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I'm trying to hang a ceiling lamp from what appears to be a reinforced concrete ceiling. My anchors require a 40 mm deep hole, but I don't succeed in drilling more than about 35 mm deep with my hammer drill, and judging from unused 35 mm deep existing holes in another room, nor did a previous resident. I thought that meant I am hitting a stud/beam (which is why I think it's probably reinforced concrete), so I would need a stud finder, but those appear to be for use in buildings using wood or drywall (see also this question); my walls and ceiling all appear to be solid concrete (with a maybe 15 mm plaster layer). The local Baumarkt rents metal detectors, which are advertised to avoid cables, wires, and pipes. Is this also the right tool to find places in my ceiling where I can drill in-between the studs of reinforced concrete, or do I need a different tool for this?

(I could also buy shorter anchors and screws, but I would still need to avoid cables)

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Do not cut the rebar in the concrete ceiling slab.

Rebar is placed in concrete slabs (and beams) for tension. Cutting the rebar eliminates the resistance to tensile stress and could cause immediate failure (collapse of the slab).

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Buy (or hire) a rebar cutter drill bit.

When rebar is encountered during hammer drilling with a normal SDS plus bit, immediately stop the rotary hammer and remove the bit from the hole.

Mount a rotary rebar cutter in the SDS plus rotary hammer with the hammer action switched OFF. Insert the rebar cutter into the hole and start it rotating above the steel. Slowly lower on to the rebar, exerting steady pressure to make sure the cutter is actually cutting and not just spinning on the top of the metal.

Source: https://www.orbitalfasteners.co.uk/en/products/relton-usa-brand-rebar-cutter-10mm-x-300-sds-plus-shank-hammer-action-turned-off-

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    Sounds good, except I think "above" and "below" are backwards when working on a ceiling.
    – manassehkatz-Moving 2 Codidact
    Commented May 5, 2019 at 21:03
  • I may have already damaged my regular bit in my attempts :(
    – gerrit
    Commented May 6, 2019 at 7:12
  • Why does the one in your link cost £86 for one but this one €6 for 5? Is what I found in the link the wrong product or just less quality? I'm in Germany and the linked product is for a "HSS Metallbohrer" which means "High Speed Steel metal bit". Will a "normal" metal drill bit not drill through rebar?
    – gerrit
    Commented May 6, 2019 at 7:51
  • I would rather prefer to drill again one or two cm away, that should avoid the rebar, but I need a detector to make sure and not turn the ceiling into Swiss cheese. I think metal detectors should detect both, unless wire detectors work by detecting electrical currents.
    – gerrit
    Commented May 6, 2019 at 8:13
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    Rebar is tough. I don't think €6 of HSS drill bit would go through it, they look intended for thin sheet steel.
    – Owain
    Commented May 6, 2019 at 16:24

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