Almost certainly nothing will fall, unless it was already so marginal as to be dangerous.

The top of the joist is in compression, and more so in the middle, and the bottom of the joist is in tension. So notching in the middle is bad, and is relegated to the edges. Clearly the top of the joist does nothing over the support, and all the load is born on the bottom, so cutting material nearer the bottom half of the joist is a no-no near the supports. In cases where a lot had to be removed, we have put another section of joist along side, and bolted and glued. This does not look like that case.

Far more care is needed with the modern joist that is a blade of plywood and 2 roof battens, than the old fashioned solid kind, which has quite a lot of excess strength.

If you drill that hole literally anywhere along the mid line of the joist it will have pretty well zero effect on the "strength" of the joist


Structurally, all you are concerned about (as Mike described above) is keep the top and bottom face of the joist in compression/tension respectively - by far the greatest quantity of material is only there to keep those faces apart - along the mid line, you are having no effect on the ability to keep those faces apart


Just crack on - the house won't fall down


Regards


OMS

Quite OMS, and I don't know where these ideas described by Chris came from but they are structurally odd to say the least. Assuming a beam is substantially evenly loaded the neutral axis (the bit which feels no forces at all) is along the geometric centre. Wooden floor joists are usually chosen for stiffness (in terms of the deflection they suffer when loads (people) move about on the floor above, and how this feels), rather than absolute bending strength to failure. With wooden joists the failure is a long way from significant deflection, unless the beam has a large knot near either edge, leading to large local deflection at that point. You have probably noticed this when trying to break up bits of scrap wood for a fire, even a bit of 2x1 (50x25 to metrics) is difficult to break across the 2" axis unless pretty long, but damage to the edges makes it much easier. Chris's diagram shows clearance around the ends where bearing takes place, which are subject to crushing but are very capable of supporting large loads as users of jacks and cars will have noticed. To see how big holes can be, examine the next steel building you are in. Often the roof beams are made by cutting a less deep beam along the centre in a strange pattern leaving round or hexagonal holes when welded back together with the correct offset. Thus one makes a lighter beam with less deflection and greater stiffness for given roof load. The same is true of trussed beams which are largely empty space across the centre, the load being on the two flanges, the upper one of which must be stiff enough between trusses to resist the compressive force. The bottom flange must resist the tensile load which is usually less of a problem. The notches are less satisfactory as they reduce the cross section exactly where the load is a maximum, and the stress is related to the square of the distance from the neutral axis, although cases of plumbers causing failure is very rare.  Holes in the centre of reasonable size (25mm) make an almost immeasurable difference to the strength of normal joists, simply because there is no stress there, but don't drill 100mm holes in a 120 mm deep beam and expect no effect! It is worth looking a a few older structures where you see the flange thickness is deliberately increased at the centre of spans, primaraly to reduce the deflection, rather than increase the failure load.

…. although the neural axis experiences no extension or compression there is a substantial shear force on the neutral axis parallel to the length of the beam, especially towards the centre of the beam where the bending moment tends to be highest.  Drilling lots of holes along the neutral axis is not a good idea in the same way that having a lot of notches in the top isn't as in the limit you would create two beams lying on top of each other.

The best way to understand this is to imagine two identical planks stacked up and supporting a load.  If the planks are free to move over each other they are less stiff than if they are rigidly connected (eg by screwing them together at regular intervals).  The connections bear the shear force which can be considerable..

So the building regs recommendations arise from considering all of the stresses in the beam not just the vertical loads but those arising from bending too.

Whilst the sheer force is important, one would need a huge number of holes along the neutral axis to make this a limiting factor. If you calculate the sheer force in a timber beam with a few holes drilled in it you will immediately see that such worries are very minor. I was trying to stay away from the technical bits, because sheer forces are not obvious to most people without the knowledge of principal forces and triaxial stresses. I would point out that most older timber beams have axial cracks, and these in most cases are not a route to structural failure.

