5.2 Suspended ground floors

Also see:

4.36.4

5.2.8 Transfer of loads: timber floors

Timber suspended ground floors, including the decking material, shall be designed and constructed to be suitable for their intended use. Issues to be taken into account include the:

  1. support of self-weight, dead and imposed loads and limited deflection
  2. safe transmission of loads to the supporting structure
  3. adverse effects of shrinkage and movement.

Support of self-weight, dead and imposed loads, and limited deflection#

Structural timber grades and sizes should be adequate for the spans and imposed loads. Where trimming is necessary, adequately sized timbers should be used.

Structural timber components should be of a suitable strength class as specified by the designer to BS EN 338. Solid structural timber should be:

  • machine graded to BS EN 14081, or visually graded to BS 4978 for softwoods or BS 5756 for hardwoods
  • assigned a strength class based on BS EN 1912 when visually graded
  • dry graded
  • marked in accordance with BS EN 14081.

Further guidance on strength classes for certain timber species can be found in PD 6693.

Engineered wood products such as I-section or metal-web joists should be assessed in accordance with Technical Requirement R3.

For guidance on floor joist deflection limits, see Clause 6.4.9.

Safe transmission of loads to the supporting structure#

Joist hangers should be suitable for:

  • the joist width and depth
  • the strength of masonry
  • the loading
  • providing adequate end bearings to joists.

Sleeper walls should adequately support the floor joists, and joists should be correctly supported at masonry separating walls.
Sleeper walls should not limit ventilation.

Shrinkage and movement#

Strutting should be provided where required following the guidance in Clause 6.4.15.

Last updated: 2nd January 2024

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