The largest components involved in the creation of a structural joist opening are the trimming joists. For substantial openings like stairwells and rooflight openings, these trimming joists are generally implemented in the form of double or triple timber beams. The role of these beams are twofold, primarily supporting the ends of any trimmer and rimmed joists that come into it, as well as defining the perimeter of the opening itself. By consulting the drawings, the dimensions and location of an opening can be discerned. When laying out a set of plates or a layout rod, the inside face of these trimming beams should be the first marks that are made. Allowances for the thickness of facing or packing material should be made to ensure the trimming beams are located correctly.

Slight adjustments in the position of the trimming beams should be made to ensure that the finished opening will be positioned in the correct location within the finished room. We have to consider the aesthetic criteria of the finished product to produce a consistent and precise outcome. Should the finished opening be centred within the room? If multiple openings are present in one joist system, should they be in line with one another? Should they be spaced evenly when viewed from the room below? In assessing these criteria, we can account for the thicknesses of drylining and facing materials to ensure that all margins will be correct when all is said and done. Working backwards with these material thicknesses in mind, we can fine tune the exact locations of the trimming beams.

With the layout for the trimming beams determined, the rest of the joist layout can take place as normal. The beams can be assembled according to the timber beam assembly procedures, and subsequently installed on the correct layout marks and fixed in place with structural coach screws. With two sides of the opening defined by the trimming beams, the trimmer timbers can be installed to complete the perimeter. For larger openings, these trimmer timbers are also typically implemented in the form of a double timber beam. The trimmer beams should be cut with millimetre precision between the trimming beams, and should be identical in size to ensure that the opening remains square. Care should be taken to ensure that the two trimmer beams remain parallel to one another. Rods cut to the inside dimensions of the opening can help to locate these beams. The intersection where a trimmer beam meets a trimming beam is supported by a suitable timber joist hanger, fastened with appropriate twist nails.

With the perimeter of the opening formed, a final check should be made to ensure that the opening is square and that all margins are correct - before any further components are installed. Using a tape measure or a layout rod, the distance between the opposite corners of the opening should be measured. If this diagonal distance is the same length in both directions, then the opening is square. If the two measurements are different, then the opening is out of square by half of the difference between the two measurements. For example, if one diagonal measures 1500mm, and the other measures 1510mm, then the opening is 5mm out of square. This can be rectified by shifting the trimmer beams by the half distance in the correct direction. If the trimmer timbers are already fastened with hangers, then the trimming beams can be unscrewed at one end and adjusted in the correct direction. It’s important to note that the distance the trimming timbers will have to move will be effected by the distance of the bearing from the structural opening. The most accurate method to make these adjustments is to incrementally move the timbers and keep checking the diagonal distances of the opening until the opening is square.

Once the opening is square and all of the margins are correct, the rest of the joist components can be installed. The regular joists can be installed on their layout, and any joists that fall within the opening are installed as trimmed joists, supported via hangers from the trimmer beams. Layout rods can be nailed to the tops of the joists to maintain their position if required, and any timber blocking can be installed in accordance with the standard procedures. At this stage, the structural opening and the joist system have been successfully formed, and the next task can be undertaken.

But what if the perimeter of the opening is not entirely defined by timber? Whilst stairwells can be formed entirely within a joist system, it’s far more common for at least 1 side of the opening to be defined by a vertical wall. This arrangement allows for greater lateral stability to the installed staircase by providing a suitable fixing for the stringer, but also allows for the total stair system to be created in a more space efficient method. By tucking the staircase into a corner or against a wall, movement throughout the building becomes more streamlined, and space within the building is maximised. Depending on the specific criteria of the staircase and the space available within the building, standard residential stairs may be installed adjacent to as many as 3 walls. So how is a structural opening formed within a joist system in these instances?

The least amount of additional trimming required occurs when the stairs are installed between 2 or 3 walls. As can be seen in the illustration, the the top of the staircase meets a beam, either a trimming or a trimmer. Either of these arrangements are installed according to standard procedures. In these instances, there are numerous structural supports in close proximity to the stairwell, allowing for the installation of minimal load bearing trimming components. When the stairwell only features 1 vertical wall adjacent to the staircase, more trimming is required. Depending on the direction of the span of the joists, the beam installed parallel to the vertical structural wall may be either a trimming or a trimmer beam. Examples of both can be seen in the illustration. In some instances, the stairwell opening is not square/rectangular, but doglegs around a corner. The perimeter of this opening is still formed through the use of timber beams, though the internal intersection of the beams must be supported via a load bearing structural support, unless a cantilever arrangement has been engineered.

Regardless of the specific arrangement, all of the standard components are installed in accordance with the regular installation procedures, and any exceptional assemblies will be highlighted on the drawings, typically accompanied by a detail drawing.

For smaller access hatches, i.e. loft hatches, the structural requirements for the opening are far less. These hatches are used far less and are intended to support a smaller load than that of a conventional structural opening. As such, the trimming and trimmer timbers are often single timbers, with the additional timber required for the installation of a beam being unnecessary. Alongside this, the openings themselves are physically smaller. Many loft hatch products are designed to accommodate the standard 400mm or 600mm centres that the joists are installed at. As such, extra steps in the layout of the opening are no necessarily required. Depending on the criteria of the opening, the location for the installation of the opening can be decided upon after the installation of the joist system. In most instances, a single joist is cut to allow for the installation of the trimmer timbers, which are supported via timber joist hangers. The now trimmed joist can then be attached to the trimmer timber, also supported via timber joist hangers.