Can ceiling joist be notched for cables?

Discussion in 'Electricians' Talk' started by JohnyB, Jun 4, 2018.

  1. JohnyB

    JohnyB Member

    Looking to replace central ceiling light with downlighters, and want to reduce the electricians cost by preparing as much as possible. So I've drilled out the downlighter holes, but as you'd expect there are joists in the way where cables need to go.

    Within the current regulations, am I able to notch bottom of joist which can then be covered with a safety plate (before plastering), or do I need to cut out sections of the ceiling to allow holes to be cut in middle of joists ? Cheers
  2. JP.

    JP. Screwfix Select

    Nope you cannot notch bottom of joist John - sorry m8 but regs are regs
    JohnyB likes this.
  3. Bazza

    Bazza Well-Known Member

    Which reg is that one, then?

    There are no zones in ceilings, and plates over notches are not necessary. The OP’s cables will be lying on top of the ceiling plasterboard in any case.
    IMO it’s best to drill in the centre of joists for cable runs, but this document gives guidance on notching etc.
    Last edited: Jun 4, 2018
  4. Pollowick

    Pollowick Well-Known Member

  5. JP.

    JP. Screwfix Select

    Bazz the chap does say that he wants to notch the bottom of the joists - this is not allowable tbqh
  6. unphased

    unphased Screwfix Select

    JP is correct. You would severely weaken the structural characteristics of the joists if you notch them. DONT!

    One way is to cut a narrow strip of the ceiling away in a straight line across the joists so you can get a drill in to drill a hole through the centre line, then patch the ceiling up afterwards.

    Welcome to the world of the electrician. Setting out recessed ceiling lights in a ceiling is a nightmare. You need to get the spacings reasonably even and miss all the joists. Good luck!
    Last edited: Jun 4, 2018
  7. DIY womble

    DIY womble Well-Known Member

    Use a magnet to trace plasterboard screws or nails and mark joist positions , check for noggins
    Josh.91 and retiredsparks like this.
  8. Sparkielev

    Sparkielev Well-Known Member

    Cut a 4" hole either side of joist with a hole cutter drill joists with angle drill replace 4" holes with the pieces that have been cut out
  9. Pollowick

    Pollowick Well-Known Member

    Why either side of joist? If the holes are chosen correctly, one hole can be used for two joists.

    And don't forget, that holes must be on the centre line and 0.25 to 0.4 of length from either end.
  10. WillyEckerslike

    WillyEckerslike Well-Known Member

    Much as my instinct (and building regs) says not to notch the underside of joists - ceiling or otherwise - the electrician's pocket guide says you can. This was aired a few months ago and Rulland (a well respected forum member) posted an excerpt from the manual showing just that. I'll try to find the thread and post a link but I'm on my mobile and have a dodgy signal....
    retiredsparks and Rulland like this.
  11. Sparkielev

    Sparkielev Well-Known Member

    Easier to get drill up and
    It the way i like to do it you never know whats clipped to joist, cutting hole either side is no big issue, making good is pretty simple
  12. JP.

    JP. Screwfix Select

    Thats the way we use to do it on lathe and plaster ceilings mostly Mr S..when everything was done we use to screw and glue pb across the holes and make good with bonding - depending on the job we either pulled down the bonding and flashed in filler and sanded flush, or usually the whole ceiling would be skimmed. Nowadays would insist on taking the whole ceiling down if lathe and plaster, and re board when cabled and stuff with 12.5mm, and in some cases double boarding or pink would be used if indicated.
    Last edited: Jun 4, 2018
    Sparkielev likes this.
  13. WillyEckerslike

    WillyEckerslike Well-Known Member

  14. Sparkielev

    Sparkielev Well-Known Member

    I screw a piece of wood across the hole inside cieling then fix the 4" cut piece to the wood the easi fill the edges
    WillyEckerslike likes this.
  15. seneca

    seneca Screwfix Select

    Same here Sparkie.
    Sparkielev likes this.
  16. Sparkielev

    Sparkielev Well-Known Member

    WillyEckerslike likes this.
  17. WillyEckerslike

    WillyEckerslike Well-Known Member

    If you read the whole thread you can see that despite lots of evidence to the contrary, the pocket guide is very clear that you can notch the bottom of a joist. You won't ever catch me doing it though - I'm not an electrician.....
  18. JP.

    JP. Screwfix Select

    It is a bodge Mr respectable trades person would notch the bottom of joists despite some guides, or whatever, saying you can
    Sparkielev likes this.
  19. unphased

    unphased Screwfix Select

    Well, take it from a structural engineer (yes me) that notching joists weakens them. The average Joe electrician and sparky does not understand the shear and bending mechanism of joists. The distances quoted by the extracts in the Regs as to where holes and notches should be placed are catch all attempts at minimising the weakening affect of holes and notches. If you notch the underside of a beam or joist in the middle third of its span you are removing the part that is in tension, on the outermost fibres, where you really NEED to have the most strength in the beam to resist the bending. Notching the top in the middle third is in a compression part which is lesser of the evil and the beam is more able to cope. Bending stress in tension or compression overall is maxed at the outer edges of the beam. The centre of the span is where tension is maximum and compression is maximum and ideally no notches or holes should ever be made along the centre of its span, ever! It is the centreline of the depth of beam across the span where bending stress is cancelled out ie the neutral axis, and this is why electricans are told to drill holes in the centreline of the beam or joist and not in the top or bottom edges. Plumbers have no choice but to notch the joist and they should know it is the top of the joist that gets notched, not the bottom. If they really need to notch the joist at the bottom then it should be close to the supported ends of the joist where shear stress is maximum and bending is at a minimum.

    A saving grace in all this is the fact that floor joists are part of a shared load system. A multiple joist system where all stresses are shared across several joists by virtue of the floor boards and ceiling boards creating a composite sandwich and the distance between the joists determines how much load can be shared. The closer the joist spacing the stronger the floor.

    There are other reasons for not using notches in a beam where wiring is concerned and that is vulnerability to damage from screws and nails when fitting floor boards and ceilings.
    Jord86 likes this.
  20. peter palmer

    peter palmer Well-Known Member

    And yet things like this are still rock solid.


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