What are rooflight openings?

Rooflight openings are structural openings created amongst the rafters of a pitched roof assembly in order to facilitate the installation of a roof light/window. These openings are created in a similar way to any trimmed structural opening. The rafters either side of the opening are doubled or tripled up depending on the size of the window and the structural criteria. Trimmers are installed on hangers between the bolted rafters to create the overall shape and size of the openings. Any rafters that are disrupted by this opening must be cut to fit between the plate and the trimmer, and the ridge and the trimmer, also supported via hangers. The height off of the floor to the bottom of the window is important here as always if the window is an opener.  

What are the criteria of a rooflight opening?

The opening must be formed in accordance with structural regulations so as not to undermine the structural integrity of the roof structure. The opening must be formed to an appropriate size to accommodate the desired size of rooflight. Structural ironmongery must be installed in accordance with the building regulations to maintain the stability of the roof.  

How are rooflight openings formed in pitched roofs?

There are a handful of ways that rooflight openings can be formed depending on the size and criteria of the roof. In a large roof assembly, the opening must be formed around the on-centre layout of the rafters. In smaller roof structures, the opening is typically formed first with the other common rafters being installed around them. In these instances, such as in small extensions, the openings are formed in a specific place within the roof, often centred within the room below, or divided equally within the visible underside of the ceiling to some extent. In a larger roof assembly, a few methods can be employed to frame the opening. If the position of the opening is negotiable, then the layout of the common rafters can be used to frame the outside of the openings. If the location of the opening is specifically stated, and the layout of the roof does not directly compliment the opening, then an alternative form of framing must be employed.  

In most instances, regardless of the style of roof, the outside rafters that form the opening are doubled or tripled up and bolted to meet the structural requirements of the roof. Doubled bolted trimmer beams are formed to the size of the opening and are installed between the doubled rafters to create the structural opening. Metal hangers are used to support the connection between the rafters and the trimmers. When framing the opening, the actual dimensions are slightly larger than the size of the roof light to allow for wiggle room in both directions to square the window up in the room. Ideally, a tolerance of 20mm all around, for a total of 40mm larger in both directions than the size of the window is preferable. If the window is framed in the ceiling surface of a room, then the vertical height up the ceiling isn't restricted by regulation and is often centred in the ceiling space. If the window is featured within proximity of a floor system, as within a loft conversion roof, then the height to the bottom of the window is restricted by regulation. A common height of between 900mm and 1100mm must be maintained if the window is a fire escape. If not, then the height must be more than 1200mm to prevent people from climbing out of it.  

If the roof light falls within the layout of a larger roof system, then an alternative method of trimming may have to be employed. To avoid altering the layout of the rafters, then a larger trimmer is installed between the shortest distance of rafters that accommodates the position of the opening. With the trimmers installed between the on-centre rafters at the correct vertical distance apart from each other, another set of vertical trimers is installed on hangers to form the side walls of the opening.

Where the rafters are disrupted by the opening, heads and tails are still installed between the plate and trimmers, and trimmer and ridge on hangers to accommodate the on-centre layout of the roof assembly. When framing openings after the rafters have been installed, timber can be braced across the top of the rafters to support them as they are cut out. When marking out the openings, remember to allow the thickness of the trimmers in order to ensure the opening is the correct size.

Splayed openings

As shown in the illustrations so far, the most common form of framing a roof light opening sees the trimmers installed at 90 degrees to the rafters to create a short tunnel that is perpendicular to the plane of the roof. When the plasterboard is installed on the underside of the roof, all the corners of the short tunnel are square and are uniformly projected out of the roof light.  

In some instances, an alternative splayed style can be implemented in order to increase the size of the opening at the point that it enters the room, increasing the amount of light coming in.  

A 45-degree angle is cut on timber spacers that are the same width as the trimmers. Realistically, achieving a true 45 on the plasterboard at the framing stage is difficult, as there are too many variants that affect the layout of the plasterboard. By cutting a timber the same size as the trimmer timber, we can always pack out the plasterboard later on to accommodate a true 45-degree angle. The double trimmers are installed further back than normal to accommodate the installation of the splayed timbers, so that the distance between the splayed timber in their final location is the true width of the roof light opening plus the 40mm tolerance. The trimmers are still installed on hangers, with the splayed timbers being supported by blocks underneath if required. If the window is particularly heavy or the opening particularly large, then a double splayed timber may be installed to support the weight of the window.  

Inside the roof, once the window is installed, the plasterboard is installed at the bottom and top of the opening at a 45-degree angle to the surface of the finished slope once the insulation and batten are installed. The plasterboard here can be packed out to accommodate a true 45 or left at the wider nonspecific angle if desired. The two sides of the small tunnel still come down at a 90-degree angle to the plane of the window.