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Passing cables through joists

Chris Pearson
I know that this has been discussed in the old forum, but I have been pondering the subject over the past couple of days.


I am more than happy to respect the exclusion zones at the ends and in the middle of joists (Electrician's Guide to the Building Regulations); but it seems odd that cables which are passed through joists must go through notches in the zone between 0.07 - 0.25 times the span, and holes in the 0.25 - 0.40 zone.


In essence, despite the removal of a substantial amount of material, the upper joist in my diagram is acceptable, but the 16 mm hole in the lower one is not.

6023a2338ff6952232f44f2cc4478e57-huge-20190802joist.jpg


It seems to me that any risk to the cable is much lower in holes than notches; and of course, if plumbers have already occupied the notch zone, there isn't much choice.


In order to remove and replace cablies descending to sockets below, I need to lift the second board in from the wall; but it would be a real PITA to have to lift another board (especially if tongue and groove) closer to the centre of the room particularly because there isn't much working space unless the furniture is removed.


I have no intention of placing holes below notches, but will the house fall down if I don't follow the Guide? ?
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    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.


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  • 0
    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.


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