What is a trimmed opening?
A trimmed opening is a gap formed in a floor of joists to let a stairwell, chimney, rooflight or service shaft pass through. The joists interrupted by the opening are carried by heavier joists alongside it. Forming such an opening is called trimming a floor opening.
The parts involved
| Part | Role in the structure |
|---|---|
| Joist alongside the opening | Continuous, heavier joist running each side of the gap |
| Cross member closing the opening | Carries the interrupted joists; in steel, a proprietary trimming section |
| Interrupted joist | Short joist landing on the cross member instead of on the wall |
The load path turns a corner: the short joists bear on the cross member, the cross member bears on the heavier joists each side, and those carry everything back to the bearings.
How to approach it
- Fix the opening first. Stairwell, chimney or shaft: settle size and position before cutting any joist.
- Upsize the joists beside the opening. The bigger the opening, the heavier they must be, because they pick up the load of the interrupted joists as well as their own floor strip.
- Have the sizing calculated. For substantial openings in an existing floor, an engineer should determine the sizes rather than a rule of thumb on site.
- Make proper connections. Use joist hangers or a sound timber joint, not screws driven into end grain.
- Keep clear of the flue. Around a chimney there is a required separation between timber and flue; it partly dictates how large the opening must be.
- Prop before cutting. Support the joists you are about to interrupt so the floor cannot drop while you work.
Where it goes wrong
The commonest mistake is leaving the joists beside the opening at the original size. After trimming, such a joist carries not only its own strip of floor but roughly half the weight of every interrupted joist. The second common fault is a weak connection between the short joists and the cross member, which leaves the floor springing near the opening.
Related terms
- Trimmer joist
- Trimming joist
- Header joist
- Binder
- Joist hanger
- Stairwell
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