Single storey houses generally have ceiling joists that span across the entire structure. In two-storey homes there is an opening between the floors for the stairwell and the joists have to be trimmed and a frame made for the opening. A trimmer joist is one of the elements of the framing that allow the load carried by the joists to be redistributed to other components. Trimmer joists are used for stairwells, around chimneys and sometimes for skylights. Each part of the framing has its own function and name.
A joist is a construction component that runs from wall to wall, or between beams or other structural elements to support a floor, ceiling or roof. They are placed parallel to each other and can be made of wood, steel, concrete and other strong materials. Joists are designed to distribute the weight of the structure in an even fashion, and engineering specifications determine how far apart and how large they need to be. Bridging joists, which run full-length, are also as common joists, or simply joists.
If an opening is required between floors for elements such as a stairwell, then the bridging joists must be cut through. The integrity of the structure is then at risk, as the resulting trimmed joists have nothing to support the loose end, and can no longer transfer their load to the structural supports. These trimmed joists have to be framed and made so that the weight they should carry can be transferred to other components.
A trimmer joist is a joist that is placed at right angles to the bridging joists and trimmed joists and is used to support the cut ends of the trimmed joists. These are often thicker than the original bridging joists as they are being used to transfer the load the trimmed joists were originally carrying to solid joists, and may require extra strength. Engineers can calculate the load based on how many bridging joists were cut, and the overall loading of the structure.
Because the trimmer joist may carry a substantial extra load, then trimming joists are used to support the ends of the trimmer joists. These have the same span as the original bridging joists, but like the trimmer joists they will need to be thicker than the bridging joists because they are carrying an extra load.