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Drilling holes in joists for cables

Updated: 2026-10-23

You have a cable in one hand and a spade bit in the other, and the joist is solid timber holding a floor. A cable hole is allowed only on the centre line, inside a small zone, and only up to a fraction of the depth. A pouch that goes to 32 mm is not a free pass through a shallow joist, and mid-span is the wrong place even when the diameter is legal. This is the buyer summary for a simple solid joist, not a sign-off. LABC says its domestic guidance is not definitive. Which listing prints 25 mm is on the [spade bit comparison](/hub/drilling-and-accessories/spade-bit).

You have a cable in one hand and a spade bit in the other, and the joist is solid timber holding a floor. A cable hole is allowed only on the centre line, inside a small zone, and only up to a fraction of the depth.

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On this page
  1. 1.Centre line, quarter depth, and the span zone
  2. 2.What a pouch set tempts you to drill
  3. 3.Not this joist
  4. 4.Cables you cannot see

Centre line, quarter depth, and the span zone

For a simply supported solid joist, LABC’s reading of Eurocode 5 (BS EN 1995-1-1) is: centre line, no more than 0.25 times the depth, and no more than 65 mm. Place the hole between 0.25 and 0.4 of the span from the support, with at least three diameters between holes. Mid-span fails. A 100 mm joist takes 25 mm, a 150 mm joist 37.5 mm, a 200 mm joist 50 mm. Measure the timber, not the nominal name. The diagram is no longer in Approved Document A. These fractions are the England explainer. Scotland, Wales and Northern Ireland have their own regulations. Involve an engineer above about 250 mm depth, beside a heavy load, or outside the table. A notch is about an eighth of the depth, top or bottom, not both. Do not drill through its end.

What a pouch set tempts you to drill

SHALL runs to 32 mm. The Bosch set states 16 mm to 32 mm and still excludes hardwood. A 25 mm bit matches the 100 mm example. A 32 mm bit needs at least 128 mm of timber. In a 100 mm joist it is already over the limit. A 25 mm pair wants 75 mm between centres. Several cables in one permitted hole are often kinder than a row of holes. A 20 mm hole can pass a small twin-and-earth cable and stay inside the cap. The joist sets the maximum. A 152 mm extension, on the Bosch set and in the BULLITT title, does not relax the zone. Confirm the printed diameter in the spade bit buying guide. A Bosch single that prints both 25 mm and 40 mm is a bad tool for a 25 mm allowance.
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Not this joist

I-joists and metal-web joists are outside the solid-timber table. Do not drill the flanges. Web holes are often limited to about 38 mm and sit where the maker’s sheet allows. Metal webs stay intact. Get the manufacturer’s chart. Rafters are not joists. East Lindsey allows a birdsmouth at the support, no deeper than 0.33 times the rafter depth, and not a casual cable hole up the slope. A notched joist, a joist under a bath, or a hole that cannot sit in the zone is the moment to stop. Filler does not put the fibres back.

Cables you cannot see

If you do not know where concealed cables are, HSE CIS65 says turn the supply off before you drill into walls. Scan the line the bit will travel. If a detector does not clear it, stop. At work, PUWER covers the drill. Goggles still matter. They do not repair a hole in the wrong zone. When the hole is allowed, the clamp and the exit are on how to use a spade bit. A self-feeding tip grabs in a joist as it grabs in a door. If you withdraw every few seconds because the chips have packed, an auger may be the other tool. Changing steel does not move the hole to mid-span.

Common questions

How big a hole can I drill in a 150 mm or 200 mm solid joist?
A 150 mm joist takes 37.5 mm and a 200 mm joist takes 50 mm, on the centre line and in the span zone. Deeper joists in LABC’s table are capped at 65 mm. Measure the actual timber. This is guidance to confirm, not an approval.
Where along the span is the hole allowed?
Between 0.25 and 0.4 of the span from the support, on the centre line. Mid-span is rejected. A 25 mm pair wants 75 mm between centres.
Is a notch the same rule as a round hole?
No. A notch is about an eighth of the depth, top or bottom but not both, near the ends. Do not drill through its root. Keep a hole about 100 mm clear.
Does this apply to an I-joist or a metal-web joist?
No. Flanges are not a drilling target. Web holes sit where the maker allows, often up to about 38 mm. Metal-web joists stay intact.
What if the only bit I have is 32 mm?
The joist must be at least 128 mm deep. In a 100 mm joist, 32 mm is over the fraction, even though several pouches include it. Use a smaller bit that passes the cable, or do not drill.
Do I turn the power off before drilling a wall?
Yes, if you do not know the cable route. HSE CIS65 says isolate before drilling into walls. A detector that has not cleared the line is a reason to stop.
When do I stop and call a structural person?
When the joist is engineered, notched, deeper than about 250 mm, next to a heavy load, or the hole cannot sit in the zone. A rafter is a birdsmouth limit, not a mid-slope cable hole. Technique does not make mid-span acceptable.

Related pages

Spade bit vs auger

Spade bit

Best spade drill bits for a hidden hole or a door lock

Spade bit buying guide for diameter, shank and timber

How to use a spade bit and keep the exit face clean

Sources

Public UK sources last checked on 19 October 2026. Shelf prices, stars and review counts are the amazon.co.uk listings read on that date.

  1. CMT Group UK, drill bits for construction materials (retrieved 19 October 2026)
  2. BuildWiz UK, drill bits (retrieved 19 October 2026)
  3. LABC, notches and holes in solid timber joists (retrieved 19 October 2026)
  4. East Lindsey Guidance Note 15, notches and holes (retrieved 19 October 2026)
  5. Professional Electrician, notches and holes in joists (retrieved 19 October 2026)
  6. HSE CIS65, concealed cables (retrieved 19 October 2026)
  7. HSE INDG291, PUWER and drilling machines (retrieved 19 October 2026)
  8. Bosch Professional GB, Expert SelfCut Speed spade bits (retrieved 19 October 2026)