Core drilling a hole up to 10 cm in a reinforced concrete panel

A diameter of up to 10 cm covers most of the everyday service penetrations in a flat. Typical situations:

• Air conditioning: the bundle of refrigerant pipes, condensate hose and cable between the indoor and outdoor unit usually fits through a hole of about 6 to 8 cm
• Smaller waste pipes: the outlet from a washbasin, kitchen sink or washing machine in smaller sizes, when you move the kitchen or bathroom one wall further
• Water and heating: insulated or sleeved pipes passing through a wall into another room
• Electrical and data: a duct for power cables, aerial or network cabling between rooms (the wiring itself is a job for a qualified electrician)
• Small ventilation: an extractor fan or air inlet with narrow ducting

Why drill rather than break out the hole? Reinforced concrete breaks unevenly when chiselled. Chunks spall off on both sides and the hole almost always ends up bigger than needed. A diamond core bit cuts a clean cylinder instead. The building is not hammered with vibration that neighbours several floors up and down would hear, and the concrete around the hole stays intact.

The price list also has a separate item for core drilling a hole up to 15 cm in a reinforced concrete panel, with its own page and its own price. What decides between them is the outside size of whatever passes through the hole, not the nominal pipe size. The hole is usually a few millimetres to a centimetre or two larger, so the pipe fits together with its insulation, sleeve or collar, and there is room left to seal it.

• Up to 10 cm is enough for air conditioning, smaller waste pipes, water and heating pipes and cables. The bit is lighter, spins faster, puts less load on the rig and motor and removes less concrete. The chance of hitting a reinforcing bar is lower, although it can never be ruled out completely.
• Up to 15 cm is needed for larger drainage, a cooker hood, bigger ventilation ducts or a single-room heat recovery unit. If the duct itself is around 10 cm across, the hole for it will almost always exceed the limit of the smaller variant.
• Impact on the structure grows with the diameter. At 15 cm, more bars are likely to be cut, and a structural engineer looks more strictly at such a hole in a load-bearing panel. At 10 cm the assessment tends to be simpler, but it is never automatic.
• Time and equipment: a larger bit drills more slowly, needs a stronger motor and firmer anchoring, and produces more slurry to vacuum up.

In practice it often happens that a customer orders the smaller hole and only finds out during installation that the insulated pipe will not pass through. Re-drilling the same spot with a larger bit is possible, but it is extra work and the edges are rarely as clean. Confirm the exact diameter beforehand with your plumber or air conditioning installer, ideally measured over the insulation.

Consent from the building and a structural check

In a block of flats, load-bearing walls, floor slabs and the external walls are usually common parts of the building, even when they are inside your flat. Before you have anything drilled, contact the building management or the owners' association. Many buildings have their own rules and ask for an assessment by a structural engineer, especially for holes in load-bearing panels and floor slabs. The engineer will tell you whether the hole is acceptable in that position, whether it should be moved and what to do if the bit meets reinforcement.

Whether your alterations need anything from the local building authority depends on local building regulations and on the overall scope of the work, not just on one hole. Do not assume that one hole is never an issue. Check with the local building authority, ideally with a description of the whole renovation.

Checking for services in the wall

Panels can carry electrical cables and sometimes water, heating or gas pipes. Before drilling, the spot should be checked with a cable and pipe detector and a rebar scanner, from both sides of the wall. Look at where the bit will come out too: the next room may have a socket, kitchen units or a light switch right there. With a floor slab, find out what is underneath in your neighbour's flat.

Nobody should drill near a gas pipe without agreeing it first with a qualified, registered gas engineer. If you ever smell gas, do not switch anything on or off, leave the flat and call the gas emergency line or 112.

Space and access

The driller needs free space around the drilling point for the rig, protected floors on both sides of the wall and a socket with residual current protection, as the work involves water. With an external wall, think about access from outside. The practical steps you can take yourself are in the DIY guide further down.

For a hole up to 10 cm the procedure is always similar. The main differences are whether it goes through a wall or a slab, and whether it leads outside through external insulation.

1. Mark the centre of the hole as agreed with the plumber and check the exit point on the other side once more.
2. Scan the spot for services and dense reinforcement, and move the hole a few centimetres if needed.
3. Anchor the drilling rig. Usually a small hole is drilled for an anchor bolt; on a smooth surface a vacuum base can be used.
4. Set the rig square to the wall or at the agreed angle. A hole for air conditioning condensate has a slight fall towards the outside, usually a few degrees.
5. Connect the water supply to the bit and get the wet vacuum or water collector ring ready.
6. Start slowly with light pressure until the bit has cut a groove and stops wandering.
7. Drill steadily with even pressure. When the bit hits reinforcement, the sound and speed change, and the work continues as agreed with the structural engineer.
8. Ease off the pressure just before breaking through so the concrete does not spall on the far side, or finish from the other side.
9. Remove the core. The concrete cylinder sometimes jams inside the bit and has to be tapped out.
10. Remove the rig, fill the anchor hole, vacuum the slurry and wipe down the area.

The drilling itself through a standard panel at this diameter usually takes from a few minutes to a few tens of minutes, depending on thickness and how much steel the bit meets. Preparation, anchoring and cleaning up often take longer than the drilling. That is why several holes in one flat on the same visit work out better, even though each hole is charged separately.

Drilling through a floor slab

With a slab, the driller works overhead or from above, and water runs along the bit. A collector ring with suction is used and the floor stays covered. Just as important is what happens below: the core and the water fall into the neighbour's room. Nobody should drill a slab without agreement with the neighbour and protection of their room. Some floor slabs have hollow cores or a different reinforcement layout, and the structural engineer decides whether and where they can be drilled.

