Not every joist is a joist
Before anything else, because it is the one that does real damage.
These limits apply to solid timber joists. They do not apply to engineered joists, and on those the difference is not a matter of degree.
An I-joist has almost all its strength in the top and bottom flanges: the web in between is doing comparatively little. Notch a flange and you have not weakened the joist, you have cut it. Web holes are allowed, but only where the manufacturer says, in the sizes and positions they publish, and every maker is different.
A metal web joist is never drilled and never notched. The open web is there so services pass through the gaps that already exist. That is the main reason anybody specifies them, and drilling one throws away the reason it was chosen while damaging a designed component.
A plumber who knows the joist rules and has never been told that distinction is exactly who this page needs to reach in the first paragraph.
The rules are proportions. Nobody can measure a proportion.
Approved Document A sets the zones out as fractions of the span and the depth, which is the right way to write a rule and a useless way to use one. Nobody standing on a joist with a tape can measure 0.07 of a span.
So that is all this tool does: turn them into millimetres for the joist in front of you, and draw them.
For a 3.6 metre span with 195mm joists:
- Notches: 252mm to 900mm from the support, top edge only, 24mm deep maximum.
- Holes: 900mm to 1440mm from the support, on the centreline, 49mm across maximum, 3 diameters apart.
Symmetrical about midspan, so the same applies from the other end. And note the zones do not overlap: the notch zone finishes exactly where the hole zone starts.
Why the top and not the bottom
A loaded joist is in compression along the top and tension along the bottom. That is why it works at all.
A shallow notch in the top edge interrupts a compression zone, which timber tolerates. A notch in the bottom edge interrupts the tension face, and timber in tension fails from a notch the way a sheet of glass fails from a score line. It is a crack with a head start.
The holes go on the centreline for the same reason from the other direction: the middle of the depth is the neutral axis, where the timber is doing the least work in bending.
The diagram is the answer
This is a spatial rule, so a table of numbers makes you work it out twice. The zones are drawn along the joist to scale with your proposed hole or notch marked in position, and it goes green or red.
Slide the position along and watch the answer change. That is a faster way to find where a pipe can actually go than reading four numbers and doing the arithmetic in your head on a ladder.
A published rule is not a check on your joist
It reports a published rule. It does not check whether your joist had any capacity to spare in the first place.
A joist already at its span limit, or carrying something unusual, is a question for an engineer whatever the zones say: the zones assume a joist with normal reserve, not one that was marginal before anybody put a hole in it.
Next to it: what C16 and C24 mean, cutting lists, and what to tell your engineer if the answer here is that you cannot do what you wanted to.
Common questions
Where can I notch a floor joist?
Top edge only, between 0.07 and 0.25 of the span from the support, no deeper than an eighth of the joist depth. On a 3.6m span with 195mm joists that is between 252mm and 900mm out, and 24mm deep. Never the bottom edge, and never in the middle third.
Where can I drill a joist?
On the centreline of the depth, between a quarter and 0.4 of the span from the support, no bigger than a quarter of the depth, and at least three diameters apart centre to centre. On a 3.6m span with 195mm joists that is between 900mm and 1440mm out, up to 49mm across.
Can I notch an I-joist?
No. Almost all the strength of an I-joist is in its flanges, so notching a flange is not weakening it: it is cutting it. Web holes are allowed only where the manufacturer says, in the sizes and positions they publish, and every maker is different. If nobody knows who made them, that is a question for an engineer.
Can I drill a metal web or posi joist?
Never. The open web exists precisely so services go through the gaps that are already there, which is the main reason people specify them. Drilling one defeats the entire point of it and damages a designed component.
Why can I not notch the bottom of a joist?
Because the bottom edge is in tension when the joist is loaded, and a notch there is a crack with a head start. The top edge is in compression, which is why a shallow notch is tolerated there and nowhere near the bottom.
Can I put a notch and a hole in the same place?
No. Each is allowed on its own within its zone, but together they take out most of a section, and the zones do not overlap anyway: the notch zone finishes where the hole zone starts. If a run needs both, they go in different places along the joist.