Flat Roof Rooflight Openings

In modern warm deck flat roofs, large structural openings are typically included to accommodate components such as sky lanterns/roof lights. Here is the procedure for creating the structural opening that receives them. 

What is a flat roof rooflight opening?

A roof light opening in a flat roof is a structural opening that is engineered to receive some type of skylight. These skylights can range from flat openers to decorative lantern styles. The structural opening is formed within the roof assembly through the use of trimmer joists, typically doubled or tripled up. In warm deck flat roofs, these openings are formed alongside the main structural components of the roof, with the rest of the assembly being built up around the opening after the fact.  

What are the criteria of a flat roof rooflight opening?

The length and width of the interior of the structural opening must fall in line with the intended size of roof light that will be fitted, as well as suiting the engineers’ calculations. As the layers of the warm deck are assembled, the structural opening must remain the same size from top to bottom, without beginning to lean or shrink towards the top. Additionally, the opening should remain square in each corner from top to bottom, without twisting occurring. Appropriate timber joist hangers should be used where applicable to support the structural members of the opening.  

How are rooflight openings formed in flat roofs?

Here is the standard procedure for a regular structural opening to receive a skylight lantern. The first step is to refer to the drawing in order to determine the required internal width of the structural opening. In this specific instance we shall look at an opening that is 2m square. Once the lantern is installed above this 2m square opening, plasterboard will be fixed to the inside of the opening and received into special slots within the lantern for a finished look. This will result in a finished opening of 1985mmx1985mm in this case. In order to create a stable surface for the plasterboard to be attached to, the inside of the structural members will be lined with plywood. The inside face of this plywood, 18mm in this case, will form the inside face of the structural opening and negate cracks in the plasterboard caused by movement in the structural timber. Therefore, in order to mark out the location for our structural timbers to sit, we must add 2 thicknesses of 18mm plywood. This means that our structural timbers will actually be located at a distance of 2036mm apart from each other.  

Due to the span of the roof and the weight of the roof light, the engineer has calculated that our roof joist should be cut from 8”x2” timber, with tripled up timbers for trimming and double trimmer joists to form the opening. After referring to the drawing for the location of the opening, we can mark out the location of the first trimming joist. The inside of the trimming joist can be marked, with another measurement pulled at 2036mm from this to denote where the inside edge of the other trimming joist will sit. Our two trimming triple joists can then be assembled in accordance with the standard procedure for forming bolted beams. Once the two beams are assembled, we can locate them in place, using coach screws to temporarily fix them in place. It's important at this stage to ensure that the trimming joists are parallel to the end of the room, as well as the OC layout of the rest of the joists. All of the other full-length joists can be installed alongside these beams, on their correct layout, and be fixed in place. Timbers can be laid across the top of the installed joists to make the rest of the assembly of the structural opening easier and safer. 

The next step is to mark out the location for the trimmer beams to sit, by referring to the location of the opening on the drawing. In this instance, the opening is in the centre of the ceiling as perceived from below, and so measurement can be made inside the room, with the interior surface finishing thickness in mind, to find the centre of the finished room. Half the width of the opening can be measured backwards in each direction from this central line to denote the hard interior face of the opening. In this case, this measurement is 1018mm in each direction from the centre point, considering the thickness of plywood again. The two double trimmer beams can be assembled at this point, cut to the correct length - 2036mm long in this instance. 

With the trimmer beams assembled, the appropriate width joist hangers can be installed on the layout marks. Once these hangers are pinned on, the trimmers can be dropped into place. Using a rod or tape measure, the diagonal measurements of the opening can be checked, in combination with the use of a large square, to ensure that the opening is square, with 2 pairs of parallel sides. The trimmer joists may need to be moved slightly to bring the opening into square, hence the hangers being pinned temporarily to begin with. Once the opening is square (with a few mm of tolerance), the hangers can be fully nailed up, with additional fixings being installed through the trimmer beam into the trimming beams. With the trimmers installed, the remaining trimmed joists can be installed on the OC layout of the roof, supporting off of the trimmer beams with hangers. 

With the main structural opening formed, the warm deck assembly around the opening can be built up. As the different layers are added to the roof, additional timbers are fixed around the perimeter of the opening to maintain the size and to receive the layers. For the primary furrings, sheathing, and insulation, an initial curb must be installed around the perimeter of the opening so that these components are retained. The total kerb once finished should project above the surface of the top layer of plywood by 100-150mm, therefore appropriate width timbers should be chosen as the kerb is built up. Alternatively, the initial kerb can be installed to a height so that the top layer of plywood sails over the kerb. An additional kerb is then installed on top of the top layer of sheathing to provide an upstand for the roof light. To fix the kerb, long coach screws are installed downwards through the timber and into the timbers below. With each layer of timber added, the corners are staggered to increase the stability of the opening. Nails should be used to fix the corners of the timbers together.  

As mentioned, the top layer of timber that forms the curb may have to be ripped down to meet the height requirements of the building, as well as maintaining an appropriate upstand above the finished roof surface. Alternatively, they may be required to be tapered, in order for the roof light to fall in the same direction as the furrings. This depends on the style of the lantern, with the details likely being displayed on the window schedule. A tilt fillet may also be required around the outside of the kerb and other vertical intersections, so remove the sharp angle in the roof covering.  

The final step to finishing the structural opening is installing the plywood facing on the interior of the opening. This plywood is cut to width from the bottom of the joists to the top of the curb and nailed in place. Care should be taken to ensure that the plywood is straight and plumb when installed, as the multiple layers of timbers forming the curb can tend to distort the size of the opening. A square should be used to ensure the corners of the opening are still square also, with minimal packing being used if required. Once the plywood facing is installed, the rooflight is ready to be fit and the roof can be weathered in. Once the lantern is fitted in place the plasterboard can then be installed on the inside face of the opening.