Bolster Chisels: How to Cut Bricks and Blocks by Hand

Your bricklayer is building the extension walls and needs to finish a course with a half-brick. He reaches for the bolster chisel, places it on the chalk line, gives it three sharp taps with a club hammer, and the brick splits cleanly along the mark. The whole operation takes about ten seconds. But when a homeowner tries the same thing without understanding the technique, bricks crack in the wrong place, blocks shatter unevenly, and you burn through materials. Worse, the striking head of the chisel starts shedding metal fragments that can cause serious eye injuries, and most people don't know to check for that.

What it is and when you need one

A bolster chisel (also called a brick bolster) is a wide, flat steel chisel designed for splitting bricks and blocks by hand. The blade is broad, typically 75mm to 100mm wide, compared to a cold chisel's narrow 12-25mm blade. That width is the point: it spans enough of the brick's face to create a clean fracture line across the full width.

You place the blade on the brick where you want the cut, bevel side facing the waste end, and strike the head with a club hammer. The force transfers through the blade into the material, cracking it along the contact line. Score all four faces first, then deliver a firm final blow to split the brick.

Beyond cutting bricks and blocks, bolsters are useful for chasing out mortar joints during repointing, removing old floor tiles, and cleaning up rough masonry edges. Electricians use narrower versions (50-55mm) for chasing cable routes into walls, though an SDS drill with a chisel attachment has largely replaced the hand bolster for that job.

Types and sizes

The two decisions that matter: blade width and whether you want a hand guard.

Blade width

For extension work where you're cutting standard UK metric bricks and 100mm dense blocks, the 100mm bolster is the right choice. It matches the brick width, so the blade contacts the full face and produces a cleaner split. If you also need to chase short cable or pipe runs, add a 75mm. You don't need the electrician's width unless you're doing extensive first-fix wiring by hand.

Click to enlarge

Guarded vs unguarded

Guarded bolsters have a rubber or EVA plastic disc fitted around the shaft, sitting between the blade and the striking head. The guard protects your hand from glancing hammer blows.

Here's the honest trade-off. Beginners should always use a guarded bolster. If you miss the head, the guard absorbs the blow instead of your knuckles. But experienced tradespeople often prefer unguarded chisels because the guard can deflect a poorly aimed hammer strike sideways into the wrist, which is arguably worse than a knuckle graze. The guard also adds bulk, making it harder to position the blade precisely in tight spots.

If you're doing occasional brickwork on a home project, buy guarded. Once you're confident with your aim and technique, the choice is yours.

How to use it properly

This is where most guides fall short. Hitting a chisel with a hammer sounds straightforward, but bad technique wastes bricks and risks injury.

Cutting a brick

1. Mark your cut line

   Use chalk, a wax crayon, or a paint marker to draw a clear line around the brick where you want it to split. Pencil lines disappear into the brick's texture. Mark all four faces if you want a clean break.

2. Score the line on all faces

   Place the bolster blade on the chalk line with the bevel facing the waste side (the piece you don't need). Tap gently with the club hammer to create a shallow groove about 1.5mm deep. Rotate the brick and score every face. This scoring creates a stress line that guides the fracture.

3. Support the brick properly

   Rest the brick on a sand bed, a pile of loose earth, or position it so the waste end overhangs a solid edge. Never cut a brick sitting flat on concrete. The hard surface underneath absorbs the impact energy instead of letting the brick split, and the shock can shatter it unpredictably.

4. Position and strike

   Place the blade back on the scored line, bevel to waste side. Hold the chisel firmly but not in a death grip. Keep the blade vertical, perpendicular to the brick face. Strike the head cleanly with a firm, vertical blow from the club hammer. Angled strikes cause crooked breaks.

5. Clean up the cut face

   The split face won't be perfectly smooth. Use the edge of the bolster or a brick hammer to knock off any protruding nibs. For blockwork that will be rendered or plastered, this rough face is fine. For exposed brickwork, take more care with the scoring stage.

Cutting dense concrete blocks

Dense blocks (7.3N/mm2) are harder to split cleanly than standard bricks. The technique is the same, but you need more patience with the scoring. Work around all four sides multiple times, deepening the groove gradually. The final strike needs more force. A 1kg (2.2lb) club hammer is the minimum; a 1.4kg (3lb) lump hammer gives better results on dense blocks.

Aerated blocks (like Celcon or Thermalite) are much softer. You can cut these with a bolster, but a hand saw designed for aerated blocks gives a cleaner, more accurate cut with less effort.

Chasing walls

For short cable or pipe runs (under a metre), a bolster and club hammer works. Score two parallel lines at the desired chase width, then chip out the material between them. It's slow, dusty, and physically tiring, but it's free if you already own the tools.

