Useful Information about Architectural Glazing

The difference between a rooflight and a roof window.

Roof glazing solutions provide many benefits, including increasing natural light, improving ventilation, and creating a feeling of spaciousness.  

Understanding the difference between the applications of various roof glazing and their thermal performance is a commonly asked question. Orientation and position of roof glazing installations can affect their thermal performance so understanding the U values  of rooflights as well as the features and benefits of different glazing solutions in roofs are important. 

When choosing a roof glazing solution, it is important to consider factors such as the pitch of the roof, the size of the room, and the intended use of the space. It is also important to ensure that the glazing solution meets building regulations. See further on for the difference in required Uw values for roof windows vs rooflights.   

Here are some of the most common types of roof glazing solutions and their differences: 

Glass rooflights 

Rooflights can also be referred to as Skylights and describe the same stye and design of roof glazing.  

Rooflights and skylights can be fixed or openable and are installed on an upstand (or kerb) within a roof structure. By elevating the rooflight away from the roof line, it provides a more robust approach to water tightness and as a result, rooflights and skylights can sit flat with little fall to the glass roof.  

Although the main characteristic of a rooflight is that it has no pitch, rooflights can be installed at an angle to suit the line of the roof if this is the preferred design.  

If ventilation is required through a rooflight, the roof glass can be an opening rooflight in a hinged or sliding design. Sliding roofights provide a larger opening for ventilation and can also be used for access. For simple ventilation, hinged rooflights are more typical. Venting roofights are most commonly found in the flat roofs of kitchen extensions for this reason.  

It is important that the external design of a rooflight is considered at the specification stage. As these rooflights are often fitted to flat roofs they are overlooked by upper rooms. You want to ensure that the external finish of a rooflight is minimal and neat whether it is opening or fixed. For example, the external design of the M.A.R.S. sliding roofight is bonded into the frame which allows the exterior design of the rooflight to be minimal and modern which was a key part of the design development of the system.  

Roof windows

Roof windows are a specific type of roof glazing and are covered under their own regulations (BS EN 14351-1:2010).  

A roof window is different to a rooflight as it has to be installed in line with the roof pitch with a minimum fall of 15 degrees. These are the types of roof glazing you tend to see on loft extensions and conversions.  

Due to the specific nature of these types of roof glass, the roof windows normally have to be UKCA marked and are only available in specific off the shelf, standardised sizes and shapes. Roof window sizes are normally smaller than rooflights and cannot achieve the sizes that rooflights can (which can be unlimited in size).    

As the weather proofing for roof windows must go around it, roof windows are most often installed with the roof construction. The roof window is then sandwiched between the roof construction.  

The thermal performance of rooflights vs roof windows  

Building regulations approved document L stipulates specific differences between the required thermal performance of a rooflight and a roof window. This is why it is so important that architects and specifiers understand the difference.  

The 2022 version of Approved Document L states (in relation to the required Uw values of gazing on a build); “In the case of roof windows, unless the measurement or calculation has been done for the actual inclination of the roof window, adjustments as given in Notes 1 and 2 to Table 6e or from BR443 (2019) should be applied. The U-values of rooflights are tested using BS EN ISO 1873 in a horizontal position, so no adjustment is needed.” 

This refers to the fact that roof windows are always installed at an angle. The thermal performance of glass changes depending on its angle, so specific Uw values will need to be calculated for each roof window installation, adjusted to the angle in which it is installed.  

Rooflights are typically always tested at horizontal angles so no adjustment is needed.  

This is a change from previous iterations of Approved Document L which stated the thermal performance of the roof glass in a vertical position and accepted that it would change depending on the actual angle of installation.  

Should I Choose a Rooflight or Roof Window?  

As both types of roof glazing have different applications, the roof glazing that is best for your project will depend on the design and intention.  

If you are looking for a very large or bespoke glass roof, then a rooflight is the best option. These rooflights can be designed and installed at almost any sizes, thanks to the bespoke nature of the detailing.  

Roof windows are best for when you need a glass roof that is small and sits in line with the pitch of the roof. These can sometimes be referred to conservation rooflights.  

If you are unsure which option will be best for your project – or what performance will be – contact the team at IQ who can help advise on the roof glazing specification.  

You may also find our bitesize roof glazing CPD course useful to advance your learning.