What are the performance considerations for rooflights and roof windows?

When specifying rooflights or roof windows, the first thing you’ll probably think about is how the items will look once they’re installed. This is certainly an important aspect to consider – after all, appearance is critical. But you will also need to analyse the performance criteria of these glazing units to ensure they serve their purpose well.

Performance characteristics for rooflights and roof windows

There are a number of performance characteristics to research when specifying roof lights and roof windows. These include:

Resistance to wind loading

Testing checks pressure and the level of frame deflection; the method for testing is set out in EN12211:2016

Resistance to snow loading

The impact of snow loading on roof windows and rooflights can be significant, and testing for this involves long-duration loading, with the center-of-glass deflection being measured

Reaction to fire

Materials are tested for combustibility and ignitability

External fire performance

Windows need to be tested for fire resistance and smoke control

Water tightness

This is tested in both shielded and non-shielded conditions and testing requirements are described in EN 1027:2016

Impact resistance

Roof windows and rooflights may be subject to impacts both from the inside and outside. The size of the window and the thickness of the glass can both affect the impact resistance, and glazing can have special treatments to improve resistance

The load-bearing capacity of safety devices

This includes elements such as retaining and reversing catches, restrictors and fixing devices.

Acoustic performance

Things that can affect the acoustic performance of windows include whether they are double – or triple-glazed, the thickness of the glass and the amount of space between panes of glass

Thermal transmittance

Also known as the U-value, this is a measure of how effective a material or building element is as an insulator. The lower a U-value is, the better the material is at keeping heat inside the building

Radiation properties

These include solar factors and light transmittance

Air permeability

The testing requirements are set out in EN 1026:2016

Which standards apply to rooflights and roof windows?

The recommended minimum performance requirements for the UK are described by various British and European standards, and in some cases, Scotland and Northern Ireland have different requirements than England and Wales.

For example, Approved Documents L1 and Approved Document L2 state that the maximum U-value for dwellings is 2.0W/M2k. Northern Ireland’s Technical Booklets F1 and F2 have the same requirements. But in Scotland, the Technical Handbook states that the maximum U-value for dwellings is 1.8W/M2k.

Here are some of the standards to consult when determining if a roof window or rooflight meets the performance requirements:

BS EN 14351-1:2006+A2:2016

This standard identifies material-independent performance characteristics for windows, including roof windows

BS 6375:2009

This a series of standards that provide advice on the selection of performance characteristics and doorsets in relation to BS EN 14351

BS 4873:2016

The specification for aluminium alloy windows and doorsets

BS 644:2012

The specification for fully-finished, factory-assembled timber windows and doorsets

BS 6510:2010

This is the specification for steel-framed windows and glazed doors

BS 7412:2007

The specification for PVC-U windows and doorsets

CE Marking and rooflight specification

CE Marking is a declaration that a product conforms to the stated harmonised European standard (hEN). It confirms that a product meets the minimum standards set out by the EU. However, it does not imply that a product conforms to national building regulations or codes.

While all windows should be CE marked, CE marking does not currently apply to out-of-plane rooflights.

Independent Testing of rooflights and roof windows

Although several standards exist to cover various roof coverings from domes, to polycarbonate systems and smoke vent AOV systems, for out of plane rooflights installed below 15 degrees there is no defined standard so it is impossible at the moment for manufacturers to test their products. One option is to look for those companies who have opted to test their products using recognised independent organisations such as the British Board of Agrément (BBA) which is one of the UK’s leading notified bodies offering approval, certification and test services to manufacturers of products and systems supplying the construction industry. BBA approval is recognised by building control, government departments, architects, local authorities, specifiers, and industry insurers like the National House Building Council (NHBC).

Not all roof windows and rooflights are equal

When specifying roof windows and rooflights, ensuring the elements meet the minimum legal requirements is a must – but, depending on your situation, you may also want the item you choose to go beyond the basics.

Roof windows and rooflights provided by different manufacturers will have different performance characteristics, and different products from the same manufacturer will also vary. With this in mind, it’s always important to check the relevant product specifications carefully. Here at Glazing Vision, we can also work with clients to ensure that the product you select meets your requirements.

To learn more about the performance considerations for rooflights and roof windows, you can request one of our RIBA-approved CPD’s.