How to drill through joists and run cable safely

Helpful video reference. The video above is GSH Electrical's "Drilling Holes in Joists to run Cables (wires) Through — Installation Considerations inc Calculations". GSH Electrical is a UK electrical training channel run by Gary Hayers, a former installation electrician who has spent 19 years teaching at college level. The channel has more than 170,000 subscribers and produces training aids for City and Guilds and EAL Level 2 and 3 courses. The video walks through the calculation for safe drilling zones and the BS 7671 limits on hole size — worth watching in full before you pick up a drill. Watch on YouTube.

1. Plan the cable route first

Before lifting any boards or picking up a drill, plan the complete cable route on paper. Identify where the cable starts (usually the consumer unit or a junction point), where it ends (the new socket or light position), and how many joists it crosses. Where possible, run parallel to the joists in the floor void rather than crossing them — that way no holes are needed at all.

When the cable must cross joists, plan to cross at right angles. A cable running diagonally across multiple joists at awkward angles is harder to route, harder to clip correctly, and harder to protect if the boards are ever lifted again.

In older homes in east Kent, the floor void is often the main cable route for upstairs socket and lighting circuits. Taking a few minutes to trace existing routes before you start will tell you where cables already pass through and how much space is available.

2. Understand the safe drilling zone

This is the critical rule. Joists work by carrying loads from above in bending — the top surface is in compression and the bottom surface is in tension. The weakest point for drilling is in the middle of the span because that is where bending stress is greatest.

BS 7671, referencing structural guidance, requires holes to be located between 25% and 40% of the joist's clear span from the nearest support. On a typical 2 m joist span (wall-to-wall), that means the hole must be placed 500–800 mm from each support. In practice, most electricians aim for roughly one-third of the span from one end — that puts them well within the safe zone.

The hole must also be drilled at the neutral axis of the joist — the horizontal centre of its depth. A 175 mm deep joist should be drilled at 87 mm from either face. Drilling high or low shifts the hole into the tension or compression zone and reduces the joist's strength more than necessary.

3. Size the hole correctly

The maximum hole diameter is 25% (one-quarter) of the joist's total depth. For common joist sizes this works out as:

• 100 mm joist: 25 mm maximum hole
• 150 mm joist: 37 mm maximum hole
• 175 mm joist: 43 mm maximum hole
• 200 mm joist: 50 mm maximum hole

Twin and earth cable is small — 2.5 mm² twin and earth is roughly 8 mm across — so even a 20 mm spade bit leaves plenty of margin on a 100 mm joist. Where multiple cables pass through the same joist, use one larger hole rather than several small ones close together, but keep the total hole size within the 25% limit.

Holes must also be spaced at least three diameters apart (centre-to-centre) where multiple holes are drilled in the same joist. For 20 mm holes that means centres at least 60 mm apart.

4. Check for pipes and existing cables

Run a cable and pipe detector across the floor above the route before lifting boards or drilling. Pipes often run in the same floor void as cables, and in properties built before 1970 you may find unsheathed cables, lead pipes, or rubber-insulated wiring that needs particular care.

Once a board is lifted, use a torch to look along the void before drilling. If you can see a pipe or existing cable in the path of the planned hole, adjust the drill position — do not risk piercing either.

5. Drill the holes

Use a long auger bit or long spade bit rated for the hole size required. A standard spade bit on an SDS drill can wander off-line through a thick joist — a long auger bit with a threaded tip stays truer. Keep the drill square to the face of the joist and drill in one steady pass. Stopping and starting mid-hole often causes the bit to drift.

Support the joist from below if you can — drilling from above pushes chips down, and the exit point of the hole can splinter if the joist is unsupported. A piece of scrap timber held against the underside prevents a rough exit hole.

6. Fit grommets before pulling cable

Push a plastic grommet or cable bush into each drilled hole before drawing any cable. The grommet protects the cable sheath from the timber edge. Without it, the sharp edge of the hole — or any splinters left from drilling — can chafe through the insulation over time, creating an insulation resistance fault years later. Grommets are cheap; replacing a cable run is not.

If the hole is too tight for a standard grommet, use a cable bushing strip that lines the inside of the hole.

7. Draw cable through and secure it

Thread a fish tape or draw string through the holes from one end of the run to the other, then tape the cable to the draw string and pull it back through. In long runs this is much easier with two people — one feeding, one pulling.

Once the cable is through, clip it at each joist to prevent it sagging in the void. Cable should not be left unsupported in long runs between joists. Where the cable runs along the side of a joist rather than through it, clip at 250 mm horizontal intervals.

8. Test before relaying boards

Before nailing or screwing floorboards back down, carry out a quick continuity test on the new cable run. Confirm continuity on the live, neutral and earth conductors and check polarity. Doing this while the cable is still accessible saves a great deal of work if there is a problem — a break in a conductor or a pinched cable is caught now rather than after the floor is closed up.

When to call us

Drilling joists and drawing cable is a reasonable DIY task in a straightforward domestic property. It becomes an electrician's job when the wiring on the other end of that cable involves the consumer unit, a new circuit, or anything that requires Part P notification. If in doubt about what is in your floor void or whether the job is notifiable, a call costs nothing.