How do I hack off blown plaster safely?

Hacking off is not difficult, but it is the point where a manageable repair turns into a fortnight of misery if it is approached badly. The three things that decide how it goes are knowing the extent before you start, containing the dust, and stopping at a straight edge in sound material.

Safety

What you'll need

• P3 mask, goggles and gloves
• Bolster chisel and club hammer
• Dust sheets, plastic sheeting and tape
• Cable and pipe detector
• Rubble sacks — more than you think
• A scutch hammer or a comb chisel (optional)
• A multi-tool or grinder with dust extraction, for cutting the edge (optional)

Step by step

1. Tap out the full extent first and mark it.Work well beyond the visible damage. Plaster is usually loose over a larger area than it looks, and discovering that after you have made good is the classic way to do the job twice.
2. Isolate the electrical circuits serving that wall.Sockets, switches and lights on and near the wall. Then scan with a detector and mark the likely cable runs — vertically and horizontally from every accessory — before any tool touches the wall.
3. Empty and seal the room.Everything out that can come out. Sheet what cannot. Plastic sheeting taped over the doorway with a zip or an overlap, and a sheet over any carpet, taped at the edges. Turn off any heating or ventilation that moves air out of the room and into the rest of the house.
4. Cut the boundary before you break anything.Score or cut a line just inside sound plaster all round the area, with a multi-tool or a grinder with dust extraction. A cut edge stops the damage propagating past where you want it and gives you something to plaster up to.
5. Start low and work upward.Loose plaster above falls away as you remove what is below it, which does most of the work for you. Starting at the top means everything you loosen lands on what you have not done yet.
6. Hold the bolster at a shallow angle to the wall.Around 20 to 30 degrees, so it slides between plaster and masonry and lifts. Held square, it chops into the brickwork and makes a hole that has to be filled before you can plaster.
7. Stop when the chisel stops lifting.Sound plaster resists. If it takes real effort, you have reached material that is bonded, and continuing just destroys good work and enlarges the area to be re-plastered.
8. Clear the debris as you go, in sacks, not in a heap.Plaster rubble is very heavy — half-fill the sacks or they cannot be lifted, and they split. Removing it as you go stops it being walked through the house.
9. Rake out and brush down the exposed background.Rake back any crumbly mortar joints, brush the brickwork thoroughly, and remove all dust. Plaster does not bond to dust. Damping down or a suitable bonding treatment comes next, according to the background.
10. Let the room and the wall settle before re-plastering.Vacuum with a dust-rated machine, let the airborne dust drop for a day, and clean again. Plastering into a dusty room drags grit into the finish.

Tips

• A scutch hammer with a comb is much faster than a bolster on stubborn areas and is easier on the wrists, but it is aggressive — keep it away from soft brick.
• An old sheet hung damp in the doorway catches far more airborne dust than a dry one.
• If the wall is a party wall in a terrace or a semi, expect the neighbours to hear every blow and tell them first. Repeated hammering on a shared wall is also a Party Wall Act question in England and Wales if you are cutting into it rather than just removing finishes.

Common mistakes

• Hacking off without isolating and detecting first — Cables in a wall are typically only just under the plaster, and a bolster finds them instantly. Isolation is what turns that from a serious injury into an inconvenience.
• Feathering out the edge instead of cutting it — A thin tapered edge of old plaster crumbles when the new coat is applied and cracks along the join later. A square cut edge gives the repair something to butt against.
• Leaving the dust to settle everywhere — Plaster dust is fine enough to get through a closed door and it goes into every fabric in the house. Containment before you start is far cheaper than cleaning afterwards.
• Hacking a textured surface without testing it — If it is a pre-2000 textured coating it may contain asbestos, and hacking is the single worst thing you can do to it.

Questions people ask