6.4.17 Multiple joists
Multiple joists shall be securely fixed together.
Joists can be doubled or tripled up to provide additional support, eg for lightweight partitions or to form trimmers. The design should specify how the joists are fixed together and be in accordance with manufacturer’s recommendations.
As a minimum the fixing centres for double member trimmer and trimming joists should be at centres of no more than twice the adjacent joist spacing.
When securing solid joists:
- fixings should be in accordance with the engineer’s specification and should be checked before the ceiling is fixed, including the tightness of bolts
- toothed plate, split ring and shear plate connectors should be provided where required
- washers or single-faced connectors should be used with bolts
- ensure that timber is not damaged by over-tightening.
Coach bolts can be used to fix multiple solid joists together. They could be located at centreline or staggered depending on the loading and design. Where located on centreline, the spacing of between S (joist centres) and 1.0m can be used. Where more than two joists are secured together or a heavy concentrated load is to be supported, then the fixing arrangement with staggered spacing shown in the table below may be used.
Engineered joists should be secured together in accordance with the manufacturers recommendation.
Last updated: 2nd January 2024