In July we moved into our bungalow. Built in the nineties it has concrete block internal walls, so far so good, fixing anything including heavy items to them should not be a problem. However, I have had to reposition a socket about to be obstructed by a built in wardrobe. Only when knocking out to sink a 35mm deep back box I found the blocks were those with the vertical hollows and in no time at all the block was crumbling due to the hollows not leaving much to fix the back box to.
In the past at our first house when we had a similar problem I just chiselled out the thin bits left and using suitably sized wood blocks stuck in with the likes of “Sticks like Sh*t” provided a suitable back to fix a back box to. Today I used a thick mix of Tourpets Masonry Repair filler sqidged into the hole and squashed the back box in and it stuck in position.
The only issue with both solutions is having to wait for the gunging material to set before anything else could be done, so I would be interested in anyone else’s solution particularly ones that would allow further work to proceed. Before you say it TNP, carbody filler was my next go to.
Richard