What is the difference between a skylight, rooflight or roof window?

There are an increasing amount of products available that offer varying degrees of natural daylight and most will be familiar with smaller scale skylights, rooflights or roof windows. We all have a predilection to well lit, inviting spaces, so increasing the use of glazing products in the built environment is almost always an efficient way of improving an internal space. But what is the difference between these glazing products: is a skylight different to a rooflight, or a rooflight different to a roof window? Skylight, rooflight or roof window? The term ‘skylight’ is used by manufacturers for a variety of different products, ranging from small scale domestic units installed on traditional pitched roofs to larger bespoke glazed units designed to be installed on flat roofs or terraces. It’s a generic term that can cause confusion among consumers as the products it refers to can differ quite drastically in terms of scale, function and application. In some cases, the use of the term skylight is actually completely inaccurate as far as the Building Regulations are concerned, where more appropriate terms should be used that are recognised in British Standards. Rooflight is another generic term that can mean different things, but generally speaking it will refer to a unit installed on a flat roof, or where installed on a pitched roof it is likely to be fitted ‘out of plane’ with the level of the tiling. Rooflights are commonly installed using an upstand or kerb system to support the actual product and act as a surface for any weathering to be fixed to; on flat roof systems the upstand will provide enough height to ensure that the rooflight remains watertight. The rooflight quite often has to be installed at a certain height to maintain any guarantees supplied by suppliers of weatherproofing systems. Roof windows are covered under BS EN 14351-1:2010 which stipulates that they must be installed in the same orientation and ‘in plane’ with the surrounding roof, typically at a minimum 15 degree pitch and weathered into the roof using a skirt or flashing. They are generally only available in standardised, smaller sizes compared to rooflights. Roof windows must be CE marked before they can be sold and manufacturers are expected to provide a declaration of performance (DOP) to advise specifiers how the unit performs under test conditions. Typically this will cover things such as tests to simulate prolonged and heavy rainfall, how the roof window withstands increased air pressure, glass deflection and monitoring any air leakage to ensure the product does not create drafts or allow in damp.

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