Permitted notches and holes in floor joists

Notches: max depth 0.125 x joist depth, only between 0.07 and 0.25 of the span. Holes: max diameter 0.25 x joist depth, on the centreline, between 0.25 and 0.4 of the span, at least 3 diameters apart

Solid timber floor joists — engineered joists follow the manufacturer's zones instead

Every figure for this topic is on one page: Floor joists: spacing, bearing, notching and drilling

Read the full answer

Where services may pass through a solid timber floor joist without weakening it.

Basis in regulation

Part A
Approved Document A
Read the source

How to check it on site

Walk the floor with first fix before decking. Any notch mid-span, any hole off the centreline, and any cut in an engineered joist web outside the printed zones is a fail.

Common failure

Plumbers notching deep at mid-span for waste runs, and holes drilled close together in a line.

Effective from 20 August 2026. Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Minimum Hot Water Storage Capacity

60 °C

Domestic hot water storage systems must provide a minimum capacity to adequately supply hot water at a target temperature, typically 60°C

Minimum

Electrical accessory spacing

100 mm

Electrical socket outlets and switches should be installed with a minimum spacing of 100mm between their centres to ensure adequate access and compliance with wiring regulations

exact

32mm waste pipe support

32 mm

Independent 32mm polypropylene discharge or waste pipes, conforming to relevant British Standards such as BS 5254 or BS EN 1451, must be adequately supported with pipe clips throughout their run to the soil stack or manifold

Minimum

Clearance from combustible materials

75 mm

Appliances that generate heat, such as solid fuel burners, must maintain a minimum separation distance of 75mm from any combustible materials to prevent fire

Minimum

Headroom over a stair

2000 mm, reducing to 1900 mm at the centre of a loft conversion stair and 1800 mm at the side

Stairs and landings within a dwelling

Maximum

Smoke alarm siting distances

Within 7.5 m of the door to every habitable room, within 3 m of every bedroom door, and at least 300 mm from any wall or light fitting

Ceiling-mounted mains-powered alarms on circulation routes