Backbox Fitting Issue?

[Mr Sparkle]

What's the best remedy when you have

1. Thin, crumbly breeze block which not allow you to get a plugs/screws in order to secure the backbox. However, the sides, upper and lower part of the breeze block still appears to be intact.


2. Concrete block (Party wall) which is hollow in half of the area the back box should be fitted. See attached image.

FYI have also turned off the power at the consumer unit and I have used wago connectors for safety

 

[ericmark]

Cheat, tape the back box in place, and use expanding foam to fill space and fix it, let it dry and trim to size. Of course that's not correct method, but much depends on time scale, using plaster to fill hole is all well and good but it takes time to dry, screw a block of wood in to fill gap and no weighting to dry, even the foam takes time to dry. Likely I would use both a lump of wood and plaster, so wood hold it in position while plaster dries.

 

[JohnD]

Crumbly?

Drill a hole big enough for your plug and screw, fill the hole with no-more-nails type of builders adhesive, press in the plug. The pressure will force it into any cracks. Leave overnight to set then screw.

Btw I prefer setting backboxes in sand and cement mortar. It fills out cavities well and adheres to brick and block (moisten the hole first to prevent fast drying and shrinkage) and has a long working time. It is much stronger than plaster.

I don't think foam is suitably fire resistant.

 

[conny]

I've used the sand and cement method over the years. Pack the hole out first then fill up with sand & cement.
Set your back box into the cement to the right depth. Mark your fixing holes and push the rawplugs into the cement. Allow cement to dry for a day or two then secure backbox by screwing into the rawlplugs. I can't recall one ever coming loose.

 

[ericmark]

I see drill a hole, fill it with asbestos you have spit on and rolled in your fingers, make a hole in it with the tool provided then screw in to position, was easy back in the day, but can't buy it any more.

But all you need is some thing to temporary hold it, and then fill with thistle, plaster, polyfiller or what ever. Once the plaster has set it's not going anywhere. Son used foam, I used thistle, I would get foam everywhere hated the stuff, but it did set fast. My thistle is OK if you can return next day to finish.

 

[chivers67]

Use Thistle Bonding. Put enough for the boxes it be pushed into place make sure it's level then fill it round the sides

 

[crystal ball]

Use Thistle Bonding. Put enough for the boxes it be pushed into place make sure it's level then fill it round the sides

Yes bonding and I used to push knockouts in or out slightly to give a key

 

[securespark]

Use Thistle Bonding. Put enough for the boxes it be pushed into place make sure it's level then fill it round the sides

This is how I have always done it.