Core drilling round holes in porcelain and ceramic floor tiles

Core drilling means cutting a round hole of a larger diameter through the full thickness of the tile. Unlike drilling a small hole for a wall plug, this is not done with an ordinary drill bit but with a core bit: a hollow cylinder with a cutting edge around its rim. The bit cuts a ring, and the disc of tile inside it, the core, is then removed.

The item is priced per piece, so each hole counts separately. Typical situations where you will need it:

• Hot and cold outlets for a wall-mounted mixer in the shower or over the bath
• Isolating valves and the waste under a basin
• Radiator or towel rail connections coming out of the wall or the floor
• Waste pipes for a WC or bidet, or a floor drain with a round body
• Back boxes for sockets and switches in a tiled wall
• Pipe risers, balustrade fixings or posts on a terrace laid with porcelain tiles

Why not simply grind the hole out with an angle grinder? It can be done, but a circle made from straight cuts is never clean. The cover plate of a mixer or radiator valve usually overlaps the hole only slightly, so a jagged edge would peek out from under it. A core bit makes an accurate circle that the cover plate hides reliably.

Standard ceramic wall tiles, with their softer, more porous body, are relatively easy to drill. Cheaper tungsten carbide hole saws or an adjustable circle cutter will work on them. Porcelain is a different matter. It is fired at a very high temperature, it is dense and hard, and according to manufacturers its water absorption is usually below half a percent. That density is exactly what makes it such a good floor material, but it also means it resists ordinary tools.

Porcelain is therefore drilled only with a diamond core bit. Drilling generates a lot of heat. If the bit is not cooled, the diamonds glaze over, stop cutting and the tile overheats. An overheated tile can develop a hairline crack that is not visible straight away and only shows up after the tile is laid or after the first heating season.

Another feature of modern porcelain is large formats and thin slabs. Tiles of 60 × 120 cm and larger are sensitive to bending and stress when handled. A hole close to the edge of such a slab is a weak point where a crack can start. With polished porcelain there is also the risk of scratching the surface around the hole if the bit skids while it is being started.

Marking out the position

Most of the work is not the drilling itself but accurate marking out. The tiler measures from a fixed point, such as a corner or the last tile laid, to the centre of the pipe, both horizontally and vertically. The width of the grout joint between the tiles must be added. This is where most mistakes happen: one forgotten millimetre per joint adds up over three or four tiles to an offset the cover plate can no longer hide. Experienced tilers therefore often hold the tile in the place where it will sit and transfer the outlet position straight onto it.

Choosing the diameter

The hole is usually drilled a few millimetres larger than the pipe. There are several reasons: the pipe may not stand perfectly square, you need a little play during fitting, and heating pipes move slightly as they warm up. At the same time, the hole must not be bigger than the cover plate or collar can hide. That is why it helps to have the actual mixer or valve on site when marking out, not just a rough idea of "some cover plate".

The drilling itself

The tile is laid face up on a flat, firm surface, such as a piece of board or another tile, so that it cannot flex while being drilled. A core bit without a centre pilot tends to wander on a smooth glaze, so either a suction-cup drilling guide is used, or the bit is first held at an angle to cut a small groove and only then brought upright. Drilling is done without hammer action, with light pressure and water cooling, or, with dry wax-filled bits, with a gentle circular motion. Towards the end the pressure is reduced further so the back of the tile does not blow out when the bit breaks through.

Cleaning up and checking

After drilling, the core is removed and the edge checked. If needed, it is lightly bevelled with a diamond hand pad. The tile is rinsed so no drilling slurry remains on it and is dry-fitted in place once more before it is fixed.

Several types of tool are available and each suits a different situation. It is useful for you to know what the tradesperson is using, because the quality of the edge depends on it.

• Dry diamond core bit with wax filling: fits an ordinary drill or cordless drill driver. The wax inside melts as it warms and cools the diamonds. Good for smaller holes and one-off pieces. It needs a circular motion and patience.
• Diamond core bit with an M14 thread for an angle grinder: fast and powerful, used by tilers when there are many holes. It runs at high speed, so cooling and firm guidance matter, otherwise the edge can chip.
• Wet core bit on a drilling rig: the most accurate option, suitable for large diameters such as a WC waste and for thick outdoor porcelain. It is used less for single holes in a bathroom.
• Tungsten carbide hole saw or circle cutter: only for soft wall tiles. It is not suitable for porcelain, blunts quickly and can break the tile.

In practice, diamond is used almost every time on porcelain floor tiles. Natural stone and glass mosaic have their own rules, and the tradesperson should tell you in advance whether they have experience with such materials.

A hole can only be drilled accurately if all pipes and boxes are accurately fixed. Before tiling starts, you should have the following sorted:

• The plumber has set the outlets at the right height and spacing, capped them and carried out a pressure test
• For a wall-mounted mixer, the outlets are level and at the centres stated by the mixer manufacturer, often 150 mm
• The electrician has set the back boxes so they will finish flush with the tiled surface, not sunk deep behind it
• In wet areas, the waterproofing is finished, including sealing collars around every pipe penetration
• You have chosen the actual mixers, valves and cover plates so the hole diameter can be decided
• There are enough spare tiles for a replacement, because hard porcelain occasionally breaks during drilling

In a block of flats, also check whether the work affects shared services, such as heating or water risers. If it does, agree it in advance with the building management or the owners' association. If the renovation also involves moving services, altering structural elements or other larger changes, check with your local building authority what local building regulations require. Drilling holes in tiles on its own does not need any approval.

If a hole is needed near a gas pipe, for example where it passes through to a boiler, only a qualified, registered gas engineer may work on the pipe. If you ever smell gas, leave immediately, do not use electrical switches and call your local emergency number or 112.

