Minimising blowout when drilling through walls

Yes, all of the above!

If you can drill from both sides of the wall that's obviously the optimum, but can be difficult/impossible to get them to meet in the middle (especially do-able with a cavity wall though).

If all else fails, I've had having a flat plate of thick plywood held really firmly against the wall where the bit was going to emerge*

David

Are you talking about an outside wall - brick on the outside, block and plaster on the inside? If so, try to drill from the outside if you can - for

• If plaster breaks out, you can repair it easily - but brick, you can't
• You can choose exactly where the hole will be on the outside - and can go through mortar at a brick corner if desired [Make sure you know where the hole will be on the inside though, and check for pipes, power cables, etc.]

Something which I haven't tried, but which just might reduce breakout on a plaster wall is to wedge (or get an assistant to hold) a sacrificial piece of plasterboard against the wall, and to drill right through that - so that the drill doesn't immediately meet fresh air when it comes through the wall.

There is no complete solution which is guaranteed to work. If you can add some support at the back of the hole - piece of wood solidly braced against something, that is the most certain way to prevent it. It is the lack of support which causes the blow out. Another way is to drill the smallest size hole then be gentle for the last inch. then keep going up a size at a time and working from both sides of the hole.

Drill as small a pilot hole as you have a drill for, going gently. Which probably means about 5mm. Then drill the required hole from both sides.

mark

If its SDS a 5mm bit will only be around 160mm long. Probably have to go up to 10 or 12 mm.

Scrapwood on the far side, with a fair bit of force behind it. Be warned that old plaster may well lose adhesion and de-laminate anyway, it just won't visibly separate.

Pilot holes won't help in a brittle material like plaster. You'll see nearly as much delamination from the pilot as from the full-size hole.

Yes - but small drills aren't too bad with an ordinary hammer drill on most internal walls. And I have a few 18" 5mm ones.

If you are using SDS then this a good one very little pressure will still get the hole drilled. Lean on an SDS and you are guranteed a saucer sized blow out. Mark the drill with a bit of tape as accurately as you can with the thickness of the wall. You can blast through most of the depth then ease of for the last inch and half or so.

That is a good method combined with the above ease off at the end. You will get a bit of blowout but the bigger drill latter may well remove all that.

Well there ain't much you can do so worrying after the event is pointless. One can do things before hand to reduce the chances of a big mess though.