Floor Joist Notching Calculator — Permitted Zones & Limits

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. A notch may be cut in the top or the bottom edge — but not both, and not in the same cross-section as a hole.

The maximum notch depth is 0.125 × D (one-eighth of the joist depth), with an overall cap of 35mm. For a 200mm joist the maximum notch is 25mm deep. Never exceed this limit as it significantly weakens the joist.

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

The maximum hole diameter is 0.25 × D (one-quarter of the joist depth), with an overall cap of 65mm. For a 200mm joist the maximum hole is 50mm. Holes must be drilled on the joist centreline (neutral axis) and kept at least 3 diameters apart, centre to centre.

The horizontal distance between any hole and any notch should be at least the depth of the joist, and a notch and a hole must not occur in the same cross-section. For a 200mm joist, keep a notch and a hole at least 200mm apart horizontally.

These deemed-to-satisfy limits are the simplified BM TRADA rules for simply-supported solid timber joists not exceeding 250mm deep, set out in section 3.2.2 of the BM TRADA 'Eurocode 5 span tables for solid timber members' guide (Part 3 — Design considerations) and historically in BS 5268-2. They sit alongside the structural design rules of Eurocode 5 (BS EN 1995-1-1), which covers notched members in clause 6.5. Approved Document A no longer publishes joist notching and drilling guidance — it now points to BS EN 1995-1-1. Engineered/I-joists are excluded; always follow the manufacturer's instructions for those.

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 install deeper joists. Anything outside the deemed-to-satisfy envelope needs a structural calculation by an engineer.