Notching timbers instead of drilling....

[TJC1]

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?

 

[JBW175]

Notching is still acceptable, I’ve pulled this from the OSG for you.

 

[ELECNEWT]

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]

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!

 

[TJC1]

Notching is still acceptable, I’ve pulled this from the OSG for you.
View attachment 59828

I will break the chisel out!!

 

[Dustydazzler]

Out of curiosity why is making notches quicker than say making 2 x 25mm holes using a decent angle drill...?

 

[Wilko]

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.

 

[telectrix]

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.
[automerge]1596266761[/automerge]

Plumbers are exempt though!

yes but plumbers do actually test the strength of the joist when notching out with an axe.

 

[littlespark]

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

notching takes more time

 

[cliffed]

Notching sounds good, if joist are close together,nothing worst pulling cables through tight joists,especially several,watch the shaffing?

 

[Andy78]

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]

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

 

[richy3333]

I will break the chisel out!!

A hole hawg is a great tool if you don’t have one. Eats through the wood like butter. Get their bits also.

 

[telectrix]

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.

 

[Dustydazzler]

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