Where can I notch a joist?

Notches are only permitted in the top of the joist, within the 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. Notches must never be cut in the bottom of a joist, as this is the tension zone.

What is the maximum notch depth in a joist?

The maximum notch depth is D/8, where D is the joist depth. For a 200mm joist, the maximum notch is 25mm deep. For a 250mm joist, it's 31mm. This limits the weakening of the joist at the point of highest shear stress. If you need a deeper notch, use joist hangers or support brackets instead.

Where can I drill holes through a joist?

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).

What is the maximum hole diameter in a joist?

The maximum hole diameter is D/4, where D is the joist depth. For a 200mm joist, the maximum hole is 50mm diameter. For a 250mm joist, it's 62mm. Multiple holes must be at least 3 diameters apart. Holes should be drilled, not punched, to avoid splitting.

What if I need to run pipes outside the permitted zones?

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.

TradeCalculator.co.uk

Joist Notching & Drilling Calculator

Check if your proposed notch or hole in a joist complies with Building Regulations. Calculates permitted zones and maximum sizes.

Joist Depth

e.g. 200mm or 250mm

Joist Span

Clear span between supports

Modification TypeNotch (top) or hole (through)

Proposed Notch Depth

Size of proposed modification

Position from Support

Distance from nearest bearing point

How We Calculate This

This calculator checks whether a proposed notch or hole in a floor/ceiling joist complies with the Building Regulations rules for zones and maximum sizes.

The rules

Notches: Zone 0.07L to 0.25L from support, max depth D/8
Holes: Zone 0.25L to 0.4L from support, max diameter D/4
Holes: Centred on neutral axis (mid-depth)
Holes: Min 3 diameters between centres