Timber and concrete upper floors

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6.4.9Timber joists

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.

Deflection and vibration limits should be:

  • designed in accordance with BS EN 1995-1-1 and its UK National Annex, or
  • „specified using the following deflection limits based on total dead and imposed loads for combined bending and shear: 0.003 x the span, with a maximum deflection of 14mm where strutting is provided, or 12mm where strutting is not provided.

Structural solid timber joists should be specified according to the strength classes in BS EN 338, e.g. 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.

Timber should be treated with preservative where it is to be built in or embedded into solid external walls.

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.