Backbox Question

[conoral11]

Hello.

I'm wanting to replace a single socket with a double socket, a fairly simply process and one I've done a few times on solid walls.

After turning the ring main off, and measuring it is as such, I removed the face plate.

Disconnected the wires and proceded to remove the metal backbox... which simply fell out. No screws into the wall.

After taking it out and clearing the rubble here is what I've got (I've reattached the faceplate for the time being)

The original 25mm metalbox didn't appear to actually be held in by anything substanial.

Behind the socket is polystyrene sheeting, which I am assuming is some sort of insulation (This is an exterior wall)

So my questions are: Is a metal backbox still the right thing to use, and do I need to 'fill in' the hole somehow?

Thank you in advance.

Conoral11

 

[sparkwright]

Yes, a metal flush box is still the right thing to do.

Assuming you can't fix into whatever is behind the polystyrene, all you can really do is cement the box in, I would think, to the surrounding masonry.

 

[conoral11]

Sparkwright,

Thanks for the reply!

Is it worth me digging out some of the polystyrene to see if there is something solid to screw into? Or just go straight for the cement?

Best Wishes

 

[Iggifer]

Bit of gripfil will do the job, just leave yourself enough depth to get some filler over it around the edges of your box, you won't paint gripfil

 

[Iggifer]

Sparkwright,

Thanks for the reply!

Is it worth me digging out some of the polystyrene to see if there is something solid to screw into? Or just go straight for the cement?

Best Wishes

There will be something there, but you want to avoid piercing the insulation if you can. It could be a few inches deep so you'd need to use a monster screw

 

[conoral11]

Sparkwright,

Thanks for the reply!

Is it worth me digging out some of the polystyrene to see if there is something solid to screw into? Or just go straight for the cement?

Best Wishes

There will be something there, but you want to avoid piercing the insulation if you can. It could be a few inches deep so you'd need to use a monster screw

Thanks Iggifer,

Looks like I'll be back to screwfix for some gripfil.

 

[JohnW2]

Bit of gripfil will do the job...

I think the OP may have to be careful "which" Gripfill he gets. IIRC, the solvent-based ones (and sometimes even just the vapours therefrom) can turn expanded polystyrene into a gooey mess!

Kind Regards, John

 

[stillp]

I'm confused. Is this wall a 25 mm+ cement render on top of insulation? Could it be that the installer of the back box had cut the recess too deep (perhaps right through the masonry) and used a piece of polystyrene to block the hole he'd created?

 

[JohnW2]

I'm confused. Is this wall a 25 mm+ cement render on top of insulation? Could it be that the installer of the back box had cut the recess too deep (perhaps right through the masonry) and used a piece of polystyrene to block the hole he'd created?

I'm not sure that there is necessarily any cement render - isn't (or couldn't it be) what we see at the bottom of the hole the polystyrene?

Could it be that the installer of the back box had cut the recess too deep (perhaps right through the masonry) and used a piece of polystyrene to block the hole he'd created?

I would say that's a distinct possibility! Dare I admit that ....

Kind Regards, John

 

[stillp]

Dare I admit that ....

Surely not! I'm shocked...

Don't ask me why I thought of that...

 

[JohnW2]

Dare I admit that ....

Surely not! I'm shocked... Don't ask me why I thought of that...

Quite so

Kind Regards, John

 

[conoral11]

Theres a solid layer of rock behind the sand coloured surface, before the polystyrene. Taking the surface back abit further and it remains the same. House was built in 1988 if that helps at all.

 

[stillp]

a solid layer of rock

D'you mean an inch of cement?

 

[JohnW2]

Theres a solid layer of rock behind the sand coloured surface, before the polystyrene. Taking the surface back abit further and it remains the same. House was built in 1988 if that helps at all.

Interesting. Is the polystyrene perhaps in the cavity of a cavity wall (albeit it probably shouldn't have been polystyrene in 1988), the 'rock' replacing the rest of the inner leaf of the wall which 'accidentally' got chopped out?

Kind Regards, John

 

[stillp]

Could you vacuum the dust out of that hole and post another picture please?