For anything longer, an SDS drill with a chisel attachment is vastly faster and produces a straighter, more consistent channel. For a full room of cable chases, hire a wall chaser with twin diamond blades. The bolster's role in chasing is really limited to short runs and pre-scoring plaster before using power tools, which reduces collateral damage to surrounding plasterwork.

How to check your chisel is safe to use

This is the safety issue that product listings bury in small print but forum users warn about repeatedly. It's more important than sharpening.

The mushrooming check

Every time you strike the head of a bolster with a hammer, the metal spreads slightly. Over dozens of strikes, the head develops a rim of flattened, work-hardened steel around its edge. This is called "mushrooming." Those thin, brittle edges can snap off under impact and fly at speed. Metal fragments in the eye is a genuine risk, not a theoretical one.

Before every session, look at the striking head. If the edges have started to flare out beyond the original diameter, dress the head with an angle grinder. Grind the mushroomed edges back to the original profile. This takes thirty seconds and prevents a trip to A&E.

Sharpening the blade

A dull blade doesn't bite into the brick surface properly. It skates across instead of scoring a clean groove, and you end up hitting harder to compensate, which increases the risk of a bad break.

Sharpen the cutting edge with an angle grinder when it starts to feel like it's sliding rather than gripping. You're not looking for a razor edge. Just a clean, consistent bevel along the full width of the blade. A quick pass on the grinder after each heavy session keeps the edge serviceable.

What to buy

Bolster chisels are cheap tools. Even the best single chisel costs less than a takeaway. Don't overthink the purchase, but don't grab the absolute cheapest unbranded option either.

The Roughneck guarded brick bolster at Screwfix (code 55448 for 100mm, code 48846 for 75mm) is the default recommendation. Drop-forged carbon steel, powder-coated shaft, soft grip with overstrike guard. Forum users consistently rate Roughneck as the best balance of quality and value for striking tools.

For a heritage "buy British" option, Footprint Tools in Sheffield manufactures bolsters from a single fully forged piece of carbon steel through 14 manufacturing processes. Their domed head design resists mushrooming better than flat-headed competitors. Available from specialist retailers like Intertools Online. They conform to BS3066:1995 (the British Standard for cold chisels and bolsters covering hardening, tempering, and dimensional requirements).

CK Tools bolsters (T3087 series) are popular among electricians for chasing work. Octagonal shank, precision-ground cutting edge, hardened and tempered to BS3066:1995. Very competitive pricing at around £7 – £10 depending on size and whether you want the grip version.

When to use something else

A bolster chisel is the right tool for cutting a manageable number of bricks or blocks by hand. But it has clear limits.

For ten cuts or fewer during a day's work, the bolster is perfect. Quick, no setup, no power needed.

For ten to thirty cuts, you'll start to feel it. The repetitive striking is tiring, and accuracy drops as fatigue sets in. An angle grinder with a diamond blade scores the brick faster and more precisely, though it generates far more dust (an FFP3 mask and outdoor cutting area become essential).

For thirty-plus cuts in a day, hire a block splitter. These hydraulic tools split blocks and bricks cleanly with a lever action, no striking required. Available from hire shops for around £30 – £50 per day. Your bricklayer will have one on site for any serious blockwork job.

For wall chasing, an SDS drill with a flat chisel attachment is the standard professional tool. It's several times faster than hand chasing and produces a more consistent channel. For a full room of chases, a dedicated wall chaser with twin diamond blades and dust extraction is the cleanest option. The bolster still has a role here: pre-scoring the plaster surface with a bolster before using the SDS reduces cracking and damage to surrounding plasterwork.

Where you'll need this

Bolster chisels appear across multiple phases of any extension or renovation project:

• Walls and blockwork - cutting bricks and blocks to length during wall construction, the primary use case
• Foundations and footings - trimming blocks at setting-out stage
• First-fix electrics - chasing short cable runs into masonry walls
• First-fix plumbing - chasing pipe routes into walls
• Kitchen installation - removing old tiles, trimming masonry for fittings
• Snagging checklist - cleaning up rough edges and checking cut quality

Safety essentials

Four things. Every time.

Safety glasses. Non-negotiable. Metal fragments from the chisel head and masonry chips both fly unpredictably. Standard safety specs from any builder's merchant cost £3 – £5.

FFP3 dust mask. Silica dust from masonry cutting causes irreversible lung damage. Even hand cutting produces enough dust to matter.

Gloves. A guarded chisel reduces the need, but gloves protect against both missed strikes and sharp brick edges. Rigger gloves are fine.

Check the head. Inspect for mushrooming before every session. Grind off any flared edges. This is the check that prevents the worst injuries.