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Drilling joists for cables/pipes

  • 07-05-2020 6:58pm
    #1
    rob w
    Registered Users, Registered Users 2 Posts: 1,164 ✭✭✭


    A query on building regs, Part A re the drilling and notching of joists.

    For holes, it states that they should be between 0.25 and 0.4 times the length of the span between supports.

    For a joist that sits on more than 2 walls, i.e.
    A joist that is spanning across 3 block walls below, 2 at its extreme ends and 1 somewhere along the middle - does the 0.25 and 0.4 apply to each section between the supports or the whole joist length in total?

    Probably over thinking it, but just want to check and be sure!

    Thanks


Comments

  • #2
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,146 Mod ✭✭✭✭BryanF


    Are the 3 walls supporting the joist? So it could be two joists resting in central wall? ..


  • #3
    rob w
    Registered Users, Registered Users 2 Posts: 1,164 ✭✭✭rob w


    BryanF wrote: »
    Are the 3 walls supporting the joist? So it could be two joists resting in central wall? ..

    Maybe a sketch will explain better?

    Can joist B be drilled in the same spot as joist A when resting on the block walls (hatched) below?
    Or does joist B need to be treated as two separate spans between the 3 walls?

    The central wall of the house has two joists resting on it spanning to the external walls.

    The source I have read (from SafeElectric) only shows a joist supported at either end - https://www.thermodial.ie/wp-content/uploads/2016/10/Holes-and-Notches-in-Joists-RECI.pdf


  • #4
    Metric Tensor
    Registered Users, Registered Users 2 Posts: 3,725 ✭✭✭Metric Tensor


    The hole and notch locations are selected based on the assumed bending moment and stresses in the joists. I won't bore you with the details but the regions shown on your drawings are selected at certain distances from the support points.

    If the joist is genuinely supported in the middle also then you should by rights stay clear of that support point also. So you need to check if the "Joist B" in your drawing is actually getting support from that middle wall or if it's actually spanning from "outer" wall to "outer" wall.

    Fair play for checking - you wouldn't believe how many joists I see that have been hacked to confetti by so-called professionals!


  • #5
    rob w
    Registered Users, Registered Users 2 Posts: 1,164 ✭✭✭rob w


    Metric Tensor wrote: »
    The hole and notch locations are selected based on the assumed bending moment and stresses in the joists. I won't bore you with the details but the regions shown on your drawings are selected at certain distances from the support points.

    If the joist is genuinely supported in the middle also then you should by rights stay clear of that support point also. So you need to check if the "Joist B" in your drawing is actually getting support from that middle wall or if it's actually spanning from "outer" wall to "outer" wall.

    Fair play for checking - you wouldn't believe how many joists I see that have been hacked to confetti by so-called professionals!

    Thanks for that, Joist B is definitely spanning to the outer wall.

    There are other joists which are the same length and size but span all the way to the outer wall with nothing else in between. So that makes me think that 'Joist B' just happens to be passing over that wall (of a room below) rather than relying on it for support.


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