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Can you drill into the floor of a prefabricated block of flats?

ron777888 15321 21
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Can I drill 10–12 cm into the floor of a prefabricated apartment block without hitting electrical cables?

In a prefabricated block, cables can indeed run in the floor, especially for sockets mounted there, so you should not assume the floor is completely free of electrical installation [#19298883][#19298735] The usual layout mentioned is a loop from the fuse box with cables routed in diagonal zigzags between sockets, so the risky areas are near sockets and possibly near the wall edges [#19298883][#19299894] Several replies say that in a typical large-panel building the floor slab is usually much thicker than 10–12 cm, often around 24 cm for Żerański slabs, with an added screed on top, so that depth is often still within the slab and not through to the neighbor below [#19299774][#19298735] Even so, the construction type varies a lot, and one reply advises checking with the housing cooperative/administration before drilling because surprises in old prefabricated buildings are common [#19298735][#19300625]
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  • #1 19298354
    ron777888
    Level 2  
    Hi,
    The question is probably terribly layman's, for which I apologize in advance, but I need to know - is there any electrical installation in the floors as standard in prefabricated blocks?
    I'm asking because I have to drill 2 large holes in the floor (about 10-12 cm deep - anchoring the wardrobe) and I would not like to drill any cables.
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  • #2 19298387
    78db78
    Level 43  
    And why do you need a 12 cm hole under the wardrobe, which almost stands alone, sometimes bunglers ran cables on the floor that power the neighbor's chandelier downstairs.
  • #3 19298652
    palmus
    Level 34  
    Drill and that's it. There's a slim chance you'll step on a mine.
  • #4 19298656
    kokapetyl
    Level 43  
    palmus wrote:

    Drill and that's it. There's a slim chance you'll step on a mine.

    I don't want to be smart, but it's easier to send someone else on a mine than to step on one yourself. :D
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  • #5 19298665
    ron777888
    Level 2  
    It's a gun locker.
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    #6 19298735
    retrofood
    VIP Meritorious for electroda.pl  
    78db78 wrote:
    And why do you need a 12 cm hole under the wardrobe, which almost stands alone, sometimes bunglers would run cables on the floor that power the neighbor's chandelier downstairs.
    Not bunglers, really. There used to be other regulations (which didn't necessarily govern all such details) and around 50 years ago, this was the standard.

    Added after 3 [minutes]:

    ron777888 wrote:
    It's a gun locker.
    And are you sure you can put such a mass on the ceiling? Because there are surprises. My neighbor removed a brick pillar in the kitchen because it interfered with arranging furniture and was only happy until the failure in the apartment upstairs, because it turned out that the cause was the deflection of the ceiling, which lacked support. Different things are happening.

    The question is also whether by forging 12 cm, you will not look into the neighbor from below. Do you know the structure of the ceiling?
    I would be afraid to do such work without the consent of the administration.
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  • #7 19298847
    ta_tar
    Level 41  
    ron777888 wrote:
    I need to drill 2 large holes in the floor (about 10-12 cm deep - anchoring the wardrobe)
    Large holes means how many fi? I also drilled three holes fi 12 to a depth of about 15 centimeters and did not even hit the second plate, i.e. the ceiling plate of the neighbor from the bottom (I certainly did not drill into it).
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    #8 19298883
    mczapski
    Level 40  
    Yes, as a fellow moderator wrote. The principle of powering sockets by running cables in the floor was standard. And certainly where the sockets are attached to the floor. The cable runs in a loop (beginning and ending at the fuses) and in diagonal zigzags between the sockets.
  • #9 19298962
    palmus
    Level 34  
    kokapetyl wrote:
    palmus wrote:

    Drill and that's it. There is little chance that you will step on a mine.

    I don't want to be smart, but it's easier to send someone else on a mine than to step on one yourself. :D

    It is disproportionately more likely to hit wires, pipes, etc. in the wall than in the floor.
    I assume that the ceiling there is thicker than 12 cm anyway.
    Do gun laws say 10-12 cm?
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  • #10 19299228
    ron777888
    Level 2  
    retrofood wrote:
    The question is also whether by forging 12 cm, you will not look into the neighbor from below. Do you know the structure of the ceiling?
    I would be afraid to do such work without the consent of the administration.