External panels and insulation

External panels are often layered: an outer concrete skin, insulation and an inner load-bearing layer. Many façades also have added external insulation. The bit must be long enough to pass through the full thickness in one go. The insulation in the middle tears during drilling, so the hole is cleaned out at the end and the penetration properly sealed. Otherwise water gets in and a cold bridge forms.

Wet or dry, rig or hand-held

Reinforced concrete is almost always drilled wet. Water cools the diamond segments, flushes out the ground concrete and binds the dust. Dry drilling produces fine dust containing quartz, which harms the lungs, and the bit overheats and wears faster. Dry drilling is used only as an exception where water must not reach the surroundings, and then with strong extraction and a respirator.

Small diameters up to 10 cm can be drilled by hand in softer materials. In a reinforced concrete panel, an experienced driller will usually still use a rig. When a hand-held machine hits a bar it can jam and twist violently, the hole goes off line and the bit is easily damaged. A rig holds the direction and angle and keeps the pressure even.

Even at the same diameter, one hole can be routine and another quite complicated. What usually helps:

• Free access to the wall and enough room for the rig
• A standard internal panel, which in many precast panel buildings is roughly 14 to 15 cm thick, depending on the building system
• A clearly marked position and consent already in place
• A socket and water close by
• Several holes on one visit

What makes it harder:

• A thick external panel with insulation, which needs a longer bit
• Dense reinforcement, which slows drilling and wears the bit
• A hole close to the ceiling, in a corner or next to a door frame, where the rig is hard to anchor
• Drilling a slab overhead with protection of the flat below
• An angled hole or a precisely specified fall
• A furnished flat where furniture, floors and tiling need protecting
• Work at height or from scaffolding, for example for an outdoor air conditioning unit

Take care with the joints between panels. A joint is not a good place for a hole, because it contains grout, connecting reinforcement and often the façade seal. Holes usually go into the face of the panel, a reasonable distance from its edges.

The driller only makes the hole. What goes into it and how it is closed is another trade's work, so it pays to have everything agreed in advance.

• Before drilling: the plumber or air conditioning installer sets the position, diameter and fall, the electrician confirms where cables run, and the structural engineer or building management gives consent.
• Right after drilling: the plumber fits the pipe, sleeve or grommet and fills the gap around it as needed, for example with sealant, a collar or mortar.
• On an external wall: the outside is sealed against rain and the façade around the penetration repaired so water runs away from the hole.
• At the end: the decorator or tiler makes good around the hole. As a drilled hole is clean, the repair is usually small.

Where a hole passes between fire compartments, for example into a service shaft or onto the common corridor, the penetration may need sealing with fire-stopping material. Ask the building management about this before work starts.

• Diameter too small. The insulated pipe will not fit and the insulation gets torn while fitting. Later, condensation forms on the pipe or mould grows in the hole.
• Exit point not checked. The bit comes out into kitchen units, a socket or the neighbour's tiling. The repairs then cost far more effort than the hole itself.
• Condensate hole without a fall. Water from the air conditioner sits in the hole, runs back inside and leaves stains on the inner wall.
• Spalled exit. Without easing off the pressure, a chunk of concrete breaks off on the far side and needs making good.
• Reinforcement cut without agreement. It is not visible straight away, but in a load-bearing panel it is an alteration to the structure, and the owner is responsible for it.
• Unsealed penetration in an external wall. Rain gets into the panel insulation, and in winter a cold, damp patch appears.
• Water through the slab. Drilling a floor slab without collecting the slurry lets water seep down and stain the neighbour's ceiling.

Drilling a load-bearing reinforced concrete panel is not a DIY job. It alters the building structure, needs consent and calls for a rig and experience. What you can do yourself is get everything ready so the driller can start straight away and nothing in your flat gets damaged.

1. Get written consent from the building management or owners' association, plus the structural engineer's opinion if requested.
2. Confirm the exact diameter, position and fall with the plumber or air conditioning installer and mark the centre on the wall.
3. Check both sides of the wall or slab for sockets, switches, units or tiling at the exit point.
4. Clear at least a metre or two of space around the drilling point on both sides.
5. Move furniture away or cover it, and lay protective sheeting and boards on the floor.
6. Make sure a nearby socket has residual current protection, or check that the driller brings a portable RCD.
7. Arrange outside access for an external wall in case someone needs to hold a board or catch the core.
8. Tell your neighbours the date and time, and keep to the working hours set by the house rules.
9. For a floor slab, agree with the neighbour below how their room will be protected and who will be present.

You can inspect the hole straight away, as nothing needs to dry. A tape measure and a spirit level are enough. Measure the diameter and compare it with what was agreed. Go to the other side and look at the exit. The hole should be in the right place, with intact edges and no significant spalling. For a condensate hole, place the level along the bottom or pour in a little water and watch whether it runs outwards.

Ask the driller whether the bit hit reinforcement and how many bars were cut. This belongs in the records of alterations to your flat, and the building management or structural engineer may ask for it. If a bar was avoided by moving the hole, check that the new position still works for the plumber.

• The diameter matches the agreed size and the insulated pipe fits through
• The position and the exit on the other side match the plan
• The hole is square or has the agreed fall, outwards for condensate
• The edges on both sides are intact without large spalls
• No pieces of core or insulation are left in the hole
• The rig anchor hole is filled
• The slurry is cleaned up and floors and furniture are undamaged
• The neighbour's side shows no water marks or damage where a slab or shared wall was drilled
• The driller has reported whether and how much reinforcement was cut
• For an external wall, it is agreed who seals the penetration and when