Can I drill a hole through a floor joist, and where?

Where the permitted zone is, why it is there, the diameter and spacing limits, and the joist types where none of this applies.

Difficulty: advanced
Time: 20 min
Read: 4 min
Safety: warning

Short answer

In an ordinary solid timber joist, drill on the centre line of the depth, between about a quarter and four tenths of the span from a support, with the hole no more than about a quarter of the joist depth in diameter and holes at least three diameters apart. Never near mid-span, never near an edge, and never in an engineered joist without the manufacturer's data.

A joist works by bending: its top edge is squeezed, its bottom edge is stretched, and the middle of its depth carries very little. That single fact explains every rule about drilling one. Material removed from the middle costs almost nothing; material removed from either edge removes exactly the fibres doing the work. The diagram shows the zone and the limits that go with it.

Safety

A joist drilled in the wrong place loses a substantial part of its strength, and the consequences appear as a bouncy or sagging floor later rather than immediately. The zones below are for ordinary solid timber domestic floor joists only. Engineered joists, I-beams, metal-web joists and trussed rafters must only be drilled to the manufacturer's own rules, and some must never be drilled at all. If the joist is already notched, damaged or you are unsure of its type, get advice before drilling.

Where a hole may be drilled through a floor joist

Step by step

Identify the joist type first.Solid rectangular timber is the case the rules below cover. An I-shaped joist with a thin web, a metal-web joist with steel struts, or a trussed rafter is engineered, and it has its own rules published by the manufacturer, often including pre-formed knockouts.
Measure the span, not the room.The span is the clear distance between the joist's supports, which may be walls, beams or steels. Everything positional is a fraction of that, so getting it wrong makes the zone wrong.
Stay on the centre line of the depth.Measure the joist's depth and drill exactly halfway. This is the single most important rule — a hole 20mm off centre is a great deal worse than one on the line.
Stay in the zone along the span.Roughly between a quarter and four tenths of the span measured from a support, at either end. The middle of the span is where bending stress is highest and is the worst place for anything.
Limit the diameter.No more than about a quarter of the joist's depth. A deep joist takes a bigger hole than a shallow one, which is why the limit is a proportion rather than a number in millimetres.
Space multiple holes.At least three diameters apart, centre to centre. Holes close together act like one large hole, and a row of them is a line of weakness.
Understand that notches are different.A notch is a cut into an edge and follows separate, tighter rules — different zones and much shallower depths. Cutting a notch where a hole would be permitted is not a substitute.
Drill straight through, square to the joist.An angled hole is effectively a larger hole. Use a right-angle drill or an auger with a long shank so you can approach it squarely in the floor void.
Check what is already there.Existing pipes, cables, previous notches and previous holes all consume the joist's allowance. A joist that a plumber has already notched at the bottom edge may have very little margin left.
Get advice when it is not routine.A large hole, a hole near mid-span, a joist supporting a masonry wall, an already-damaged joist, or anything structural: this is a question for a structural engineer, and the fee is trivial compared with a floor that needs rebuilding.

Tips

Cables and pipes running through joists must be positioned to protect them from later nails and screws through the floor. That is a separate set of requirements and it is why the centre of the depth suits both purposes.
Where the run genuinely will not fit inside the permitted zone, the answer is usually to route the service differently — along the joist, under the ceiling in a boxed run, or through a different bay — rather than to compromise the joist.
Building regulations approval may be required for work affecting structure. Cutting joists as part of a larger job is frequently notifiable, and this is worth checking before the floor comes up.

Common mistakes

Drilling near the bottom edge because it is easier to reach — The bottom edge is in tension and carries the most load. A hole there removes exactly the fibres holding the floor up.
Drilling near mid-span — Bending stress peaks in the middle. It is the one place along the length where a hole does the most harm.
Applying solid timber rules to an engineered joist — An I-joist's strength is in its thin flanges; the web is not equivalent to the middle of a solid joist. Drilling one without the manufacturer's data can be seriously unsafe.

Questions people ask

Can I drill a joist for a 40mm waste pipe?

Only if the joist is deep enough that 40mm stays within about a quarter of its depth, and only within the permitted zone. On a shallow joist that is very often not possible, and the pipe has to be routed elsewhere.

What if a previous owner has already notched the joists badly?

Treat it as a defect to assess rather than a precedent to follow. Badly notched joists are a common cause of bouncy floors, and adding to the damage makes it worse.

Do the same rules apply to ceiling joists and rafters?

Ceiling joists carry less load but are still structural, and rafters and trussed rafters are a different case again. Trussed rafters must never be cut or drilled without the designer's approval.