Choosing the most suitable rooflight for your build can be anything but straightforward, with a variety of important aspects to consider including shape, size, and the materials it’s made from. Paul Trace from Stella Rooflight offers some practical advice to self-builders

The first thing to consider is the type of materials that are available for the rooflight frame. There are many different types of rooflight on the market catering from the modern contemporary style through to a more traditional look and feel.  

Steel is the obvious choice for period properties and barn conversions, but they are now also being more frequently used in sleek new contemporary houses due to their appearance, low profile and ability to let in more natural daylight. Timber is a long-time favourite on homes of any style, but getting onto the roof to maintain the timber can be an awkward task. PVCu is also an option and is often the cheapest, but it will not provide the slim frames that metal is capable of, nor the natural look of wood. 

The important thing is to select a rooflight that matches the look and feel that you are trying to achieve with your home, as it should blend in with your roof and the materials used throughout the rest of the house. 

GLAZING OPTIONS  

Once you have chosen the most appropriate material for your rooflight, you then need to consider the glazing options. Aside from thermal performance you need to decide if you need toughened glass, for additional safety and protection against falling objects. 

Self-cleaning glass is another option and can be the ideal choice for situations where cleaning will be costly or difficult. You also have the option for double or triple glazing. This is probably fairly obvious, but you should always opt for triple glazing if budget allows as this will have a significant impact on the thermal performance of the rooflight. 

It is also possible to have solar control glazing, which incorporates invisible layers of special materials on the glass that has the dual effect of allowing sunlight in, while repelling solar heat. 

ENVIRONMENTAL PERFORMANCE 

Consideration must be given to thermal performance and improving carbon and energy reduction in buildings. Part L of the Building Regulations is one of the tools used by the Government to achieve these objectives. These regulations are set to change this month, as the UK Government continues its drive towards greater thermal efficiency in housebuilding. 

For rooflights, skylights and roof windows (of which the definition is crucially important), the relevant consideration is the thermal transmittance. This is measured as a U-value in units of W/m²K (which stands for Watts per metre squared Kelvin). The lower the U-value the more efficient the construction is at keeping heat flow through the structure to a minimum. 

The new regulations deem the worst acceptable U-values to be 2.2 W/(m²K) for rooflights and 1.6 W(m²K) for roof windows. To correctly assess whether an element meets the new limiting U-value figure, the U-value must be calculated for the element in the appropriate plane – either horizontal or vertical. 

Now this makes a big difference, as testing the same product in either a horizontal or vertical position will make a significant difference to the resulting U-value figure – with the vertical position providing a much lower (better) U-value figure. This is where the definition of the terms and roof window prove crucial in determining how they should be tested and what the relevant U-value should be for each. 

According to the Approved Document, the following definitions apply:

• Rooflight – A glazed unit installed out of plane with the surface of the roof on a kerb or upstand. Also sometimes referred to as a skylight. 
• Roof window – A window installed in the same orientation as, and in plane with, the surrounding roof. 

It is important to understand that U-value calculations for roof windows and rooflights must be undertaken by an approved UKAS accredited product certification agency, who will calculate accurate thermal performance figures based on the individual make up of components in the product’s construction. It’s always worth checking with your supplier to see proof of performance as U-values are often misquoted, and can refer to the centre glass pane rather than the rooflight as
a whole. 

LIKE-FOR-LIKE

As with most things in life, you tend to get what you pay for, and this is particularly true when comparing rooflights in the market. Prices often reflect the quality of manufacturing and materials used to produce roof windows. While prices may vary considerably, there is a good reason for this and the difference in quality between the bottom and high end of the rooflight market can be night and day.  

The rooflight may be one of the last things that you budget for, but it is also one of the most important finishing touches to your home. Opting for the cheapest product may come back to haunt you in the long run. 

Paul Trace is director of Stella Rooflight