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Good morning all

Is there any reg to say about drilling cables into joists instead of notching?

I have been an electrician for 12 years now and have always drilled out timbers, we are starting a HUGGGGGGGGGGE rewire with long runs. It would save a lot of time i think to notch the top of the timbers and use a protective metal plate to go over after the install

Thoughts?
 
The strength of a rectangular beam loaded in the vertical direction depends on the second moment of inertia I. Where I= B*D(cubed)/12 . Where B is the width of beam and D is the depth.
You need to know the exact dimensions but as an example: for a six-inch-deep beam with a one-inch- deep notch, the beam becomes effectively five inches deep. So, the original D(cubed) was 216 and the modified D(cubed) becomes 125. The strength is now 125/216 or 58% of the original.
For changes in stress levels, other factors such as distance from supports, are relevant.

For interest:
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.
 

FatAlan

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The strength of a rectangular beam loaded in the vertical direction depends on the second moment of inertia I. Where I= B*D(cubed)/12 . Where B is the width of beam and D is the depth.
You need to know the exact dimensions but as an example: for a six-inch-deep beam with a one-inch- deep notch, the beam becomes effectively five inches deep. So, the original D(cubed) was 216 and the modified D(cubed) becomes 125. The strength is now 125/216 or 58% of the original.
For changes in stress levels, other factors such as distance from supports, are relevant.

For interest:
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.
Plumbers are exempt though! ;)
 
Out of curiosity why is making notches quicker than say making 2 x 25mm holes using a decent angle drill...?
 

Wilko

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Personally I don’t notch beams. Over the years I’ve been unlucky enough to damage a few cables installed this method.

Someone has designed the beam to meet a load and as @ELECNEWT has mentioned notching will not help. Also, any top protection plate needs to be properly effective as any future damages or injury caused by penetrating the plate will be down to the installer, in my view. I haven’t seen a protection plate I trust so if I work on a circuit that has notching I re do it. This one was yesterday. Probably why I’m poor.

7C5E18AB-8CB9-4E7C-994A-81DC36140394.jpeg
 

telectrix

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if. like me, you've worked on a load of terraced houses, top of stairs you often find a 3ft square "landing" between fron and rear bedrooms. the floor joists are 4" x 2" and as such drilling centre of joist ,cables will be <50mm from both surfaces. here, the only compliant option is to notch and plate.
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Plumbers are exempt though! ;)
yes but plumbers do actually test the strength of the joist when notching out with an axe.
 
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littlespark

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A new sharp drill bit and a fully charged drill, you’ll be through in seconds.

notching takes more time
 

Andy78

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I never notch unless I have to. Personally my battery combi and self feed drill bits fly through joists in seconds. Notching would take much longer and wiring would require more daffy dressing with notches.

If I had a huuuuuge rewire to do I'd treat myself to something like the Milwaukee hole hawg 👍
 

stevethesparks

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Mark up the joists perfectly in line before drilling and the cables will glide through from one end to the other.
 

telectrix

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Mark up the joists perfectly in line before drilling and the cables will glide through from one end to the other.
except when the joists are too close together so you need to drill at a bit of an ange. in this case, my way is to drill first joist from 1 side, the next from the other side and so on alternate joist drilled left-right, right - left, etc. makes pulling a lot easier.
 
Been thinking about this , I’m thinking once a row of say 2 inch wide notches are cut into the joist , the cables can all be run and then simply laid into the notch. Then cover plates put over them.
If you had a lot of cables , laying them directly into a notch will be quicker than pulling in cables through holes one by one.

i still would opt for a row of 2 x 25mm holes
but that’s just me
 

davesparks

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Notching is allowed, but holes are preferable where possible.
I have notched joists in the past for running SWA or 16mm T&E because getting that stuff through holes ain't no fun sometimes.
 
Having notched and used holes... I reckon, if you're organised, it's far quicker with holes... would be fun to have a race !

A laser level, hole hawg and a combination square would be my weapons of choice.
 
