Ways of routing twin and earth across joists......

[3george]

I'm trying to route some 1mm twin and earth cable across about 5ft of the the ceiling of a ground floor utility room. I have no access from above so my only option is to go across the ceiling. My goal is to hide the cable as much as possible and therefore I don't wish to surface mount or use mini-trunking etc.

The cable needs to run against the direction of the joists and three are in the way. The joists are wood and about 2 inches wide with about 14 inches in between each one. I wondered if anybody has any good ideas as to the least disruptive way of routing the cable. I had thought of using a cutter to make an access hole mid-way between each joist and then using a long reach wood bit such as this : -

[url]http://www.screwfix.com/prods...Bits/Bosch-Self-Cut-Flat-Wood-Bits-22-x-400mm

to make a hole in each joist through which I could route the cable, with the help of bits of wire / string etc.

I'd only be left to make good the holes in the ceiling then the job would be finished......anyone have any other ways they have tried....

Many thanks - 3george...

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[GaryMo]

If we need to install from downstairs then we would cut a strip of plasterboard out of the ceiling against the run of joists and use an angle drill with a stubby auger bit bearing in mind drilling zones for joists.
The strip can then be screwed back into place or even better a new strip of plaster board and skim over the damage.

Holes in joists should be 50mm from the top or bottom. By drilling with a long reach flat bit you can't guarantee that.
Holes should be drilled within a zone which is between 0.25 and 0.4 of the span.
Holes in the same joist should be 3 diameters apart and maximum diameter of each hole should be 0.25 x joist depth.

 

[3george]

Thanks for the advice Gary....could I use a small notch and a joist protection plate ?

 

[SparkyNinja]

Thanks for the advice Gary....could I use a small notch and a joist protection plate ?

I can't think of any reason why not, other than that it cant be easily recognized. But if it provides mechanical protection, then why not?
I'm gonna look this up later, I should imagine that if it conflicted with anything then it would be part P.
If anyone comes up with an exact reason do tell as I would have learned something.

I myself use Gary's method, although I call it 'keyhole surgery'

 

[ban-all-sheds]

Not sure you're allowed to notch joists at the bottom. The bottom edge is in tension, which wood isn't as good at as compression, and you could get cracks developing from a notch....

 

[GaryMo]

Agree with BAS.
The Part P for Building Regulations book only mentions notches at the top of a joist.

 

[ban-all-sheds]

It's intuitive if you think of it this way:

You've been asked to break a piece of wood by putting it over your knee and pulling on each end.

You're told you may weaken it by cutting a notch.

Where would you put your knee - on the edge with the notch, or the opposite edge?

 

[GaryMo]

Opposite for fear of the notch leaving a noticeable indent on my knee!

 

[seasickstevie]

Can't understand the logic of not being able to go less than 0.25 x length from end of span.

Does anyone else understand the theory behind this rule?

I mean, if you're trying to break BAS's piece of wood over your knee, would you put your knee less than 1/4 of the way from the end?

 

[ban-all-sheds]

It's not less than, it's more than. Can't go more than 0.25 x span away from the end for a notch.

As you said, if you were trying to weaken a piece of wood, you'd want the notch in the middle...

 

[GaryMo]

BAS, I assume he's referring to the zones of which you can drill within a joist.

For the benefit of others:

You can only drill between 0.25 and 0.4 x the length of span. The area's you can't drill are either end 25% and the middle 20%.

For example, a 3M joist. Drill between 0.75M and 1.2M (from either end) but don't drill within the first 0.75M and past 1.2M.

BAS is quite correct in that notching areas are different. You can only notch between 0.07 and 0.25 x the length of span. Maximum depth of 0.125 x the joist depth.

 

[big-all]

can you not go with the joists and around the wall in coving or in the safe zone!!!

i think going accross the joist will leave a 25mm gap in the plasterboard that you then need to fill in my humble opinion the edges [joints] will keep on showing because you have in effect 2 unsuported joints side by side that will flex with movement on the floor above

 

[seasickstevie]

...don't drill within the first 0.75M...

Like I say - what's the logic?

I mean, a joist ain't gonna snap in the first 25% is it? Just don't get it.

 

[stargazer]

...don't drill within the first 0.75M...

Like I say - what's the logic?

I mean, a joist ain't gonna snap in the first 25% is it? Just don't get it.

Those are the regulations, somebody has done the calculations.
Last weekend I drilled a 13mm hole in the centre of a 7inch joist 0.3 meters from the edge of the wall in order to run some 2.5mm twin and earth for a power socket in our bedroom.
Now, do you think I am going to spend the rest of my life wondering if our bedroom is going to collapse into the dining room below?

 

[big-all]

a joist in general is under compression[squeeze] on the top under tension[stretch]underneath with a neutral accsess along the centre

in general the safest place to remove material is along the neutral accsess

now when you get towards the centre of the span the neutral accsess comes under compression in its self as the underside and top are trying to squash together also at the end of the span the neutral acsess is also under load as the end of the joist is transferring the load of the whole structure to the wall and needs a few inches before the load in the neutral acsess reducess to a safe level

clear as mud ehhh