Back Box In Hollow Wall Installation

mackatoon · Aug 1, 2022

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

[ElectriciansForums.net] Back Box In Hollow Wall Installation

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

SparkyChick · Aug 1, 2022

It's looks like lath and plaster on the first take, but now I'm not sure... what is it actually made of?

snowhead · Aug 1, 2022

It looks like the hollow earthenware blocks as used in Europe.

Same sort of thing as this but not this exact one,

SparkySy · Aug 1, 2022

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

snowhead · Aug 1, 2022

Closer

SparkySy · Aug 1, 2022

They look more like the sort of thing I was thinking of!

mackatoon · Aug 1, 2022

SparkySy said:
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.

SparkySy said:
They look more like the sort of thing I was thinking of!

mackatoon · Aug 1, 2022

mackatoon said:
Yeah that's exactly what it feels like clay tile, I think your spot on.

SparkySy said:
They look more like the sort of thing I was thinking of!

I think every socket i need to chase in will need filling then afterwards. Jeez what a nightmare this job will be

SparkySy · Aug 1, 2022

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

telectrix · Aug 5, 2022

dot & dab adhesive. works a treat. or if hole is too big, epanding foam. goo the back box in before foam cures.

littlespark · Aug 5, 2022

I don’t know if anyones mentioned it…. But what about surface trunking and a pattress box?

It’s good enough for some council rewires…..

telectrix · Aug 5, 2022

but as electricians, we should be proud of our work. if it don't fit using normal methods, sort it properly. there are no problems, only solutions.

Mike Johnson · Aug 5, 2022

What is preventing you using a dry lining box?

GBDamo · Aug 5, 2022

Mike Johnson said:
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 · Aug 5, 2022

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.