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3.36.4.9 Timber joist performance
Floor joists shall be of an appropriate size and quality, and be suitably durable.
I-joists and metal web joists should not be used in situations where any part of the joist is exposed to external conditions, and be:
- in accordance with Technical Requirement R3
- used in accordance with the manufacturer’s recommendations
- protected from adverse weather conditions during transport and storage
- stored clear of the ground and stacked vertically
- not used where damaged
- designed in accordance with BS EN 1995-1-1 and its UK National Annex.
Instantaneous deflection of floor joists should be:
- no more than 0.003 x the span for the combined bending and shear based on the total dead and imposed loads, with a maximum deflection of 14mm where strutting is provided, or 12mm where strutting is not provided.
Floors formed by the bottom chords of attic trusses are required to meet the above guidance.
Structural solid timber joists should be specified according to the strength classes in BS EN 338, eg C16 or C24 and marked with:
- the strength class, or evidence of species and grade made available so as to determine the strength class
- the identification of the company responsible for the grading (when graded to BS 4978 or BS EN 14081).
When graded to BS 4978:
- the species should be included in accordance with BS EN 1912 or the class strength specified
- BS EN 338 can be used to determine strength class.
Regularised timber should be used for solid timber joists, and be:
- dry graded to BS 4978 or BS EN 14081
- marked ‘DRY’ or ‘KD’.
Materials should be checked on delivery for conformity with the design.
Joists should be stored on bearers or in racks and be protected.
Timber should not be used where:
- it is excessively bowed, twisted or cambered
- it has large edge knots or shakes
- it has a waney edge more than half the thickness
- it is damaged or has any sign of rot.
Last updated: 2nd January 2024