Paul Trace, director of Stella Rooflight discusses

the unique properties of a genuine conservation rooflight

What is a conservation rooflight?

If you know that you need conservation rooflights for your project, the chances are that you have searched online and found plenty of choice. But what is a conservation rooflight and are they all the same?

To better understand what makes a rooflight a conservation style, it is important to understand the history behind this type of glazing and why the design is so sought after, not just on period properties but also more modern projects.

Without the ingenious concept of a rooflight the vision of transforming unconventional space into a well-lit property would be a daunting task, and in some cases an impossible option. For that reason, the conservation rooflight could be considered a highly influential building feature, which has given the construction industry a solution to introducing natural light into a property.

Although rooflights, or skylights as they are sometimes known as, have been around for centuries they became more prominent during the Victorian era as technology and building aspirations were stretched and roof glazing boomed. One of the most famous Victorian building projects was the Crystal Palace, which in 1851 used glazing on an unprecedented scale to showcase just what could be achieved.

Mass-produced Victorian rooflights for residential use tended to be made from cast iron and the earliest examples would have smaller, lighter panes of glass. This was partially down to limits of glass technology at the time but also because of excise duties, which were imposed on glass by weight in the mid-18th century. These slim, single glazed rooflights with multiple panels of glass were unobtrusive in design and sat flush in the roof. Today it is this minimalist appearance that many people are seeking to achieve in their glazing designs.

As a result of their popularity, there are lots of conservation roof windows on the market, which can make choosing the right one virtually impossible unless one can identify what the differentiations are. An effective way to make this distinction is to look closer at the attributes of a true replica of a Victorian conservation rooflight.

What material is the conservation rooflight manufactured from?

If a conservation rooflight is all frame, then there is little point in having one. Genuine conservation designs should be manufactured with slim, clean lines and a low-profile to match the roofline. A number of skylight companies try to produce conservation rooflights using modern bulky aluminium profiles, which sit proud of the roofline, particularly slate. It is widely accepted that most authentic conservation rooflights are manufactured from steel because it provides great strength while offering a slim profile and excellent glass to frame ratios.

Single or double glazed?

Victorian rooflights would have been single glazed, however, today’s modern building standards are much higher and so single glazing does not meet the minimum requirements for thermal efficiency (Part L). Double glazing is now the most popular option for genuine conservation rooflights because glazing technology is such that a modern double glazed unit can provide a number of benefits while remaining reasonably slender.

Some conservation rooflight suppliers are keen to boast about offering triple glazing in their products, however, while this does offer a slightly improved thermal performance it comes at the expense of appearance. The optimal spacer bar thickness is 16mm so any decent triple glazed unit is going to be almost 50% thicker than a double glazed version. Now with a flush fitting profile being one of the main requirements of a conservation rooflight, the introduction of triple glazing makes that almost impossible on some roof types.

Glazing bars?

It is often a stipulation from the Conservation Officer that a conservation rooflight should have a glazing bar to replicate that original Victorian appearance. It is not always the case but it is definitely worth checking whether you need them before purchasing any conservation rooflight.

If your conservation rooflight does require a glazing bar then it should be a genuine one. This is an area that separates those producing close replicas to the original Victorian rooflights and those who are trying to pass off modern skylights as something more traditional. A genuine glazing bar should be something which not only divides the glazing but also provides additional strength to the casement.

Top hung or centre pivot?

Once again, if you are looking for a close replica of a Victorian rooflight then a top hung profile will be the one you should opt for. Not only does a top hung design offer a more authentic appearance, it maximises the space below because the casement doesn’t stick into the room. Smaller top hung rooflights also utilise beautiful brass ironmongery to operate the casement whereas centre pivot designs tend to rely on modern plastic handles, which are out of reach and offer nothing to enhance the internal aesthetics.

Is any old conservation rooflight suitable for my project?

Just because something is sold as a conservation rooflight, that doesn’t automatically make it suitable for all building types. If your building is Listed or in a Conservation Area then the criteria for using conservation rooflights are much stricter and you should always gain approval, not only for their use but also the manufacturer that you would want to use.

