Timber and concrete upper floors

6.4

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

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

protected from adverse weather conditions during transport and storage

stored clear of the ground and stacked vertically

not used where damaged.

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 519).

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 519

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.