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We have thin concrete walls in our flat, ~7cm/2.7inch thick. I need to drill some 6.5cm/2.5inch deep holes with a diameter of either 10mm/0.4inch or 8mm/0.3inch (I prefer the wider but can live with the slimmer) in order to attach a wall bracket for a heavy floating board. So almost drilled through. I can live with drilling through though - applying some filler and adding a piece of wallpaper afterwards is something I can accept.

But how do I make sure that no debris breaks off from the other side of the wall? Or, at least, lower the chances?

We made shorter holes in such walls before (I think it was 5.5cm/2.1inch deep with diameter 8mm/0.3inch), and it already occurred twice that a piece of concrete broke off from the other side. One piece was as large as a palm of a hand, the other was a bit smaller. In both cases we had to apply a lot of filler to the wall and were not able to conceal that damage was done - the filler shrinks when drying, leaving a visible dent. It's not that the dents catch the eye immediately, but they are still easy to spot.

I am not afraid of statics issues. I merely want to have a nice-looking flat and not one with construction site vibes.

We use a rotary hammer for drilling the holes (this one), and we cannot use anything else. I know there are concrete walls that allow usage of hammer drills, but our walls are too hard: My husband once tried it; it took him 45min (and a lot of musclepower and sweat) to get 3-4cm/1.2-1.5inch into the wall, then he couldn't get any further. And this was just 6mm/0.23inch diameter.

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  • Are you using the depth gauge on the drill ? SDS drills certainly work better when you push on them, to give the action something to push against, but you can go gentle at the end. The depth gauge (often referred to as a stop but it's not really) can show when you're getting close.
    – Chris H
    Commented Aug 6, 2023 at 11:41
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    And if you have to fill, overfill then sand back when it's dry, finishing with quite fine sandpaper. Then the shrinkage won't be an issue
    – Chris H
    Commented Aug 6, 2023 at 11:42
  • I find it difficult to believe your walls are only 2.7" thick.
    – Huesmann
    Commented Aug 6, 2023 at 16:31
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    @Huesmann That's completely standard for reinforced concrete panels used in millions of apartment blocks all over (mainly the eastern half of) Europe some decades ago.
    – TooTea
    Commented Aug 7, 2023 at 9:17
  • What are the holes meant for? Can't you just make them shallower to avoid spalling on the other side? There are many kinds of wall anchors on the market shorter than 6 cm, and in solid concrete any single of them will easily hold an elephant.
    – TooTea
    Commented Aug 7, 2023 at 9:20

3 Answers 3

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My answer would be: Don't attempt. There are basically two ways of drilling concrete:

  • With hollow bits, but this usually requires water cooling, which might not be what you want.
  • With power/pneumatic drilling, but then you're basically guaranteed to damage the wall on the other side.

My solution (as I have even thinner concrete walls, 6cm) is to simly drill through with a 5mm or 6mm SDS drill, then enlarge to the required size, and repair the wall on the other side. (I prefer this over drilling straight to 10mm as this way the damage on the other side is usually smaller. To mitigate the damage further, you can drill 1--2cm with the 10mm from the other side.¹) Then, I keep a bit of white plaster, and some ready-to-use ceramic stucco, and a bit of the wall paint, and I repair the damage on the other side. Gets the job done, and I don't have to fear whether I make the damage or not.

¹ Thanks to the commentators for the pro-tip!

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    Pilot hole, then you do some from both sides with the real size bit\core. If it spalls out to the outside when you're not trying to penetrate, then you're trying to drill a hole for a fastener that's too long. And if that's the required length of your fastener then the wall is unsuitable. - Masonry core bits would only require water for dust control, and do require a hammer drill. But they don't make 1/2" core bits anyway.
    – Mazura
    Commented Aug 6, 2023 at 19:07
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    @yo' even a shallow how from the back before doing most from the front would help, and would minimise misalignment. You could even go up in steps (6/8/10mm) to maximise the centring effect of the original hole. When enlarging you can be quite gentle too.
    – Chris H
    Commented Aug 7, 2023 at 16:58
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    I love the idea of an "aircraft-masonry" drill bit - one that is super long for its width, but has a carbide tip for hammering masonry. Two words that really don't go together.
    – Criggie
    Commented Aug 7, 2023 at 22:04
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    @Criggie I've got a couple of moderately long ones for my SDS (30-40cm, which will go through my outside walls). When I've had contractors out they often have 60-100cm-long bits, for even the stone walls in some of the old houses round here.
    – Chris H
    Commented Aug 8, 2023 at 8:59
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    @Criggie I once had an idea for a concrete airplane, but I could never get it off the ground.
    – JimmyJames
    Commented Aug 8, 2023 at 16:28
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Something that is very common in wood working is to back the piece you want to drill a hole into with another scrap piece of wood. Tearout happens at the back of the work piece when the drill literally pushes the remaining material into thin air, having a back board gives that material something to brace against.

I have not yet tried that method with concrete. In your case I'd try a very soft piece of wood, that can conform to any bumps in the concrete wall, to give maximum surface contact. Locate the hole position on both sides of the wall, then have somebody hold that board against the wall while you drill. Make sure your hands are out of the way.

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    Definitely worth a shot
    – FreeMan
    Commented Aug 7, 2023 at 12:58
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    The difficulty will be pressing it hard enough against the wall. In woodworking you'd clamp it or press down onto it against the bench
    – Chris H
    Commented Aug 7, 2023 at 17:02
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    @ChrisH No problem. First you drill holes to anchor the backing piece... /s
    – Wayne Conrad
    Commented Aug 7, 2023 at 18:24
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    @yo' The comment was sarcasm, as indicated by the use of the tone tag (wikipedia link) "/s", used in text communcation to replace vocal intonation.
    – B-K
    Commented Aug 8, 2023 at 10:27
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    Nah, @WayneConrad, you just have a couple of really big friends lean against it! ;)
    – FreeMan
    Commented Aug 8, 2023 at 13:26
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If you want to minimise the chances of breaking out then you need to use a drill on a stand where you can control the feed rate and use minimal pressure to achieve the cut.

However, even an odd lump or piece of grit may cause a break out...

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