There are only a handful of companies that specifically make conservation rooflights and even fewer who design, manufacture and assemble in the UK. Many conservation rooflights available online are simply other products which have been spruced up to look like they meet the requirements of that type of product. If you ask a supplier what the main difference is between their conservation rooflight and those used on modern buildings and the answer is a stuck on glazing bar, then you should avoid at all costs. Likewise, there are many elements which go into a genuine conservation design and price is always a reflection on quality.

Is there anything else I should consider when choosing my conservation rooflight?

With the UK Government pursuing a carbon neutral environment it is imperative that every action is taken to reduce energy consumption. Rooflights are energy efficient as they let in large amounts of natural light thus reducing the need for artificial lighting. Bringing natural daylight into your home is about much more than creating a bright, welcoming environment, it’s about protecting your health and wellbeing and achieving a more positive way of life.

One way to ensure that you maximise the amount of available light is to increase the size of your rooflights… or is it? Just because you have a large rooflight this does not always guarantee lots of light and you should always check what the finished viewable (often referred to as clear viewable) area of the rooflight will be. You might think that a conservation rooflight with a whole frame size of 900mm (w) x 1200mm (h) would have a similar clear viewable area regardless of the manufacturer, but you would be wrong and bulky framed modern types or the flat rooflights posing as pitched conservation styles will let in considerably less light than a genuine steel framed version.

With so many choices available, choosing the right conservation rooflight can be a bit of a minefield but with the right guidance and advice it need not be a stressful experience.


For more information download ‘The Ultimate Guide to Conservation Rooflights’

An independent guide to everything you need to know about specifying conservation rooflights – available exclusively from Stella Rooflight




The Abbey Hill Academy in Stockton-on-Tees is an educational establishment for students aged 11-16 with learning difficulties and disabilities.

The learning environment is never more critical for such students and since the Horizons Specialist Academy Trust became custodians of the school in 2013 refurbishments have been taking place to restore the 1970’s building.

In addition, to heating and hot water upgrades and a new roof, the striking walkway that had a rooflight canopy was replaced and upgraded to enhance the exterior aesthetics of the building. The unusual shape of the building meant that the upgraded rooflights allowed a greater degree of light into the body of the school enhancing both the practical elements and also pupil and staff wellbeing due to the increased availability of natural light.

“The rooflight canopy had definitely seen better days and was no longer fit for purpose having been exposed to the elements for so many years,” said Jim Lowther Sales Director Xtralite. “We were able to specify the latest rooflight technologies to replace the existing ones to ensure a quality restoration was undertaken.”

Four 20m x 2.2m, one 9 x 2.2m and other elements were all fitted from the X-Span range of products, a thermally enhanced self-supporting rooflight. All were double glazed with a toughened outer and laminated internal glass.

“Glass is a good choice for atria, canopies and walkways and in this situation allowed the almost exact replacement of product albeit by those that are far superior,” said Jim. “Furthermore X-Span complies with BS6399 system Part 1 (Code of Practice for dead and imposed loads), Part 2 (Code of Practice for wind loads) and Part 3 (Code of Practice for imposed/snow loads) delivering a premium solution of aesthetics and practicality.”

Further information can be found by visiting or by calling 01670 354 157.

Around 8 million people a year are expected to be sheltering under Kalwall at the beautifully-designed new bus station at West Croydon. Designed by Transport for London Architects, this is an unusual example of how translucent Kalwall can offer many different advantages above its normal use for translucent traditional building cladding and rooflighting.

The brief for this project was to create a user-friendly waiting and assembly shelter to service the thousands of passengers using this important and busy transport hub linking the 150 buses an hour with the adjacent tram stop and West Croydon railway station providing routes to Canada Water and east London, and via the tram network to Beckenham and Wimbledon.

The normal choice of construction for a translucent and weatherproof canopy would be to glaze the roof with glass, However in-house architect Martin Eriksson and the project team at TfL realised that Kalwall offered a better solution in this location which would not only solve the brief and contribute to a better design but would offer many other benefits over traditional glazing.

For example, since Kalwall is much lighter than glass it meant that the supporting structure needed to be less strong and far less chunky. In additional, not only would the shelter be less high but the vertical supports would be less obstructive and open up a better view of the environment including the very attractive church nearby which had previously been blocked from the view of waiting passengers.

Unlike conventional glazing, highly insulating Kalwall is far more attractive than glass while eliminating shadows and glare and the stark contrasts of light and shade; improving the experience for passengers below.

