Routing cable through ceiling joists

Discussion in 'Electricians' Talk' started by DO IT MYSELF, Jan 5, 2005.

  1. DO IT MYSELF

    DO IT MYSELF New Member

    Hi can anyone tell me is it safe to drill through ceiling joists (approx 350mm x 900mm) to route the lighing circuit cables. If so what is the maximum diameter hoel permissible? The joist centres are 600mm if this is relevant.

    Thanks
     
  2. The Trician

    The Trician New Member

    Do a search in the top left hand box on this page mate - plenty of info there if you care to look and read hard enough.

    TT
     
  3. charliex

    charliex New Member

    if youre drilling holes in joists then they must be a minimum of 50mm from the surface of the joist. hole size dependent on how many cables going through it, but should be kept to a minimum
     
  4. DO IT MYSELF

    DO IT MYSELF New Member

    Er sorry those measurements should have been 35mm x 90mm.
     
  5. The Trician

    The Trician New Member

    The words Neutral Axis are the ones you need to bear in mind. There's already been a recent thread about this, together with a supplied URL to a site which provides the info you need.

    TT
     
  6. DO IT MYSELF

    DO IT MYSELF New Member

    I've tried doing a search for Neutral Axis as I assume this is the trem you are talking about (obviously this means nothing to me!) but can't find anything that makes any sense on the search. However I did find a link to a PDF for Zurich which describes drilling and notching. Is that the kiddy?
     
  7. The Trician

    The Trician New Member

    Indeed it is.

    Happy reading - any terms you don't understand, come back here.

    TT
     
  8. Cornish Crofter

    Cornish Crofter Active Member

    Hi there.

    I am no expert but I am just quoting from the On site guide Page 52. This is an IEE publication and is used in conjunction with the 16th edition.

    7.3.1 Floors and Ceilings

    When a cable is installed under a floor or above a ceiling it shall be run in such a position that it isnot liable to damage by contact the floor or ceiling or their fixings. Unarmoured cables passing through a joist shall be at least 50mm from the top or bottom as appropriate or enclosed in earthed steel conduit. Alternitively, the cables can be provided with mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like. (Note, the requirement to prevent penetration is difficult to meet.)

    Notes:

    1. Maximum diameter of hole should be 0.25 of joist depth

    2. Hole on centre line in a zone between 0.25 and 0.40 x span

    3. Maximum depth of notch should be 0.125 x joist depth

    4. Notches on top in a zone between 0.1 and 0.25 span

    5. Holes in the same joist should be 3 metres apart.

    OK there you have it. Unless you're using steel conduit that you won't be able to drive a nail through it's at least 50mm below the boards (Assume below the underside of the boards).

    Whatever you do DO NOT, I repeat DO NOT put notches on the underside of the joists or have holes really close to the underside (50mm is absolute max, I would personally leave more). Removing timber on the underside of the joists will seriously weaken your floor and kill you if it lands on you, or if you're on top of it when it goes.

    If you're exstensively rewiring, you're probably doing walls as well. There are more requirements here as well. Page 53 of the same book has a diagram of permitted routes for cables in walls. I would suggest you buy one of these around £20 IIRC, or even better, get yourself on a 2381 course a your local college of further education, where you can learn about the 16th edition, and hopefully pass an exam on it.

    In any case your work will now have to be subject to an installation by a competent electrician (part P building regs contact LA Building control), so don't go planstering over any cables and make sure the ones in your floors are accessable.

    I bet you thought this was going to be easy :)

    Good luck

    CC
     
  9. The Trician

    The Trician New Member

    I bet you thought this was going to be easy


    It's as hard or as easy as you want to make it.

    Hopefully common sense shall prevail and no joist shall be seriously weakened by drilling 5" holes in 4" joists!

    TT
     
  10. Cornish Crofter

    Cornish Crofter Active Member

    Just read some comments on Neutral Axis, when I started my last post there were NO replies to this question. Now there are 7!

    The Neutral Axis is the horizontal line that runs parallel to and in between the top (floor level) and the bottom level (where the downstairs ceiling is screwed onto). If your joist is 200mm in depth, then the Neutral Axis will lie 100mm above the ceiling and 100mm below the floor. It is alon this axis that few if any forces are exerted, hence this is the safest place to drill your holes.

    The 50mm I mentioned earlier apply to any part of the hole, not the centreline, so you must have 50mm min of solid uncut wood below the lowermost part of your hole and the same above the uppermost part of your hole. However, best to try and get the hole in the middle if you can.

    CC
     
  11. charliex

    charliex New Member

    seriously, all that info will fry your brain. just drill the middle of the joist!!!!!!!!!!!!!
     
  12. Cornish Crofter

    Cornish Crofter Active Member

    90mm depth, are you sure, that's not even 4 inches.

    That would not leave you any room to put the hole in.

    What is the span of these joists?

    CC
     
  13. Cornish Crofter

    Cornish Crofter Active Member

    Charlie, I hate to sound pedantic, but that info is straight out of the OSG! If he drills through the joist at the wrong point he could seriously weaken the floor.

    CC
     
  14. Kipster

    Kipster New Member

  15. l00pd0g

    l00pd0g New Member

    OMG, that made me cringe watching that. Somehow, I don't think that's going to take off over here. I hope not.

    Andy
     
  16. slapstyxx

    slapstyxx Member

    "90mm depth, are you sure, that's not even 4 inches."

    Guys, are we missing something here? 90mm x 35mm sounds like either (a) prestressed roof trusses, which would mean they're in the roofspace and shouldn't need drilling, or (b)the same dimensions as CLS timber, which suggests an outbuilding of some sort. Maybe DIY can give us more info about the proposed installation?

    Mark
     
  17. sparks

    sparks New Member

    "90mm depth, are you sure, that's not even 4
    inches."

    Guys, are we missing something here? 90mm x 35mm
    sounds like either (a) prestressed roof trusses,
    which would mean they're in the roofspace and
    shouldn't need drilling, or (b)the same dimensions as
    CLS timber, which suggests an outbuilding of some
    sort. Maybe DIY can give us more info about the
    proposed installation?

    Mark

    My thoughts exactly the dimensions given work out to more like 3 1/2" x 1 1/2" sounds more like ceiling joists or roof trusses, certainly doesn't sound like a load bearing floor, and as you say if its a roof space then it shouldn't need any holes drilled in it, need a bit more information I think.

    TC
     

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