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Joist Notching & Drilling Calculator
Check if your proposed notch or hole in a joist sits within the BS 5268-2 / Eurocode 5 (BM TRADA) deemed-to-satisfy zones and maximum sizes. For simply-supported solid timber joists ≤250mm deep.
Simplified rules apply to joists ≤250mm deep
Clear span between supports
Size of proposed modification
Distance from nearest bearing point
For calculating minimum hole spacing
Minimum 3 diameters apart
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How We Calculate This
This calculator checks whether a proposed notch or hole in a floor/ceiling joist sits within the deemed-to-satisfy zones and maximum sizes given in BS 5268-2 / BS EN 1995-1-1:2004 (Eurocode 5), as reproduced by BM TRADA. Approved Document A no longer contains these notch and hole diagrams, so the guidance is now drawn from Eurocode 5 rather than the Building Regulations directly.
The rules
Notches: Zone 0.07L to 0.25L from support, max depth D/8 (35mm absolute cap)
Holes: Zone 0.25L to 0.4L from support, max diameter D/4 (65mm absolute cap)
Holes: Centred on neutral axis (mid-depth)
Holes: Min 3 diameters between centres
Key principles
- Notches in the top or bottom of the joist, not both — kept within the 0.07L–0.25L end zone (top is the usual, safer default)
- Holes drilled at neutral axis (minimum stress point)
- Never notch or drill near mid-span (maximum bending)
- A notch and a hole in the same joist should be at least 100mm apart horizontally
- These rules apply to simply-supported solid timber joists not greater than 250mm deep
- Deeper joists, engineered/I-joists, and trimmers must be designed to Eurocode 5 by a structural engineer
Working figures for this are collected in our Notching and Drilling Rules for Joists reference table.
Frequently Asked Questions
Notches are permitted in the top OR bottom of the joist (not both), within the end zone from 0.07 to 0.25 of the span measured from the support (bearing point). For a 4m span joist, this means the notch zone is 280mm to 1,000mm from the wall. In practice notches are usually cut in the top, which is the simpler and safer default for first-fix services. Never notch outside this end zone — and never near mid-span, where bending stress is highest.
The maximum notch depth is 0.125 × the joist depth (D/8), subject to an absolute cap of 35mm. For a 200mm joist, the maximum notch is 25mm deep; for a 250mm joist it is 31mm. The 35mm cap only bites on joists deeper than 280mm, which fall outside the simplified guidance anyway. This limits weakening of the joist at the point of highest shear stress. If you need a deeper notch, use joist hangers or support brackets instead.
Holes must be drilled within the zone from 0.25 to 0.4 of the span from the support. For a 4m joist, this is 1,000mm to 1,600mm from the wall. Holes must be centred on the neutral axis (mid-depth of the joist). Holes must not be closer than 3 diameters apart (centre to centre).
The maximum hole diameter is 0.25 × the joist depth (D/4), subject to an absolute cap of 65mm. For a 200mm joist, the maximum hole is 50mm diameter; for a 250mm joist it is 62mm. The 65mm cap only bites on joists deeper than 260mm, which fall outside the simplified guidance anyway. Multiple holes must be at least 3 diameters apart (centre to centre). Holes should be drilled on the centreline (neutral axis), not punched, to avoid splitting.
If pipes or cables must pass through joists outside the permitted zones, use proprietary joist reinforcement plates (such as Speed-E-Trim) or install the services below the joists in a suspended ceiling void. Alternatively, use engineered joists (I-joists) which have pre-designed service holes. Never cut or drill outside the regulations without structural advice. For overall timber quantities, use our First Fix Timber Calculator.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.