You should not let n architect or engineer design the floor joist layout for a house, it’s a job for the carpenter and technician.


You select the depth and breadth of joists required to span the longest run, typically the lounge, along with appropriate spacing to suit both the load and the length of the plasterboard on the ceiling along with the floor sheets.


So in a typical house you will have floor joists over the lounge  at 400 mm centres, you don’t want steps in the floor or ceiling levels of 50 mm  or so as you go from room to room. Therefore it is general practice that whatever depth of joist is used across the longest span is used throughout the house to avoid steps in the floor and ceiling levels.


If you just carried on with the same joist specification across the whole house the joists will be oversized, possibly massively so in some areas of the house. You don’t want to vary the height, so you can reduce the width of each joists or you could increase the spacing to 600 mm reducing the amount of timber used, but still suitable for 2400 mm sheets of plasterboard and chipboard. But often the same spacing is retained to give structural strength by using the joists to tie the building together.


So as an electrician you need to be careful drilling the joists over the longest span as those joists are designed to just do their job, but drilling that short span under the landing is unlikely to cause issues as the joists are probably well oversized.


I have seen NHBC inspectors make electricians strip out the cables in badly placed holes in joists with long spans, then the electricians have been contra charged for the carpenter repairing the joists with plywood gussets before the electricians reinstalled their cables.


I have never seen anyone get worried about holes in the short joist spans, as they are generally oversized.


Andy Betteridge.

I see that you miss the important thing Andy, and that is not breaking load but floor stiffness. BTW I assure you that Engineers do understand structures, whereas the others tend to follow various rules, a bit like the OSG! If you stand on a floor with 600 mm spacing between the chipboard sheets you will notice that it moves significantly as you move, and you may be able to crack the chipboard by jumping up and down or add heavy items carelessly like a bed or big bookcase with small leg diameter. Next time you have one handy get a scaffold board and support it either end on a couple of bricks. Stand in the middle and note the deflection (several inches). The board does not break but is reasonably close. A floor joist could have a similar deflection without breaking but they are much stiffer, so that the deflection is reasonable. Some modern designs have very poor floor stiffness and it is horrible, but then it saves a few quid. Now you should understand the design criterion. I prefer concrete floors upstairs because as well as the stiffness they have much better sound insulation, but again it is a matter of a few pound notes rather than ultimate building quality.


BTW you might like to try drilling a few holes in a beam on the centre line and verify that it makes little difference to the stiffness or deflection, a useful lesson.

Bridging is installed to stop the joists deflecting under load and as the joist span is increased so is the thickness of the flooring. 


Increasing the spacing of the joists may add to the overall cost as saving a few joists may increase the cost of the flooring throughout the house. 


Leave the design to the architect and they will put the joists at 450 mm centres and neither the flooring or plaster board will fit without them being cut down. 


Andy Betteridge

Thank you, gentlemen, for the interesting comments.


Concerning span v joist depth ...


It had occurred to me that not all rooms need the same specification of joists, but the floor does need to be reasonably level (let's forget ancient buildings!). Any smaller joists could be packed to sit level with larger ones, but then the ceiling heats would differ. I am not sure that that would matter.


Which leads to redundancy ...


The 3 m wide room in question is a bedroom. The bed is against the middle of the far wall, so there is very little loading in the middle of the room. There isn't really enough room for furniture to the sides of the bed, so at most two people standing on one side or the other. On the wall which contains the door, there is room for a small chest of drawers and a single wardrobe, so again, not much loading.


Alternatively, the bedroom could become a bathroom with a cast iron bath in the middle. That might change things a bit.


When we had a structural engineer at home for another matter, I asked him whether a bathroom with joists spanning 4.4 m could support a cast iron bath in the middle. All he did was to jump up and down and then said, "You'll be fine." Whether there were notches and holes close to the centre of the joists (which there are) didn't seem to bother him.


I have a feeling that the Electrician's Guide keeps it simple deliberately, but it doesn't seem to be a very helpful approach.