What specifiers need to know about installing roof windows and rooflights Back to all blogs | Glazing Vision Europe | Tutorials The benefits of specifying roof windows and rooflights are clear: they provide high levels of natural light, they can be wonderful sources of ventilation, and they contribute to the thermal comfort of a building. Roof windows and rooflights also provide an aesthetically pleasing addition to the external appearance of a property. But what else should you consider to ensure the specification runs smoothly once it gets to site? Roof windows and rooflights are generally easy to install, but knowing what to expect can help to ensure things go smoothly. Glazing Vision recently spoke to Clarkson Builders about how to ensure proper specification and keep work on-site progressing as expected. Here are the things they recommend specifiers should be aware of: Window size Roof windows and rooflights come in an array of shapes and sizes, and choosing the right dimensions for your project is critical. The ‘standard’ for roof windows is 900x600mm, and these are installed between rafters at 600mm centres. Other common sizes are 980x980mm and 900x1200mm – the latter is designed to fit in a double-rafter space. They can also be provided in much larger sizes – for example, Glazing Vision Fixed Flushglaze windows can measure up to 4000 x 1500mm. Structural support The most effective way of supporting a roof window is to use double trimmers for horizontal support and double rafters for vertical support. Double hangers are placed in each corner, and single hangers are located around connected rafters. The double supports comprise two pieces of rafter timber with 600mm centres (either C16 or C24 between 100x50mm or 225x50mm, depending on specification and structural calculations). The rafters should be fixed together with bulldog timber connectors and M12 bolts. When fixing the timber, the head of the bolt should be recessed on the side of the wood to which the rooflight will be fixed. Otherwise, it could protrude into the area beneath the glazing. As with all structural requirements, relevant calculations should be carried out by a qualified structural engineer. Preparing the window aperture Remember that the size of the window aperture may not be the size of the window itself. The actual requirements will depend on the individual roof window, so be sure to consult the installation guide early in the planning stages. When preparing the rafters, consider how elements like the insulation sarking and the ends of the battens will be supported. The roofing underlay should be folded back and fixed appropriately. Other elements will also need to be added to the roof, such as tilting fillets, battens and flashing. Out-of-plane rooflights will also require the construction of a window kerb.