Timber and concrete upper floors

6.4.18Notching and drilling

Notching and drilling shall be carried out within recognised limits.

Solid timber joists

Notching and drilling should be designed by an engineer where:

  • the joist is deeper than 250mm
  • it does not meet the guidelines in this chapter, or
  • it is close to heavy loads, such as those from partitions, cisterns, cylinders and stair trimming.

Notching and drilling should:

  • have a minimum horizontal separation of 100mm
  • be in accordance with Table 6.

Table 6: Notching and drilling solid timber joists

LocationMaximum size
Notching joists up to 250mm depthTop edge 0.1-0.2 x span0.15 x depth of joist
Drilling joists up to 250mm depthCentre line 0.25-0.4 x span0.25 x depth of joist


Preformed holes should be provided in the web and:

  • holes or notches should not be cut without the approval of the manufacturer
  • restraint straps can be slotted into webs immediately below the top flange.

Metal web joists

In metal web joists:

  • service conduits should run in the gaps between the metal webs
  • maximum duct sizes should be in accordance with the manufacturer’s recommendations
  • large service ducts may have to be inserted before fixing the joists, as it may not be possible after the joists have been fixed.