Also, due to the way it diffuses natural daylight downwards and at night reduces vertical illumination and reflection upwards, it controls light pollution on the surrounding high rise buildings. Soil and detritus are less obvious on Kalwall than on glass and maintenance and cleaning is much simpler because access scaffolding is not required and maintenance staff can safely walk across its surface.

One of the main attractions of specifying high performance Kalwall for conventional buildings is its unique effect on both the interior and exterior. Internally, rooms are flooded with diffused natural daylight which creates a stimulating and very attractive environment. Although translucent, it also offers the big advantage of privacy while the elevations appear crisp, simple and inviting. When illuminated at night they emit a very attractive ethereal glow.

Apart from being specified for all types of new build, Kalwall is increasingly used for the refurbishment of cladding or rooflights on aged buildings. Case studies and technical information are available from Structura UK Ltd, Tel: 01233 501 504 or visit
Structura UK is the exclusive distributor of the Kalwall translucent daylight building system for England, Wales and Northern Ireland and a leading supplier, fabricator and installer of glass curtain walling, rainscreens, glass atria, windows and other architectural glass building products.

Will you see the light?

Since the late 1950’s GRP has been a feature of many commercial, industrial and agricultural buildings across the UK, bringing the free resource of natural daylight into the workplace.
GRP is a strong thermoset material with good impact resistance and consists of, among other components, polyester resin which is reinforced by a glass strand mat.

The success of Filon rooflights resulted in several UK based manufacturers continuously producing GRP. Eventually, this involved the use of alternative plastic materials such as PVC. As a brittle material, this was at times subject to damage due to storms, foot traffic on the roof and UV degradation.

This didn’t stop the efforts of thermoplastic manufacturers to try to compete with ‘Filons’ and other GRP rooflights though and polycarbonate in-plane rooflights were born and have since steadily encroached on the GRP rooflight market. There are many positive attributes to polycarbonate such as it providing high levels of light transmission, being very strong and having a good fire rating. There are however, many aspects that should be considered in rooflight material choice for profiled roof applications and we shall discuss them now.

Thermal movement

A thermoplastic material such as polycarbonate has much greater thermal movement than GRP and over 5 times more than the surrounding steel sheets.

If no allowance has been made for this movement such as oversized fixing holes, it could create some problems particularly around the fasteners such as the sheet cracking and at the end laps with seals potentially failing. It is also possible for rooflights with an insulating box detail, such as those used in composite panel roofs, for the rooflight to expand but find resistance. The material will have no room to move as it will be constrained by the surrounding metal roof panels and so could belly out – bulge out of shape between the purlins.

Light Distribution

Another significant consideration is the type of illumination required in the building. GRP has high levels of light transmission and is also a naturally diffusing product. It will provide an even distribution of natural daylight across the area to create a balanced illumination reducing bright spots, shadows and hot spots.

Thermoplastics like polycarbonate, when used as in-plane rooflights however, tend to be clear or colour tinted. They are much less diffusing and allow more light to pass directly through the rooflight. This can create localised bright spots with solar glare and also hotspots due to the nature of direct sunlight.

The first images show two very similarly constructed equestrian centres but one is fitted with polycarbonate rooflights and the other with Filon GRP rooflights. The images clearly show a very different lighting pattern: the polycarbonate rooflights allow light to pass directly through so that their position is clearly replicated on the floor – even the purlins are casting shadows on the ground; the GRP rooflights in comparison provide a very even light distribution, so much so that there are not even any shadows visible around the horse and rider – the perfect conditions for easily spooked horses.

The second pair of photographs show a supermarket distribution centre, firstly with polycarbonate rooflights and secondly after the rooflights have been replaced with Filon GRP. Again, in the first picture, the position of the rooflights is clearly visible by the bright spots on the floor. The picture with new GRP rooflights has eliminated all of the bright spots and reduced localised internal temperatures without compromising lux levels – much more suitable conditions for storing some supermarket goods and foodstuffs.

The example projects highlight the importance of selecting the appropriate rooflight material. For your next industrial, commercial or agricultural building, please give careful consideration about the type of light distribution required. If an even spread of diffused light, without shafts of light, hotspots or dark corners is preferred, then GRP is likely to be the most appropriate choice.

Written by Mark Wilcox, Sales Director, Filon Products Ltd