What are ‘in plane’ and ‘out of plane’ rooflights?

Predominately an ‘in plane’ rooflight is a rooflight that is in plane (by which we mean flush) with the rest of the roof that it sits on. An in plane rooflight doesn’t require any changes to the pitch of the roof, for example a Velux roof window. Other examples would be polycarbonate or GRP sheeting which are manufactured to the same profile as the rest of the roof sheets, often seen on large commercial buildings. Out of plane rooflights mean that the rooflight itself sits out of plane. By this we mean that it is not in line or flush with the roof covering. Although some rooflights appear to be flush with the roof, they are fixed to a weathered upstand of 150mm housed within a concealed/free draining deck or with a secret gutter surrounding the rooflight. The upstand height is mainly based on minimum dimensions that meet the weathering requirements of the waterproofing manufacturer. For example, a walk on rooflight on a flat roof may appear to be an in plane rooflight. However, although it may look as though it’s flush, the rooflight is only flush to the finished surface and not flush to the roof waterproofing layer, making it an out of plane rooflight. There are other products available from rooflight manufacturers such as sliding box units used for access or eaves units which feature glass in horizontal and vertical planes; these products are also considered to be out of plane as they are usually fixed to a wall and therefore separated from the roof itself. Aesthetically speaking, customers often demand that their rooflight product appears flush fitting. Most rooflights require fitting to a kerb but can give the effect of a flush rooflight, but it is important to note that the appearance of flush fitting and being in the same plane are not always the same thing. To understand more about the design and specification of glass rooflights, please call our technical team or request a CPD.

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