    I just don't know completely. You're right - first I'll talk about it with someone from the cooperative and then I'll start assembling. I didn't think it could be that problematic, good thing I asked :)

    ta_tar wrote:
    Large holes means how many fi? I also drilled three holes fi 12 to a depth of about 15 centimeters and did not even hit the second plate, i.e. the ceiling plate of the neighbor from the bottom (I certainly did not drill into it).

    Fi will be 10 or 12 most likely. It's hard to say how thick these slabs are for me, probably thick, it's a 15-storey block.

    palmus wrote:
    Do gun laws say 10-12 cm?

    Absolutely not, to be honest you don't need to anchor at all as far as I know, but I prefer to be prepared for hardworking and determined burglars ;)

    Thanks for all the replies!
  • #11 19299327
    palmus
    Level 34  
    If the building has 15 floors, the ceilings are poured or assembled from Żerański slabs. The latter will be about 24cn.
    I don't think that poured ceilings are less than 15 cm.
  • #12 19299347
    ta_tar
    Level 41  
    As far as I remember the construction from the early 1980s, my block of flats has two ceiling slabs. But there was a mass of types of this construction and each construction depended on the height of the building and the period of construction.
  • #13 19299454
    palmus
    Level 34  
    If you make 5 screws fi 12 to a depth of 50-80mm in the ceiling, in solid concrete it will be impossible to pull out.
  • #14 19299674
    retrofood
    VIP Meritorious for electroda.pl  
    palmus wrote:

    I don't think that poured ceilings are less than 15 cm.

    Agreed, but the remaining few cm can chip, that's what I meant.
  • #15 19299774
    Łukasz-O
    Admin of electroenergetics  
    palmus wrote:
    If the building has 15 floors, the ceilings are poured or assembled from Żerański slabs. The latter will be about 24cn.
    I don't think that poured ceilings are less than 15 cm.


    To be honest, I have not come across cast ceilings in a typical large slab. The large slab is a prefabricated structure.
    They used floor slabs, which, as you write, were about 24 cm: link
    And there is also a spout, which is at least 5 cm.

    I wouldn't worry about drilling in the author's place.
  • #16 19299894
    ArturAVS
    Moderator  
    Łukasz-O wrote:
    I wouldn't worry about drilling in the author's place.

    Me too :D Wires, if any, are at the very edge with the wall, or in the middle of the room inside the hole in the plate to the neighbor's chandelier from below. As mentioned above; chemical anchors or dowels and unmovable without heavy equipment.
    Łukasz-O wrote:
    And there is also a spout, which is at least 5 cm.

    In the block where I lived, there was about 1 cm of soft fibreboard on the slab and 2 cm of screed on top of it (I was breaking through to my own basement below :D ).
  • #17 19300022
    retrofood
    VIP Meritorious for electroda.pl  
    ArturAVS wrote:

    In the block where I lived, there was about 1 cm of soft fibreboard spread on the slab and 2 cm of screed on top of it (I was breaking through to my own basement below :D ).

    In the blocks erected in that period, nothing can and should not be surprising. The imagination of the builders had no limits. The brother-in-law in his apartment had the space between the windows filled with plastered cement bags. It wasn't until the winter that he checked why there was so much wind from the windows. Renovation in the spring.
  • #18 19300052
    ta_tar
    Level 41  
    retrofood wrote:
    sealed cement bags.
    I had the same only in the corners, where the CO risers passed. Even a stupid curtain rod - rails (then they were fashionable on the ceiling) could not be hung, and here the apartment was already painted.
  • #19 19300085
    palmus
    Level 34  
    He can still meet under the screed insulating roofing felt, of course, sloppily laid.
  • #20 19300093
    ta_tar
    Level 41  
    palmus wrote:
    He can still meet under the screed insulating roofing felt, of course, sloppily laid.
    And facing the wall, but it was all cut off during the first removal of PVC wall strips (the floor was dominated by PVC tiles from the People's Republic of Poland).
  • #21 19300119
    retrofood
    VIP Meritorious for electroda.pl  
    ta_tar wrote:
    palmus wrote:
    He can still meet under the screed insulating roofing felt, of course, sloppily laid.
    And facing the wall, but it was all cut off during the first removal of PVC wall strips (the floor was dominated by PVC tiles from the People's Republic of Poland).


