Notching and Drilling Suspended Timber Floor Joists: The Rules

Plumbers and electricians routinely need to route pipes and cables through timber floor joists. As long as work stays within defined zones, this does not affect structural performance. Outside those zones, the joist section is reduced and the joist may become inadequate.

Notches

Notches are cuts taken from the top or bottom edge of a joist to allow a pipe or cable to pass through.

Where notches are permitted:
– Only within the outer quarter of the joist span — i.e. within the zone from each support to L/4 from that support
– Notch depth must not exceed 0.125 × the joist depth (12.5% of total depth)
– For a 200 mm deep joist, the maximum notch depth is 25 mm

Why the zone matters:
Bending moment is highest at mid-span and lowest near the supports. By restricting notches to the outer quarters — where bending is low — the structural capacity of the joist is maintained. A notch at mid-span would reduce the section exactly where maximum bending stress occurs.

Top vs bottom edge:
Notches in the top edge reduce the compression zone. Notches in the bottom edge reduce the tension zone, which is structurally more critical. Avoid bottom-edge notches where possible. Where unavoidable, ensure they comply with the depth limit.

Drilled Holes

Drilled holes cause less stress concentration than notches because the section material is removed from within the depth rather than from the edge.

Where holes are permitted:
– Only within the middle third of the joist span — i.e. in the zone between L/3 and 2L/3 from each support
– Holes must be located within the middle third of the joist depth vertically — i.e. centred between d/3 and 2d/3 from either face
– Maximum hole diameter: 0.25 × the joist depth (25% of total depth)
– For a 200 mm deep joist, maximum hole diameter is 50 mm
– Minimum clear spacing between holes: 3 × the hole diameter

Manufactured I-Joists and Engineered Timber

Manufactured I-joists (e.g. Metsä Wood Finnjoist, Posi-Joists) have specific notching and drilling rules set by the manufacturer. These are more restrictive than the rules for solid timber:

– Never notch the flanges of an I-joist — this destroys the composite section
– Holes in the web are generally acceptable within manufacturer-specified zones and diameter limits
– Always check the manufacturer’s technical literature before drilling I-joists

For engineered LVL or glulam beams, do not drill or notch without specific engineering approval.

What to Do if You Need to Deviate

If your pipe routing requires a hole or notch outside the permitted zones, or larger than the limits allow, options are:

1. Re-route the pipe to stay within the permitted areas
2. Replace the joist with a deeper section that provides the required clearance
3. Raise a Change Request with PorthouseDean — we can check whether the specific joist in question can accommodate the required opening with a local strengthening measure

Contact us or raise a Change Request before cutting outside the permitted zones.