Specifying large format glazing On your head be it!

21 May 2014 by Matt Trace in Construction/Architectural

Specifying large format glazing On your head be it!

As technology and thermal performance continues to improve, the trend in recent years has been for homeowners to opt for aesthetically pleasing large format glazing panels. After all, why wouldn’t you? They look great and let in larger quantities of natural daylight.

However when it comes to specifying large format glazing there seems to be huge disparities between what is achievable and acceptable and there is no one size fits all guidelines. The advice given to architects and specifiers will vary, depending on the application and type of installation, and on the company that you speak to.

When it comes to rooflights, which are designed to lay flat or at an angle on a roof, this adds the challenge of exposure to the elements. When you consider that a cubic meter of snow can weigh around 100kgs, then it’s easy to understand the potential dangers involved in specifying suitable rooflight glazing.

Specifying glazing for rooflight units is particularly important for obvious reasons, the main one being that glass installed high above your head can be potentially life threatening if it fails.

Glass, be it double glazed, toughened, or treated, has a breaking point and simple physics determines that the larger the piece of glass the weaker it becomes. Believe it or not glass does bend and the deflection in a unit can be as much as 25mm in some cases before it becomes unsuitable. This deflection is sometimes referred to as the ashtray effect and can be quite daunting when you have 150kgs of glass above your head.

The difference between what glazing companies deem ‘acceptable’ in terms of a single sheet of glazing vary greatly and it’s often left to the architect to specify what they determine as safe. In turn, architects will seek the advice of rooflight manufacturers, who in turn rely on the advice of the glazing manufacturing.

It’s not as straightforward as looking at the toughness of the glass. Architects should be asking more questions when specifying rooflights, such as what is the pitch of the roof, the pane size, the method of support, the wind and snow loads, type of glazing, etc.

One of the UK’s leading glass suppliers, Pilkington, requires manufacturers to complete a 2 page specification document before they can supply the glazing unit. If the glazing specification does not meet the criteria they deem to be ‘safe’ then they simply will not manufacture the glass.

For example, a single sheet of glazing 3.5m x 1.5m being installed into 5 degree pitched roof, made from toughened glass, might be deemed as unsuitable by Pilkington, yet there are other manufacturers out there willing to supply this.

According to Pilkington, 75% of UK urban properties can work to an average wind load of 1200 kNm2 and a snow load of 750 kNm2. The other 25% will require smaller glazed areas or a more expensive and thicker glass option.  Thicker glass is also heavier and may have an impact on the materials used to manufacture the rooflight.  A unit made from two pieces of 4mm toughened would weigh 20kgs per m2, whereas a unit made from two pieces of 6mm toughened would weigh 30kgs per m2.

Ideally large units should be supported on all 4 sides.  Where support is only on two or three sides, the maximum unit size decreases.

Location also plays an important factor when specifying large format glazing. A large rooflight made for a project in London might not be suitable for a project in Brighton, despite the design and size of the rooflight being exactly the same.  Don’t be fooled into thinking that because a supplier can make a single unit at 5m2, that it will be right for your project.

When things go wrong, liability can be a tricky one to sort out, with the builder, architect, manufacturer, and glazing companies all left exposed.

Not all projects have a JCT and in reality, structural engineers are often bypassed on smaller projects due to cost or time constraints and glazing units are often specified with little or no insurance policy for their design.

The issue of liability is one that can be avoided by taking the advice of a reputable glazing company. If a glazing company won’t manufacture the glass you want, it’s probably not a good idea to shop around until you find one that will.

Just because it can be done doesn’t make it a good idea! The advice would be to err on the side of caution and seek the advice of Lumen on your bespoke rooflight projects.

For further information on rooflight glazing specification contact Lumen on 0330 300 1090, email enquiries@lumenrooflight.co.uk or visit www.lumenrooflight.co.uk