Where can I notch a floor joist?

Notches are only permitted in the zone between 0.07 and 0.25 of the joist span, measured from the support. For a 4m span, the permitted notch zone is 0.28m to 1.0m from the wall. Notches must be on the top edge only.

How deep can a joist notch be?

The maximum notch depth is D/8 (one-eighth of the joist depth). For a 200mm joist, the maximum notch is 25mm deep. Never exceed this limit as it significantly weakens the joist.

Where can I drill holes in a floor joist?

Holes are permitted in the zone between 0.25 and 0.4 of the span (the neutral axis zone where bending stress is lowest). For a 4m span, holes can be drilled between 1.0m and 1.6m from the support.

How big can a hole be in a floor joist?

The maximum hole diameter is D/4 (one-quarter of the joist depth). For a 200mm joist, the maximum hole is 50mm. Holes must be centred on the neutral axis (mid-depth of the joist) and at least 3 diameters apart.

What happens if I notch or drill outside the permitted zones?

Notching or drilling outside permitted zones or exceeding size limits can cause joist failure. If you need to run services outside these zones, use alternative methods such as running pipes below the joists or installing deeper joists.

TradeCalculator

Floor Joist Notching Calculator — Permitted Zones & Limits

Check notch and hole positions against Building Regulations: notches in 0.07\u20130.25 span (max D/8 deep), holes in 0.25\u20130.4 span (max D/4 diameter).

Joist Depth

Depth of the floor joist

Joist Span

Clear span between supports

Notch Check

Proposed Notch Depth

Depth of notch from top edge

Notch Position from Support

Distance from nearest support

Hole Check

Proposed Hole Diameter

Diameter of hole

Hole Position from Support

How We Calculate This

This calculator checks your proposed notch or hole against the Building Regulations (Approved Document A) permitted zones and size limits.

The rules

Notches: Top edge only, 0.07–0.25 of span, max depth D/8
Holes: On neutral axis, 0.25–0.4 of span, max diameter D/4
Hole spacing: Minimum 3 diameters centre to centre

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.