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Topic Title: Metal Web Joists
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Created On: 22 April 2013 05:53 PM
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 22 April 2013 05:53 PM
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synapse

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I have only just encountered these only recently, in my observation of some new build houses. I noticed that no particular cable management had been employed by the first fix electricians. The cables were losely tie wrapped in bundles and lay on top of the lower batten.

google metal web joist if you are not familiar..

The bottom batten is 47mm deep which means that any cable resting on it will not be 50mm away from the bottom edge as required.

I thought this new technology is bound to have been properly considered and thought guidance from the NICEIC would be helpful. The NICEIC Engineer had not even heard of metal web joists but agreed with me that just resting the cabless on the bottom batten would strictly not be compliant. I was not offered any guidance as the NICEIC had not even considered the issue. Technology may move forward hopefully the NICEIC will catch up.

Views and Opinions please............. (on metal web joists)
 22 April 2013 06:41 PM
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daveparry1

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So what's wrong with the cables laying on the bottom battens then? no different to cables laying on top of a plasterboard ceiling surely, also if the bottom battens are 47mm by the time there's a layer of plasterboard beneath, the cables will be more than 50mm above the surface. Apologies if I've got this wrong but i've never seen these metal joists,

Dave.
 22 April 2013 06:57 PM
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rocknroll

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They are called by a number of names that include Easi-Joists, Posi-Joists etc; for your information regarding depths, before the plasterboard is fitted a 16mm resilient bar is fitted at right angles to the joists and between the ground and first floor ceiling there should be 2 x 19mm or 2 x 12.5mm plasterboard sheets and on the first floor below the roof space 1 x 19mm plasterboard sheet, but generally only applicable to intermediate floors, although this method has been used if the loft is to be used and a floor laid, hope this helps.

Just a quick note if the ceiling is CT 1 then as above but if CT 2 then substitute for 2 x 15mm sheets.

On the whole plenty of depth for your cables either way.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 22 April 2013 at 11:30 PM by rocknroll
 22 April 2013 07:08 PM
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synapse

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Dave,

Good point - if you include 9 or 12 mm of plaster board then the 50mm is met. However the regs do not include the plasterboard.

Sorry if I'm being picky but given web joists are likely to be standard on the majority of new build houses I feel some thought to cable management is needed. Its feels overlooked to me ......

I feel the risk of mechanical damage is increased by not placing cables in the middle of the joist space.....
 22 April 2013 07:19 PM
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Smith249

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50mm depth for cables only relevant on walls.
 22 April 2013 07:30 PM
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daveparry1

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No, that's only for non use of rcd protection Smith, there's no minimum regulation cable depth for walls,

Dave.
 22 April 2013 07:53 PM
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Smith249

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Yeap, I know Dave, what I was saying is that if it was relevant, it would only be relevant to walls, not ceilings.
 22 April 2013 09:08 PM
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prophet

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Guess you could wire it in MIMS/metal conduit/swa,
or add 3mm of wood at each joist,
or decide that 3mm isn't enough to loose sleep over.

Out of interest, how did the installers run the cables when they were going parallel with the joists?
 22 April 2013 10:56 PM
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rocknroll

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You do realise that the metal webs have rounded edges and generally a hole in the centre so you could hang cables from them, the rounded edge gives the webs strength but the added advantage is it will not damage pvc pipes, cables etc.

There is nothing wrong with the way the cables are laid in the OP's post but there is generally plenty of places you can secure them if you wish besides the way I suggested above, the 3 x 2 or 4 x 1 strongbacks that travel the length of the floor or rim boards or even ring beam at the ends.

An important issue to note is that under no circumstances must the metal webs be cut, drilled, modified or removed and the timber chords must not be cut, notched or drilled, if so the unit will need to be replaced and as most are now a complete floor or cassette as they are called and delivered on-site it could be expensive for the perpetrator or their insurance.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 22 April 2013 10:59 PM
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Martynduerden

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Originally posted by: rocknroll

They are called by a number of names that include Easi-Joists, Posi-Joists etc; for your information regarding depths, before the plasterboard is fitted a 16mm resilient bar is fitted at right angles to the joists and between the ground and first floor ceiling there should be 2 x 19mm plasterboard sheets and on the first floor below the roof space 1 x 19mm plasterboard sheet, hope this helps.

Just a quick note if the ceiling is Cat 1 then as above but if Cat 2 then substitute for 2 x 15mm sheets.

On the whole plenty of depth for your cables either way.

regards


Is this robust details for separating floors? I have come across loads with 2 sheets of 12.5 fireline directly fixed to ceiling both pozi and I beams (sterling board/mdf web).

Obv less of an issue on the latter.

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Martyn.

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www.electrical contractors uk.com
 22 April 2013 11:45 PM
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DOUGIE1000

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Originally posted by: synapse
I thought this new technology is bound to have been properly considered and thought guidance from the NICEIC would be helpful.


We have a local housing estate quite local to me, all metal webbed joists, houses are from 70's.

I never seen any other way other than cables fed through the joist same for plumbers pipes.

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 22 April 2013 11:51 PM
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DOUGIE1000

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http://s207.photobucket.com/us...0100722-1655.jpg.html

http://s207.photobucket.com/us...0100722-1654.jpg.html

i really need to stop taking pictures of almost every job i go to and get a hobby

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 23 April 2013 08:10 AM
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dickllewellyn

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Originally posted by: DOUGIE1000

http://s207.photobucket.com/us...2-1655.jpg.html



http://s207.photobucket.com/us...2-1654.jpg.html




i really need to stop taking pictures of almost every job i go to and get a hobby


I think there's room in anyone's life for both dougie. Perhaps the photographs could form part of a hobby? I know there is the aerial photographers association, so maybe start with photos of aerials and try to encourage cables too?!!

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 23 April 2013 08:25 AM
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synapse

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Prophet,

I'm not sure it's about loosing sleep. I'm not even sure it's about about meeting the minimum requirements.

A new build house should imho be an exemplar of electrical installation.

In the build I mentioned cables running parallel to the joists were laid loose with a few tie wraps on the metal webs and would have ended up sitting on the ceiling.
 23 April 2013 09:48 AM
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perspicacious

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Where in 522.6.100 (i) does the 50 mm include the additional layer(s) on the bottom of the joist?

Regards

BOD
 23 April 2013 10:31 AM
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synapse

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As previously stated - it does'nt
 23 April 2013 11:25 AM
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BigRed

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ok, look at the joist. it this was a rafter in a ceiling, what woulds you do? think as the bottom as a rafter, would you worry about the cable going over the rafter in an un occupied loft space? at worst, drop a piece of 2 x 2 into the bottom of the loops to clip the cable to. 50mm guarenteed, if someone screws in from below, it lifts the wood first. failing that, rest a small plank on the angle of the preform and clip to that.
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