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I'm trying to work out the construction of a wall that partitions two flats (maisonette). Most things on the internet deal with internal walls (with wood studs, found with a stud finder), or solid external walls. However this is kind of both, as it's internal, but partitions two separate residences.

The odd thing is that the stud finder (a powerful magnet in this case) finds nothing. But does find studs easily on the adjacent internal wall. Using the tapping technique I can find parts that sound solid, but not in vertical runs. As I tap across, I can find a solid sound, but then as I tap upwards the sound changes to hollow, and then solid again, alternating. I've drilled into the "solid" part, and couldn't discern wood, and appears to be just powder coming out. Unfortunately there's no wall sockets to inspect either. Construction was in 2013 in the UK (if that helps).

Anyone got an idea what's behind this wall? I'm trying to put up a heavy load bracket. Any help greatly appreciated!

Edit: I'm starting to think it may be a "Dot and Dab" fixing of plasterboard over a solid wall. This would explain what I'm seeing/hearing.

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    Based on 1) not finding anything with a stud finder, 2) the alternating solid and hollow sounds you're hearing as you move vertically, and 3) the image in your link, I'd guess that there's a cinder block wall in between the flats. That's commonly done in the US as a code requirement to act as a fire stop between the two residences.
    – FreeMan
    Oct 15, 2019 at 19:32
  • You probably wish to check with building management (or a suitably qualified engineer if building management is stumped) as to a way of fixing to this wall that does not compromise its integrity as a fire and smoke barrier, by the way
    – ThreePhaseEel
    Oct 16, 2019 at 2:03
  • I did not think of issues of fire/smoke safety.. how deep would I need to go before this is a serious concern? Would an inch or so still be considered safe?
    – Coran
    Oct 16, 2019 at 11:51
  • *By inch I mean an inch into the solid back of the block
    – Coran
    Oct 16, 2019 at 11:58

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Concrete block wall and dot and dab plasterboard would be pretty standard between two dwellings because of fire regs.

Grab some post-it notes and stick them on the wall where it sounds solid. See if you discern a pattern, i.e rows of 'dots' (or 'dabs', I never did figure out the difference!). It will (or should be) solid along the bottom and the top (a continuous dot or dab).

Make sure your bracket sits on solid areas or you'll crush the plasterboard into the 10mm (or so) gap behind the boards.

If you have to go where it's hollow, drill an oversized hole and pump/push plasterboard adhesive into each hole forcing it in to spread it out behind the board as much as you can. We push the plastic disk out of an empty caulk tube with a long dowel etc., fill it up with board adhesive and pop a fresh nozzle on and you're good to go. Pump away!

Wait until it's dried for a day or three and then drill your holes into the concrete block.

Make sure your fastener is wholly holding in the block, i.e. not the plasterboard or its adhesive.

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  • Thanks for the detailed response! This is very useful.
    – Coran
    Oct 16, 2019 at 11:52

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