Timber and concrete upper floors

6.4

6.4.8Timber joist spans

Timber floor joists shall be adequate for the spans and loads, and be correctly installed.

Solid timber joist sizes are provided in the BS 8103-3 span tables. Where the tables do not apply, or where there are concentrated loads, floor joists should be designed by an engineer in accordance with Technical Requirement R5.

Span tables for solid timber joists

Tables 1 and 2 in this chapter are derived from the TRADA Technology Ltd. ‘Eurocode 5 span Tables for solid timber members in floors, ceilings and roofs for dwellings (3rd edition)’. The section sizes are based on regularised ALS or CLS timber.

For timber floors between homes:

  • to meet acoustic performance, the dead load of the construction is likely to be 0.6-0.7kN/m2
  • use the three right-hand columns from Tables 1 and 2.

For upper floors with 22mm thick chipboard decking and a 12.5mm plasterboard ceiling:

  • a dead load of between 0.25kN/m2 and 0.5kN/m2 may be assumed
  • use the centre three columns from Tables 1 and 2.

Where lightweight non load-bearing partitions weigh up to 1.0kN (101.9kg) per metre run and are parallel to the joists, the following applies:

  • Partitions may be directly supported by one or two additional joists.
  • Partitions should be fixed through the floor decking into the joist(s) beneath.
  • Where similar lightweight partitions run at right-angles to the joists, the maximum spans in Tables 1 and 2 should be reduced by 10%.
  • For all other additional loads, joist sizes should be designed by an engineer in accordance with Technical Requirement R5.

Table 1: Permissible clear spans for domestic floor joists. Strength class C16

Imposed load not exceeding 1.5 kN/m2.
Service class 1 or 2.

* Two additional joists required
Bold text = normal bearing of 40mm to be doubled

Table 2: Permissible clear spans for domestic floor joists. Strength class C24

Imposed load not exceeding qk = 1.5 kN/m2 or qk = 0.90 kN.
Service class 1 or 2.

* Two additional joists required
Bold text = normal bearing of 40mm to be doubled