Do not cut a notch or drill a hole wherever it is convenient. Solid timber joists have permitted zones intended to keep services away from the areas where the joist is most vulnerable.
Drill holes on the centreline between 0.25 and 0.40 of the span
LABC guidance for simply supported solid timber joists places holes on the neutral axis between 25% and 40% of the span measured from a support. The hole diameter should not exceed 25% of the joist depth, with a maximum diameter of 65mm. Separate holes by at least three hole diameters measured centre to centre.
Cut notches between 0.07 and 0.25 of the span
Notches should be no deeper than 12.5% of the joist depth, with a maximum depth of 35mm. Keep them within the permitted zone near the support and do not notch both the top and bottom of the same joist.
The rules are for solid timber joists
Do not apply these dimensions to I-joists, metal-web joists or other engineered products. Follow the joist manufacturer's instructions for engineered systems.
An oversized existing notch or hole is a repair question
If a joist has already been heavily notched, drilled outside the permitted zone, split or otherwise weakened, do not simply add another cut or assume a short timber patch restores its capacity. The correct repair depends on the remaining section, span and loading.
See the LABC notch and hole guidance. For repair options, read Sistering Floor Joists. If replacement timber is specified, browse structural timber.