Good morning all

Is there any reg to say about drilling cables into joists instead of notching?

I have been an electrician for 12 years now and have always drilled out timbers, we are starting a HUGGGGGGGGGGE rewire with long runs. It would save a lot of time i think to notch the top of the timbers and use a protective metal plate to go over after the install

Thoughts?
Notching over holes wins hands down. And laying the cables in rather than pulling through holes in joists is, nt just a great time saver, it's kinder on the cable Insulation. My only problem with it is I don't get to "notch" often enough. Ideally I try to first fix before the upstairs flooring goes down. In agreement with the builder I " notch" the joists where they are resting on a stud or wall. I never do it otherwise. To avoid waking joists
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Notching over holes wins hands down. And laying the cables in rather than pulling through holes in joists is, nt just a great time saver, it's kinder on the cable Insulation. My only problem with it is I don't get to "notch" often enough. Ideally I try to first fix before the upstairs flooring goes down. In agreement with the builder I " notch" the joists where they are resting on a stud or wall. I never do it otherwise. To avoid waking joists
Sorry "weakening"
 

Mike Johnson

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From a structural point of view drilling holes in the neutral zone of the joint is preferable i.e. in the centre of the joist between the tension and compression area, notching reduce's the compression area of the joist which is its major work zone.
 
From a structural point of view drilling holes in the neutral zone of the joint is preferable i.e. in the centre of the joist between the tension and compression area, notching reduce's the compression area of the joist which is its major work zone.
Does that still apply if you have notched over a stud or wall and by that I mean the notch is sitting middle of the wall?
 

Mike Johnson

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Yes, makes no difference especially if the stud or wall are a structural support member and you have reduced the end bearing of the joist.
 
The existing joists are probably already peppered with holes and notches form previous installation and plumbing, and not always in the zones indicated in the OSG, so god knows how one is ever to calculate what strength any joist has left!
The maths might work for a new joists but 100+ year old properties ....! especially where irregular (oak) beams have been used.
 

davesparks

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Does that still apply if you have notched over a stud or wall and by that I mean the notch is sitting middle of the wall?
Notching the joist where it sits on a wall is not permitted in building regulations as far as I know.
There are specific places where notches are permitted and limits to their size.

Holes are ideally placed at 1/3 the span of the joist, on the centreline and subject to size and spacing limits. From memory holes are permitted between 0.25 and 0.4 of the span in building regulations, must be spaced by at least 3x the diameter of the largest hole, can't remeber the limit of diameter but it is related to the depth of the joist.
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The existing joists are probably already peppered with holes and notches form previous installation and plumbing, and not always in the zones indicated in the OSG, so god knows how one is ever to calculate what strength any joist has left!
The maths might work for a new joists but 100+ year old properties ....! especially where irregular (oak) beams have been used.
Older properties had the joists oversized to allow for service holes to be cut out.
It's more modern builds where joist sizes can be calculated down to a minimum (cheapest) permitted size that the rules on hole locations become more important.
 

Mike Johnson

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@DefyG Sounds like you are just making an excuse to do whatever you want, rather than what is correct or safe, is that how you think of your electrical work.
 
Yes, makes no difference especially if the stud or wall are a structural support member and you have reduced the end bearing of the joist.
Then I bow to your greater expertise on this one. In practice we would actually drill a hole first, then slot. After laying in the cables, reinsert the cut out slot and leave a neat and tidy job behind.
 

davesparks

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This diagram shows the rules quite well

upload_2019-4-8_14-46-14.png
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Then I bow to your greater expertise on this one. In practice we would actually drill a hole first, then slot. After laying in the cables, reinsert the cut out slot and leave a neat and tidy job behind.
What's the point in putting the cutout bit back in?

Notches for cables should have metal plates fitted to prevent nails or screws penetrating.
 

Mike Johnson

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@ Edmond I hope I have misunderstood your above comment, drill a hole, cut down to it, lay in the cable and then put the bit back?
 
