6.4.8 Timber 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 BM TRADA Technology Ltd. 'Eurocode 5 span Tables for solid timber members in floors, ceilings and roofs for dwellings (4th edition)'. Tables 1 and 2 give permissible clear spans of simply supported domestic floor joists of solid timber for specified loadings, sizes and spacings calculated in accordance with EC5. The tables make no allowance for concentrated or line loads applied by partitions, trimmers or other similar loads. The minimum bearing length assumed for the tables is 40mm unless noted otherwise, and longer bearing lengths may be required for other practical reasons. The section sizes are based on regularised ALS or CLS timber.
For timber floors between homes (compartment floors):
- to meet acoustic performance, the dead load of the construction of up to 1.25kN/m² may be assumed
- 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 up to 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 qk = 1.5 kN/m² or Qk = 2.0kN.
Service class 1 or 2.
* Two additional joists required
Bold text = normal bearing increased to 50mm
Table 2: Permissible clear spans for domestic floor joists. Strength class C24
Imposed load not exceeding qk = 1.5 kN/m2 or Qk = 2.0 kN.
Service class 1 or 2.
* Two additional joists required
Bold text = normal bearing increased to 50mm
Last updated: 2nd January 2024