What is the minimum roof pitch for a rooflight or a roof window?

Rooflights and roof windows are both popular choices for bringing natural daylight into a property, and rooflights can also provide a variety of other benefits, such as natural ventilation and access to roof terraces. When selecting a rooflight or roof window, there are several specification considerations to think about – and one factor that could play an important part in your decision is the pitch of the roof. Roof pitch affects drainage, determines how the units should be installed and even what type of products you can use. Below, we explain the different requirements for rooflights and roof windows; this should help you to decide which product is most appropriate for your project. Installing a rooflight on a flat roof When installing a rooflight on a flat roof you should ensure that the product is not fitted completely flat itself. Glass inherently has a degree of flex when installed flat (as opposed to vertically as you would find in traditional windows and doors). This is known as ‘deflection’ and means that without pitching the rooflight up slightly at one end, rainwater will accumulate and begin to ‘pond’ on the glass. When this evaporates it can leave unsightly marks and stains behind. Manufacturers therefore recommend a ‘minimum pitch’ to install their product so that rainwater and debris runs off the glass more effectively, keeping the rooflight cleaner for longer. The recommended pitch for rooflight installation on a flat roof For many rooflight applications on flat roofs, there should be a minimum pitch or fall of three degrees. This is enough to ensure that water will drain off the surface of the glass and avoid ponding. The higher the pitch, the more effective it is at draining off the water, so although three degrees is often the recommended minimum pitch, installing the unit slightly higher – for example at five degrees will prove more efficient. To achieve the required pitch, an upstand or kerb is usually constructed around the aperture in the roof, which will accommodate the slope into it for the rooflight to be structurally fixed to. If you are unsure, the rooflight manufacturer should provide drawings to indicate minimum kerb heights, which will allow your builder to calculate how high the top of the slope needs to be to achieve the desired pitch.

For more information on What is the minimum roof pitch for a rooflight or a roof window? talk to Glazing Vision Ltd