@DefyG Sounds like you are just making an excuse to do whatever you want, rather than what is correct or safe, is that how you think of your electrical work.
@DefyG Sounds like you are just making an excuse to do whatever you want, rather than what is correct or safe, is that how you think of your electrical work.
Mike I find your conclusion there a little odd. I, m not on here making excuses for "whatever I want". Firstly, notches are allowed. Secondly if you are going to notch, would, nt it be common sense to do it at a point where the joist has rock solid support? or do you think notching a joist where it is freestanding is a better idea?
 
@ Edmond I hope I have misunderstood your above comment, drill a hole, cut down to it, lay in the cable and then put the bit back?
No Mike. You did, nt understand a jot of what I said.
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No Mike. You did, nt understand a jot of what I said.
Apologies Mike. It's clearly past my bedtime
 

davesparks

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Mike I find your conclusion there a little odd. I, m not on here making excuses for "whatever I want". Firstly, notches are allowed. Secondly if you are going to notch, would, nt it be common sense to do it at a point where the joist has rock solid support? or do you think notching a joist where it is freestanding is a better idea?
No thinning the joist at the point where it is supported is not a good idea at all.

It is common sense to do it at the point prescribed in the building regulations which is where engineers have worked out it is safe to do so.
 
@DefyG Sounds like you are just making an excuse to do whatever you want, rather than what is correct or safe, is that how you think of your electrical work.
????? I was merely pointing out what joists of a lot of older properties are like after many years of work and re-work and that to try and calculate the strength of such timbers would be 'impossible'! .......
 
No thinning the joist at the point where it is supported is not a good idea at all.

It is common sense to do it at the point prescribed in the building regulations which is where engineers have worked out it is safe to do so.
"thinning the joist at the point where it is SUPPORTED is not a good idea at all". Classic
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@Edmond just follow the guide lines posted by davesparks above and you wont go wrong.
Thanks Mike.
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No thinning the joist at the point where it is supported is not a good idea at all.

It is common sense to do it at the point prescribed in the building regulations which is where engineers have worked out it is safe to do so.
No thinning the joist at the point where it is supported is not a good idea at all.

It is common sense to do it at the point prescribed in the building regulations which is where engineers have worked out it is safe to do so.
DAVE, you made a very valid point earlier about electricians using their intelligence. Whichever engineer worked out that it is safer to slot a joist where it an sag rather than where it can be supported, well.......
 
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Mike Johnson

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End bearings, maximum compression and no help from continuous fibres.
 

davesparks

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DAVE, you made a very valid point earlier about electricians using their intelligence. Whichever engineer worked out that it is safer to slot a joist where it an sag rather than where it can be supported, well.......
Yes, have you studied the stress diagrams for beams?
Have you considered what the effect on the strength of the joist will be by thinning it at the point where it is supported?

Have you ever noticed how splits in timber tend to start where someone has cut through the fibres of the grain?

I learned a lot of this stuff from my father who was highly qualified in structural and building work and a very well respected building control officer.

If you look at the actual stress patterns, areas of tension and compression etc in a joist you can remove much of the middle of the joist at 1/3 of the span leaving a relatively small amount of the top and the bottom of the joist with minimal effect on its strength, but a hole drilled through the centre at the end or middle of the span can have a much bigger affect on its strength.
 

Mike Johnson

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Just think of Castellated beams to get the idea, but note the shear webs near the bearing points.
 
Yes, have you studied the stress diagrams for beams?
Have you considered what the effect on the strength of the joist will be by thinning it at the point where it is supported?

Have you ever noticed how splits in timber tend to start where someone has cut through the fibres of the grain?

I learned a lot of this stuff from my father who was highly qualified in structural and building work and a very well respected building control officer.

If you look at the actual stress patterns, areas of tension and compression etc in a joist you can remove much of the middle of the joist at 1/3 of the span leaving a relatively small amount of the top and the bottom of the joist with minimal effect on its strength, but a hole drilled through the centre at the end or middle of the span can have a much bigger affect on its strength.
That is a persuasive post and one I will not ignore
 
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