Allowable notching

TRADA (The Timber Research and Development Association) and BS5268-2 give guidance on how large holes and notches can be cut out of standard floor joists, and still remain within safe working limits. The information here has been duplicated for ease, but is a summary of the full document. Refer to TRADA and the British Standards for complete information.

Prefabricated metal web joists can avoid this issue as they have large spaces between top and bottom chords where services can easily pass.

Notches and holes in simply supported floor and ceiling joists should be within the following limits:

  • Notches should be not deeper than 0.125 times the depth of a joist and should not be cut closer to the support than 0.07 of the span, nor further away than 0.25 times the span.
  • Holes should have a diameter not greater than 0.25 times the depth of a joist and should be drilled at the joist centreline. They should be not less than 3 diameters (centre to centre) apart and should be located between 0.25 and 0.4 times the span from the support.

In addition to the limitations above, holes and notches on the same ends of rafters should be kept 100mm apart when measuring horizontally. Notches can be on the top or the bottom of the joist, but not both at the same end.

If notches are limited to the area between 0.1 and 0.2 of the span only (as opposed to 0.07-0.25) then the depth of the notch can be increased slightly to 0.15

Notches or holes should not be cut in rafters, purlins or binders unless approved by the building designer.

Rafters restrained by ceiling ties at eaves level may be birds-mouthed at supports to a depth not exceeding one third of the rafter depth.

Any holes or notches that need to be larger than these guidelines needs to be agreed with a structural engineer.