Discuss Back Box In Hollow Wall Installation in the Electrical Forum area at ElectriciansForums.net

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Hi Guys, I've completed rewires with hollow walls before and was able to use a dry line Box. No chance of being able to use them this time.

I've never seen walls like this though. Just wondering how people have installed back boxes in this circumstance? I've attached a pic

Back Box In Hollow Wall Installation 20220801_165208 - EletriciansForums.net

Back Box In Hollow Wall Installation 20220801_170514 - EletriciansForums.net


I'm assuming Fire resistant Expanding foam or Plaster? Prob need the version which dries the quickest as ill need to start runnung cables asap.

Your helps appreciated

Andy
 

SparkySy

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They look like clay hollow tiles to me, similar to above but with nothing in the middle! There are some houses in Meriden built with those! Complete pain to work with. They don't have much integrity once you have smashed the middle out of them so use cement or something strong to bed your boxes to give the block some of it's strength back! I don't think they are meant to be chased out at all!
The last one like that I rewired we didn't sink the box any deeper than the Plaster and then the whole house was overboarded to make up the difference!
Sy
 
OP
M
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They look like clay hollow tiles to me, similar to above but with nothing in the middle! There are some houses in Meriden built with those! Complete pain to work with. They don't have much integrity once you have smashed the middle out of them so use cement or something strong to bed your boxes to give the block some of it's strength back! I don't think they are meant to be chased out at all!
The last one like that I rewired we didn't sink the box any deeper than the Plaster and then the whole house was overboarded to make up the difference!
Sy
Yeah that's exactly what it feels like clay tile, I think your spot on.
They look more like the sort of thing I was thinking of!
 

SparkySy

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They are rubbish, they were a very cheap building material though at the time!
Horrible job you may need to pre fill the void somewhat before you fit the box as trying to drill the remaining block will deteriorate it further and won't provide a fixing! Then once the box is in make good around the box and chase.
Sy
 

GBDamo

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What is preventing you using a dry lining box?
You'd have to diamond cut/grind each one precisely and hope you hit a void perfectly every time, nigh impossible.

As mentioned above, I'd cut them out as in the OP and then set the boxes in some bonding or drywall adhesive. If they're set well they shouldn't need any further fixings.

It's not going to be quick though, it's one of those jobs that if you try to rush it it'll take twice as long.

Needed to chase and set the backboxes then given them 24hr to go off before trying to pull cables through. I fear they'll just come lose otherwise and then you'll be spending twice as long trying to secure them.

Absolute nightmare stuff. We had one wall to deal with, tried batten and board but could get a fixing for the batten, ended up using drywall adhesive to gob the backboxes to the wall then dot and dab board up to the back boxes.

If there are more than a couple to deal with then I'd be revisiting the price with the customer.
 

Mike Johnson

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Easy enough to cut and take out any obstructions with a Multi tool, it will only be a dividing flange, as long as the hole is accurate in width the sliding side fixings will fit and hold well, even used dry lining box's in a ceiling, yes terracotta hollow block ceilings, pull cord for the lights in the shower and the fan isolator switch.

Don't chase for the conduit/cable feed from above and below through the hollow block, just knock through each top and bottom with a long rod and yes I have done this as its common practice in countries that use the hollow block wall construction.
 

Pretty Mouth

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Don't chase for the conduit/cable feed from above and below through the hollow block, just knock through each top and bottom with a long rod and yes I have done this as its common practice in countries that use the hollow block wall construction.
Can you describe this in more detail please Mike? I can't quite visualise how this works (the cells run horizontally), but it sounds like it would be usefull.

I come across this type of blockwork fairly regularly. It seems like it was a popular material round here sometime in the 60s, usually used for the inner leaf of cavity walls, but also internal walls too. It's a pain to work with, particularly when it has only a thin coat of plaster covering it and you have to chase it out.

The cells run horizontally, so
 
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All day everyday doing council kitchens, I use the double+single dual 35mm backboxes because their so wide you can get at least 2 screws into the protruding parts.

I have to fill all the holes and chases with bonding anyway, so the other option is run the cables, fill the cavity with bonding/hardwall and just push the BB in until it extrudes out like toothpaste.

If you're good you can drill vertically through all the voids and pop out the bottom, although watch out for it being one layer thick sometimes, the rear part being what's plastered for the room next-door. It's still preferable to solid red brick chasing
 

Mike Johnson

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Can you describe this in more detail please Mike? I can't quite visualise how this works (the cells run horizontally), but it sounds like it would be usefull.

I come across this type of blockwork fairly regularly. It seems like it was a popular material round here sometime in the 60s, usually used for the inner leaf of cavity walls, but also internal walls too. It's a pain to work with, particularly when it has only a thin coat of plaster covering it and you have to chase it out.

The cells run horizontally, so
I thought that was enough detail, but I use a piece of reinforcing rod and a club hammer, according to how far up or down you need to go, it may be necessary to drill in an intermediate rodding point, you only have to go through the top and bottom of the block, bit of a pain to do, but not as much work afterwards to make good.
 
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Plasterboard adhesive - doesn't shrink when it dries so it's perfect for this application. Whack it in there, sink your metal box into it so it's flush and let it dry. Make sure to grommet your entry hole before putting the box in or make sure the entry hole is completely free of adhesive before you walk away from it.
 

Welchyboy1

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Just done a Rewire exactly like this, literally finished it yesterday

knock the horizontal lips off the adjoining blocks flat to the back then
just cut 2x 4 inch piece of 1x1 wood batten(stuff that’s used in packaging for integrated ovens is best😀) and liberally no nails them to the flat back of the blocks(one above and below the horizontal lip of the block is best but it’s gonna depend on whereabouts you chase in) don’t try to glue them to the rough lip only the smooth flat bit!!)

leave it until the next morning then just fix your metal standard back boxes with 1in x8 screws

sorted and solid as a rock, if you use 35mm boxes it should work out near as damn it to flush of the wall

when I bond the chases afterwards I always make sure I squeeze loads in around the socket boxes to give it that bit of extra support but it’s fine honest!!

if you think your gonna cut a plasterboard box Into it your dreaming 🤣
 
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