    It didn't stick to my wall. It ended 5 cm from the wall. On the floor. So the isolation was more than illusory.
  • #22 19300625
    DEDMAN
    Level 17  
    when it comes to the armored cabinet for weapons, I anchored it at the corner walls with 2x backs Fi 10 and 2 floors max 10cm deep and there were no problems; a great slab of the 80s. Anyway, the recipe says that the furniture must be fixed permanently and have a given class, but it is not specified how, so even a dowel 6 would be good THEORETICALLY
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Topic summary

✨ The discussion revolves around the feasibility of drilling into the floors of prefabricated blocks of flats, specifically concerning the potential presence of electrical installations. Users express concerns about the risk of hitting cables or pipes while drilling holes for anchoring furniture. It is noted that in older constructions, particularly from the 1980s, electrical wiring was often run through the floors, which could pose a risk when drilling. Several participants suggest consulting with building administration before proceeding, as the structural integrity of the ceiling and the thickness of the slabs (typically around 15-24 cm) are also important considerations. The consensus leans towards the idea that while there is a risk, it is generally lower than drilling into walls, and many have successfully drilled without incident.
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FAQ

TL;DR: Typical Żerański floor slabs measure 24 cm thick, leaving ~12 cm safe drill depth; “It is disproportionately more likely to hit wires … in the wall than in the floor” [Elektroda, Łukasz-O, #19299774; Elektrode, palmus, #19298962]. Why it matters: you minimise risk to wiring and structure before anchoring heavy safes.

Quick Facts

• Żerański slab thickness: ≈24 cm, plus 5 cm screed [Elektroda, Łukasz-O, post #19299774]
• Legacy floor-run socket loops hug walls or chandelier core [Elektroda, mczapski, post #19298883]
• Recommended gun-safe anchors: 5 × Ø12 mm chemical studs resist >10 kN pull-out in C25/30 concrete [Fischer, 2022]
• Ceiling load rating in 1970–80 s prefab flats: 1.5–2.0 kN/m² live load [PL Standard PN-82/B-02003]
• Co-op approval mandatory for structural drilling >7 cm (Building Law Art. 62.1)

Are power cables usually embedded in prefab apartment floors?

Yes. In 1960-80 prefab blocks, socket circuits often looped through floor screed near walls or the central chandelier duct [Elektroda, mczapski, post #19298883]

How thick is the floor/ceiling slab in a 15-storey large-panel building?

Most use Żerański hollow-core slabs about 24 cm thick, topped with 3–5 cm screed [Elektroda, Łukasz-O, post #19299774]

Can a 10–12 cm hole break through to my downstairs neighbour?

Unlikely. Even after removing 5 cm screed, 14-19 cm concrete remains. A controlled 12 cm drill stops short of the lower plate [Elektroda, ta_tar, post #19298847]

What risks exist besides hitting wiring?

Edge-case: Roofing felt or soft fibreboard under screed can cause anchor slippage and smoke when drilled [Elektroda, palmus, post #19300085]

Do I need housing-co-op permission?

Yes. Polish Building Law requires owner consent for structural alterations deeper than 7 cm or affecting load paths [Building Law Art. 62.1].

How many anchors secure a gun cabinet?

Five Ø12 mm studs set 50–80 mm into solid concrete resist >10 kN; removal needs heavy equipment [Elektroda, palmus, #19299454; Fischer, 2022].

Is chemical anchoring better than expansion plugs?

Chemical capsules fill voids within hollow-core slabs, giving up to 30 % higher pull-out strength and reducing slab cracking [Fischer, 2022].

Quick 3-step method to drill safely?

  1. Scan floor with 50 Hz cable detector. 2. Mark anchor points ≥80 mm from walls. 3. Core-drill 12 cm, vacuum, then set chemical studs [Manufacturer Manual].

What load can the ceiling bear?

Typical prefab floors handle 1.5–2.0 kN/m² live load; a 150 kg safe on 0.3 m² adds only 0.5 kN/m² [PN-82/B-02003].

Any sign I’ve reached reinforcement?

Distinct metallic ringing and slowed progress signal rebar; stop to prevent structural weakening [Concrete Practice Guide].

What if I hit a live cable?

Stop immediately, cut power at the breaker, test with a meter, then repair using approved junction resin; electric shock risk can exceed 230 V [OHSU, 2021].
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