⏱ Read time: 8 min | 🔨 Difficulty: Intermediate | ⏰ Time: Half a day (4 hrs) | 👷 1 person
Your drill is screaming, the bit is glowing orange and the wall has barely blinked. Welcome to engineering brick. It can be five times stronger than the brick in your average house wall, and natural stone is often tougher still.
Here is how to get through both cleanly, and the point where a diamond core drill stops being overkill and becomes the only sensible option.
New to masonry altogether? Start with our beginners' guide to drilling into brick, then come back here for the hard stuff.
What is engineering brick, and why is it so hard to drill?
Engineering brick is the same clay as an ordinary house brick, just fired hotter and for longer until it turns almost glass like. Dense, nearly waterproof, brilliant for a retaining wall and miserable for a drill bit, because there is barely any soft material for a masonry bit to chip away.
There are two grades:
- Class A is the harder, usually a dark blue grey.
- Class B is slightly softer and usually red. Both give themselves away when tapped: engineering brick rings like a tile, ordinary brick sounds dull. Wienerberger’s range shows the classes side by side.
You will find it below the damp proof course, in garden and retaining walls, and all over Victorian outhouses. Stone behaves differently. Granite is brutally hard, while sandstone and limestone are brittle and crumble at the face.
How do I know when I need a diamond core drill?
Short answer: if the hole is wider than about 25 mm, or it has to come out the other side of the wall, hire a diamond core drill. Anything smaller is SDS territory.
Diameter and material both matter, though diameter usually settles it. Most guidance on how to drill into brick assumes a soft house brick that a percussion bit can chip through.
Engineering brick barely chips, so the bit stops cutting and starts polishing. Leaning on it harder just cooks the tip.
Most advice on how to drill into brick assumes a standard house brick. Engineering brick changes the maths. Past roughly 25 mm across, or once the hole has to cross a full 215 mm wall, a percussion bit is the wrong tool.
Which core size do I need?
What kit will I need?
How do I drill into engineering brick, step by step?
Be sure to work through these in order and don’t skip anything!
Step 1: check what is behind the wall
Scan the drill line with a cable and pipe detector, then again from a different angle. Old houses ignore the safe zone rules, so treat every wall as guilty.
Step 2: mark the full circle, not a dot
When you’re satisfied there is nothing behind the wall, hold the core bit against the wall and pencil around it. A 117 mm core removes a circle the size of a tea plate, so you can see exactly what disappears.
Step 3: seal the room and rig the extractor
Connect an M Class extractor to the drill shroud, then tape polythene across doorways and cover the furniture. Dry coring produces silica dust in surprising volumes so you’ll want to avoid it collecting on your furniture.
Step 4: get the drill square
Anchor the rig and check it plumb above 100 mm, or fit the pilot drill for smaller handheld cores. A core that starts crooked stays crooked.
Step 5: switch hammer action off, then start slow
Score a 5 mm groove on the lowest speed, then withdraw the pilot. Diamond segments grind rather than smash, so one burst of percussion writes off the bit.
Step 6: let the drill set the pace
Use firm, even pressure, and listen to the motor rather than pushing through a drop in note. Forcing it glazes the diamond smooth, and a glazed core stops cutting.
Step 7: clear the dust every 25 mm
Pull the bit right out, let the extractor clear the dust, then carry on. Packed dust is the most common reason a bit stops cutting.
Step 8: break through from both sides, then make good
Cut two thirds of the way from inside, then move outside and finish from there, or you will punch a crater out of the brick everyone can see. Fit the sleeve, seal it and repair the mortar.
How do I drill into natural stone without cracking it?
One thing to understand about stone is that it punishes you for being impatient. Here are four habits keep it intact.
Working on a listed building or conservation area?
Check with your local authority before you cut anything. Consent rules bite harder than the brick does. Full Approved Documents for Building Regulations are on GOV.UK, and Part F covers ventilation while Part J covers combustion appliances and flues. For listed or period stonework, see Historic England's homeowner advice. Gas flues need a Gas Safe engineer.
Should I hire or buy?
It all comes down to how many holes you need to make, and how big they are.
Worth knowing: the core bit wears out, not the drill. Cheap cores in engineering brick are false economy. Our core drill accessories range covers arbors, drive bars and replacements.
What health and safety points should I not skip?
Kit yourself out properly. Browse PPE and workwear at HSS DIY. HSE also publish free construction dust guidance covering silica and respiratory risks.
How long will the job take?
Your core drilling checklist
Print it out or stick it on the wall, so you can tick as you go to keep on track of your drilling project.
Before you start
☐ Brick or stone identified, core drill confirmed
☐ Wall scanned for cables and pipes, twice
☐ No lintel, joist end or DPC in the path
☐ Building Regs or Gas Safe checked
☐ Sleeve measured, core size chosen one size up
☐ Drill, rig, cores and transformer booked
On the day
☐ Dust sheets up, furnishings covered
☐ M Class extractor connected and tested
☐ Goggles, FFP3 mask, ear defenders, boots on
☐ Hammer action switched OFF and double checked
☐ Rig anchored square, or pilot drill fitted
☐ Far side cleared of people and pets
☐ RCD in line, cables clear of the cut
Finishing up
☐ Core plug removed and disposed of safely
☐ Sleeve fitted and sealed, grilles secured
☐ Mortar repairs done above 5 degrees
☐ Dust vacuumed rather than swept
Frequently asked questions
Can I drill engineering brick with a normal cordless drill?
For a small fixing, maybe, with a sharp bit and patience. Above 10 mm you want an SDS plus hammer drill. A combi will overheat and blunt the bit first.
What size core do I need for an extractor fan or tumble dryer vent?
Standard 100 mm ducting needs a 117 mm or 127 mm core, because the sleeve needs room to sit. Measure the sleeve you are fitting.
Do I need a wet or a dry diamond core drill?
Dry cores suit engineering brick and blockwork, and keep water out of your cavity. Go wet for granite and reinforced concrete, on 110 volt kit.
Why has my diamond core stopped cutting?
It has glazed over. The diamonds are there but the bond has polished smooth. Cut a few centimetres into an aerated block to expose fresh diamond.
Can I use a diamond core bit with hammer action on?
No, and it is the fastest way to ruin one. Percussion knocks the segments clean off the barrel.
Is autumn a good time to tackle this job?
One of the better windows in the UK calendar. Vents and dryer ducting matter once the heating goes on, and mortar still cures above 5 degrees.
The bottom line on drilling into engineering brick and stone
And there we have it!
Engineering brick and stone are not difficult, just unforgiving of the wrong tool. Work out the diameter, check what is lurking in the wall, keep hammer action off the core and let the machine grind at its own pace. The wall that shrugged off your masonry bit will hand you a hole tidy enough to photograph.
At HSS DIY, you can browse the diamond core drill hire range, add PPE and consumables, and take next day delivery. Order online 24/7, grab a project bundle if it suits, and bear in mind a multi item order may arrive separately. More ideas sit in the HSS DIY blog and our DIY projects hub.
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