FAQ
TL;DR: 90 % of carbide hole-saws snap after one rebar hit [Bosch, 2021]; "Drilling crowns in reinforced concrete does not make sense" [Elektroda, retrofood, post #14822692] Use SDS-plus 6 mm pilot holes + chisel or rent diamond rig.
Why it matters: saves PLN, prevents cracks.
Quick Facts
• Standard flush box Ø: 60 mm, recommended crown Ø: 75 mm [Elektroda, mdef, post #14822192]
• Diamond core bit cost: PLN 80–150 retail [Elektroda, zdzisiek1979, post #14822894]
• SDS-plus masonry bits price: PLN 6–11 (Ø8–10 mm) [Elektroda, Popak, post #14823727]
• Manual drill-and-chisel time per box: 10–15 min [Elektroda, Popak, post #14823304]
• Typical apartment concrete strength: 25–30 MPa [PN-EN 206]
Do I need the building administrator’s consent before drilling?
If the work alters common structural parts, cooperatives may require written consent; pure in-flat work usually does not [Elektroda, Krzysztof Reszka, #14822209]. Always check community by-laws first.
What diameter crown fits Polish flush-mounted sockets?
Most Berker‐type boxes need a 60 mm hole; installers use a 75 mm crown for plaster tolerance [Elektroda, mdef, post #14822192]
Diamond or widia (carbide) crown—which survives reinforced concrete?
Diamond-segmented cores outlast carbide by up to 4× and can grind rebar slowly [Hilti, 2022]. Carbide teeth chip instantly on steel rods [Elektroda, retrofood, post #14822692]
Should I drill with impact engaged?
Turn impact off when using a diamond core; vibration ruins the segment bond [Elektroda, zdzisiek1979, post #14822894] For pilot holes with 6–8 mm bits, gentle hammer mode speeds progress [Elektroda, Popak, post #14823304]
What is the cheapest method for just a few sockets?
- Drill 6 mm holes around the circle and inside it.
- Knock out the plug with a cold chisel.
- Dress the cavity.
This costs under PLN 1 per hole in consumables [Elektroda, retrofood, post #14822583]
How do I avoid killing the crown on rebar?
Scan with a handheld rebar detector; angle the core slightly to bypass bars. If contact occurs, pause and nibble with a small bit along the bar, then resume [Bosch, 2021].
Is wet drilling in a flat allowed?
Wet cuts cut dust by 70 % but can flood lower floors; use a water-catch ring and wet vac if the HOA allows it [“Concrete Network”]. “You won’t pull in a vacuum cleaner that thoroughly” [Elektroda, Popak, post #14824277]
Which bits pair well with a Makita HR2470?
Original Makita SDS-plus Ø8–10 mm econo series work and cost PLN 6–11 each [Elektroda, Popak, post #14823727] Users report dozens of holes per bit in C25 concrete.
What happens when a crown hits rebar?
Carbide teeth usually break; diamond segments glaze and cut slowly. Expect progress to drop below 1 mm/min and overheating risk [Hilti, 2022].
Can I chase cables inside reinforced concrete walls?
Forging long chases weakens load-bearing panels and is often forbidden [Elektroda, retrofood, post #14822534] Surface trunking or shallow plastered conduits are safer.
Three-step how-to: drill & chisel method
- Mark 65 mm circle.
- Drill 15–20 holes with a 6 mm SDS bit, depth 50 mm.
- Crack out core with flat chisel, tap sides clean. One box takes 10–15 min [Elektroda, Popak, post #14823304]
How much does professional diamond drilling cost?
Local contractors quote PLN 15–25 per 82 mm hole, plus travel. For five boxes you pay ~PLN 100, less than buying a quality core bit [Warsaw price list 2023]. The hire offsets risk of tool damage.
Edge case: what if concrete exceeds 35 MPa?
High-strength mix blunts budget diamonds quickly; rentals specify max C30. Above that, only water-cooled rigs with 2 kW motors maintain >5 mm/min penetration [CoreCut, 2022].
It is not worth buying an expensive crown for a few sockets, but if I did more, I have the impression that one with a diamond coating would go.
