Trimmer Joist
/TRIM-er joyst/
Also known as: trimmer, doubled joist, parallel trimmer, trimmer beam
Definition
A joist doubled up alongside an opening in a floor or roof - such as a stairwell, hatch, or chimney breast - to carry the additional load transferred from the header joist that frames across the opening.
In Practice
Trimmers run parallel to the regular joists on each side of an opening. The header joist spans between the two trimmers to close off the opening. For openings up to 3m the trimmer is typically a doubled joist of the same depth as the floor joists. Wider openings may require engineered timber or steel. All connections should be made with joist hangers.
UK Building Regulations
Approved Document A - Structure
Trimmer sizing should be verified against TRADA span tables or a structural engineer's calculation, particularly where the opening is large or loads are concentrated.
See Also