Even though each hole is "one piece", the difficulty varies a great deal. That affects both the time needed and the risk of damaging the tile.

Simpler situations

• A loose tile before laying that can be drilled on a bench
• A smaller diameter and a hole in the middle of the tile, well away from the edge
• Matt porcelain of normal thickness, roughly 8 to 10 mm
• Several identical holes at once, once the tools are set up

More demanding situations

• A hole in tiles that are already laid, for example when a radiator is added later. The tile must be fully bedded, otherwise it can crack during drilling, and there may be pipes or cables behind it
• A hole close to the tile edge. It should usually be at least roughly 1 to 2 cm from the edge, and more on thin large-format slabs
• A hole across a joint, where half the circle falls on one tile and half on the other. The tiles must be clamped firmly together while drilling
• A large diameter, such as a WC waste or a balustrade post
• Thick outdoor porcelain of around 20 mm, a polished surface or an anti-slip texture
• A large-format slab that is awkward to handle and costs the most work if something goes wrong

With laid tiles, it is wise to wait until the adhesive has fully cured. According to adhesive manufacturers this is usually at least 24 hours, and longer for a thicker bed. Drilling into a freshly laid tile can loosen it.

Besides this item, the price list includes a related job, cutting openings in wall and floor tiles. They sound similar, but the method and the tools are different.

• Core drilling always produces a round hole, made with a diamond core bit, with a clean circle as the result. It suits pipes, outlets, wastes and back boxes.
• Cutting an opening means a square or irregular hole or notch, for example for a square floor drain, an access panel, a cut around a door frame, a wall corner or a plinth. It is done with an angle grinder, a tile cutter or a wet saw.
• With core drilling, the diameter, the material and the position of the hole matter most. With cutting, it is the length and number of cuts, and whether the opening is in the middle of the tile or from the edge.
• A square opening in the middle of a tile is often started by drilling small holes in the corners so cracks do not run from sharp corners. That is part of the cutting job, not a separate core-drilled hole.

So if you need a hole for a radiator pipe, you are ordering core drilling. If you need an opening for a square floor drain, it is cutting. If in doubt, show the tiler the actual product that will pass through the tiles.

Most problems with holes do not appear straight away, but only when the fittings go on or after a few months of use.

• Hole in the wrong place: poor marking out or a forgotten joint. The cover plate then sits off-centre or does not cover the hole at all, and the tile has to be replaced.
• Hole too small: the pipe has to be forced through, the tile is under stress and cracks over time. This happens more often with heating pipes, which expand as they heat up.
• Hairline crack from overheating: caused by drilling without cooling or with a glazed bit. At first it is invisible, later it lengthens and collects dirt.
• Chipped edge: too much pressure at the end or drilling with hammer action. Under a cover plate it sometimes gets away with it, around back boxes and wastes it often does not.
• Rigid filling around the pipe: the gap between pipe and tile is filled with grout. Movement of the pipe then crushes the grout or damages the tile. The gap should be filled with a permanently flexible sealant, usually sanitary silicone.
• Broken waterproofing: when drilling later in a shower, the waterproof membrane or collar under the tile is damaged and water starts to get behind the tiling.
• Drilled pipe or cable: drilling into laid tiling without checking what lies behind it. Before such drilling, you need to know the service routes or check them with a detector.

If you need one or two holes in a loose tile, for example for a towel rail connection, you can manage it yourself. For holes in laid tiles in a shower, large diameters and large-format slabs, it is wiser to call a tiler. Drilling creates fine dust containing quartz, so wear at least an FFP2 respirator and safety glasses.

1. Get a diamond core bit of the right diameter, a drill with hammer action switched off, a drilling guide or a piece of board with a pilot hole, and a container of water.
2. Hold the tile in its final position, transfer the centre of the outlet onto it and add the joint width.
3. Stick masking tape on the glaze and mark the centre of the hole on it with a felt pen.
4. Lay the tile face up on a flat, firm surface so it cannot flex.
5. Make sure hammer action is off and set a medium speed according to the bit manufacturer's instructions.
6. Start the bit in the guide or at an angle, cut a shallow groove, then bring the bit upright.
7. Drill with light pressure, cooling with water as you go, or with a wax bit, keep a gentle circular motion.
8. Ease off the pressure towards the end so the back of the tile does not chip.
9. Remove the core, check the edge and smooth it lightly with a diamond hand pad.
10. Rinse the tile, let it dry and dry-fit it once more before fixing.

If the bit stops cutting and only hums, it has glazed over. Drilling briefly into a dressing stone or a piece of aerated concrete block exposes the diamonds again.

Drilling one hole in a loose tile usually takes only a few minutes. Most of the time goes on marking out, setting up the tools and checking. In laid tiles, with a large diameter or thick porcelain, it takes considerably longer. If the gap around the pipes is filled with silicone, bear in mind that according to manufacturers it usually takes several days to cure fully, so avoid soaking it in the shower until then.

When checking, look at the hole up close with light coming from the side, which reveals even fine cracks. Run a finger along the edge: it must not be sharp or chipped. Tap gently around a laid tile, as a hollow sound means poor bedding. Above all, try the cover plate or collar that will hide the hole.

• Every hole matches the position of the fixed outlets and pipes
• Cover plates and collars hide the holes completely and sit centrally
• Mixer outlets are symmetrical, at the same height and correct spacing
• Hole edges are clean, without chips or sharp fragments
• No hairline cracks are visible on the tiles under side lighting
• The glaze or polished surface around the holes is not scratched
• Tiles with holes do not sound hollow when tapped
• The gap between pipe and tile is filled with flexible sealant, not grout
• Waterproofing and sealing collars around penetrations in the shower and by the bath are intact
• The surrounding area is cleaned of drilling slurry and dust