rooflights weighing in excess of 200kg.

The reality is that when you scale things up, even the simplest plans can start to become complex. But before drawing that large rectangle on a roof plan, it is important to consider how the rooflight is going to be lifted from the ground to roof level and what the weight implications might be for the structure. Stella can weld stainless eyelets to the rooflight frame to aid the lifting process, and we would suggest doing this on frames which weigh in excess of 100kg, which is the point where we expect a crane might be required. However, does the site have suitable access for any specialist lifting equipment, and has this additional cost been budgeted for?

The weight of an opening rooflight will also determine whether it is manually operated or requires electric actuation. We would suggest that our largest manually operated rooflight be around 1000mm wide or 1400mm high. Anything over this size will generally require electric actuation to lift the weight and in the instance of wider casements, to provide a tight seal. Manual operating casements are less expensive than electric, so there is a cost element that also needs to be considered when specifying larger opening rooflights. There is no maximum size in terms of what is possible to manufacture, although it is worth keeping in mind that a single piece of glass with an area over 5m2 becomes significantly more expensive.

Fortunately, there are alternatives to using large, expensive single panes of glazing, such as introducing glazing bars to reduce the unit sizes, linking frames and having more than one casement. This not only makes the rooflights easier to transport, lift and install but can also reduce the price.

If a large, single rooflight is the only option for your project, then you must also make sure that it is safe to install. While rooflight manufacturers will be able to provide advice on a suitable specification for the job in terms of materials, glazing thickness, functionality and installation, it is ultimately the responsibility of a structural engineer to ensure that the product being specified is fit (and safe) for the building that it is fitted into.

To avoid any ambiguity, it is fairly essential that the architect, structural engineer and rooflight manufacturer discuss large rooflight installations, likely weights, load-bearings, site access and lifting capacity at an early stage to avoid any problems down the line. It is not advisable to leave this for the builder to deal with at the last minute.

Another area for consideration should be the safety of large areas of glazing situated high up in a roof structure. Again, much like an elephant, there is a lot of grey area here.

Regardless of its size, your rooflight should meet the BS 5516-2: 2004 patent glazing and sloping glazing for buildings standard. This code of practice for sloping glazing defines that inner panes must always be laminated wherever rooflights are more than 5m above floor level (increased to 13m for panes less than 3m2) or are located over water (e.g. swimming pools). The standard permits the use of toughened inner panes in other applications (for example, where rooflights are less than 5m from floor level), but only where a stringent risk assessment for the particular application has been completed and has concluded that the use of toughened glass does not give any additional risk to those below the rooflight.

No glass is impervious to breakage, so it is important to remember that roof glazing can, and often does, sit high above common areas in a home or office, so there will always be an element of concern if a unit were to break.

Certain industry bodies are calling for all rooflight glazing to include a laminated inner pane to provide greater security in the event that the glass breaks. Much the same as how car windscreens are required by law to be laminated to protect passengers from shattered glass in the event of an accident. There can be no dispute that laminated glass is safer because it forms a net when broken, which remains in one piece, whereas toughened glass breaks into little cubes and will fall down onto whatever is below.

While there is a growing trend for projects to include more and larger rooflights, it is not as straightforward as just adding them to the plans. Clearly, there is so much more to specifying large rooflights than meets the eye and while your rooflight manufacturer will be able to provide recommendations, ultimately having a better understanding of what glazing is required and involving a qualified structural engineer in the early phases will resolve any issues you may have further down the line.

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