Core Drilling Holes in Reinforced Concrete Panels up to 15 cm

A core drill does not hammer the concrete. The tube-shaped diamond bit slowly grinds a narrow ring, and a solid cylinder of concrete, called the core, then comes out of the wall in one piece. The panel does not shake and the concrete around the hole does not crack. The bit can also cut through steel reinforcement.

1. Check the position of the hole against the drawings or the planned pipe route, and mark the centre.
2. Scan the wall with a detector for cables, pipes and reinforcement. If possible, move the hole away from dense rebar.
3. Fix the drill stand to the wall or ceiling with an anchor or a vacuum base so the bit cannot wander.
4. Connect the water cooling and set up a way to catch the water below the hole, or use a slurry vacuum.
5. Drill with steady, moderate pressure. The bit gradually cuts through both concrete and reinforcement.
6. Remove the core, clean the hole and check its diameter and direction.

For a single hole in a standard panel, the drilling itself is usually quick. Most of the time goes into preparation, anchoring the machine and cleaning up afterwards.

The two options differ in the size of the core bit, and so in what will fit through the hole. The hole should always be slightly larger than the pipe or duct. The gap leaves room for a sleeve, a seal or insulation, and it also allows a small correction in position.

• A hole up to 10 cm is enough for cables, water pipes, smaller waste pipes, and air conditioning or condensate lines. It takes less out of the panel, and it is easier to find a good spot between the reinforcing bars.
• A hole up to 15 cm suits larger penetrations: a cooker hood duct, ventilation ducting, the main bathroom waste or a pipe with thick insulation. The bit cuts through more reinforcement, so on a load-bearing wall the work needs even more careful assessment.

Before you book the drilling, measure the outside diameter of the pipe including its insulation, and check what hole size the ventilation or air conditioning manufacturer recommends. In practice it often happens that a hole is drilled "to fit the pipe" and then the insulation or sleeve will not go in.

How demanding the work is, and so what it ends up costing, depends on several things:

• Panel thickness. A thicker panel means longer drilling and more wear on the bit.
• Amount of reinforcement. When the bit hits thick bars, progress slows down.
• Wall or ceiling. Drilling upwards into a floor slab makes it harder to anchor the machine and to manage the water.
• Access. A hole close to a corner, above a kitchen worktop or in a narrow bathroom makes it harder to fix the stand.
• A furnished flat. Furniture, floors and plaster need protecting from water and slurry.
• Number of holes. Several holes in one visit are more efficient than separate call-outs.

You can make the job easier by clearing space around the wall, covering the floor and making sure water and electricity are available nearby.

In a panel block, most walls and all floor slabs carry the load of the whole building. Even a round hole is an intervention in a shared part of the structure. That is why you should not drill it yourself, and why you should talk to the building management or owners' association before you book anyone. For a larger hole, several holes close together or drilling near the edge of a panel, an assessment by a structural engineer is usually needed.

Rules on alterations to load-bearing structures differ from country to country and sometimes between local authorities. Check with your local building authority whether your work needs a notification, approval or permit. Do not assume in advance that nothing is required.

If there could be electrical cables in the wall, isolating and rerouting them is work for a qualified electrician. The same principle applies to gas pipes near the drilling point: work only starts once the route has been checked. If a gas pipe is damaged despite this, leave the area at once, do not switch anything on or off, and call your local emergency number or 112, or your gas supplier's emergency line. Any repair belongs to a qualified, registered gas engineer.

• A hammer drill or chiselling instead of core drilling. The impacts break the edge of the hole, can cause cracks and can damage the reinforcement.
• Drilling without checking for services. A cut cable or pipe turns one hole into an expensive repair.
• The wrong diameter. A hole that is too small leaves no room for insulation, and one that is too large weakens the panel for no reason.
• An unanchored machine. The bit slips and the hole ends up angled or oval.
• Underestimating the water. Cooling slurry runs under the flooring or into the flat below, especially when drilling into a ceiling.
• Leaving the penetration unsealed. The gap around the pipe lets through noise, smells and, in a fire, smoke. Penetrations between flats and through floors are therefore sealed with a suitable material, and with a fire-rated one where the hole passes through a fire compartment wall or floor.

Inspect the hole before the pipe goes in and the wall is made good. Mistakes are much harder to fix later.

• The hole is in the agreed position and at the height shown on the drawing
• The diameter suits the pipe including insulation or sleeve
• The hole is straight, without noticeable deviation, and comes out on the other side where it should
• The edges are clean, with no broken-off pieces and no cracks around them
• Any cut reinforcement was approved beforehand by a structural engineer where required
• Nearby services are undamaged and working
• Slurry and water have been removed, including from the floor and the area below the ceiling
• The core and concrete waste have been taken away, or it is clear who will dispose of them