Structural Joist Openings
Within any joist system, be it a floor, ceiling, or flat roof assembly, structural openings must often be formed to facilitate the installation of the other components. Though each scenario differs in criteria and desired outcome, the steps undertaken to form any variety of structural joist opening remain largely unchanged in approach. Here we shall look specifically at the formation of structural rooflight openings, access hatches, and stairwell openings.
What is a structural joist opening?
Without the presence of a structural opening within a standard joist system, the main bulk, if not all of the joists within the O/C layout span between two vertical structural supports. In order to facilitate the formation of an opening within the system, a structural perimeter must be established to define the outer edges of the opening and support the ends of the trimmed joists that become interrupted by the assembly. This perimeter is typically formed through the use of trimming and trimmer joists - most often implemented as timber beams - though flitch beams or larger RSJ’s may be called for depending on the load bearing criteria. This criteria must be calculated to include the weight of the joist system itself, the installation of any sheathing materials, load applied to the the joist system through habitation, as well as load applied through the installation of secondary components such as a staircase of roof light.
What are the criteria for the installation of a structural joist opening?
Despite the variation in the desired outcomes of structural joist openings, the criteria for their formation remain largely unchanged between applications. Seeing as most structural openings are formed from standard joist components and timber beams, all of the standard criteria such as bearing, direction of bow, direction of cup, and layout are applicable here. The exact location and dimensions of the opening take priority over the rest of the joist structure, with any interruptions being adequately supported or altered to comply with standard joist installation criteria. For rooflights and stairwells, the thickness of sheathing and the total run should be considered respectively. All timber intersections should be suitably supported with timber joist hangers, and reinforced with strapping where necessary.