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Discuss Hole in joist - question on span in the DIY Electrical Forum area at ElectrciansForums.co.uk.

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  1. Sloth
    Offline

    Sloth New EF Member

    Location:
    Dorset
    I need to make a hole in a floor joist - I know it needs to be within 0.25 to 0.4 x the span, and in the middle of the joist etc, but I can't find a clear answer on what the span is when there is a supporting wall in the middle. Is the span seen to be between the outer wall, and the supporting wall in the middle? Thus you have a span either side?
     
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Top Poster Of Month

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    the span is between 2 supporting walls, so in your case you have 2 spans.
     
    • Agree Agree x 3
  3. buzzlightyear
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    buzzlightyear still kicking sand, before pushing up daisies . Electrician's Arms

    Location:
    star command
    new build or old build?
     
  4. Sloth
    Offline

    Sloth New EF Member

    Location:
    Dorset
    Thanks guys.

    Old 60s ex-council house. There are holes in various places, both original and added-at-some-point in the past that are very much not in places they should be. I'd rather not add any others where they shouldn't.
     
  5. Midwest
    Online

    Midwest Electrician's Arms

    Location:
    Oxfordshire
    I wouldn't worry too much about it.
     
    • Agree Agree x 1
  6. willy fixit
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    willy fixit Regular EF Member

    Location:
    Exeter - a glorious place to live!
    Business Name:
    Mike Willmott
    Remember sizing of joists is not to do with overall strength but rigidity.
    you could halve the size of all the timbers in your house and it would still stay up. Just wobble a bit more... then fatigue and fall down a bit sooner than it would have if you'd left it as the designer intended.
    If your hole is for one cable (say 10mm dia.)and if its in the centre go the joist it'll make no difference at all. Or rather the affect will be negligible.
     
    • Agree Agree x 1
  7. anthonybragg
    Online

    anthonybragg Electrician's Arms

    Location:
    NORTHAMPTON
    Business Name:
    CHARLTEC ELECTRICAL LTD
    what are you trying to put through?
     
  8. ELECNEWT
    Offline

    ELECNEWT EF Member

    Location:
    Lancashire
    I'm sure you were only exaggerating to illustrate a point, but his house would fall down fairly quickly.

    If you halve the width of a beam you reduce its strength by a factor of two
    If you halve the depth of a beam you reduce its strength by a factor of eight
    If you halve both width and depth you reduce its strength by a factor of sixteen
     
    • Agree Agree x 1
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