How to Knock a Tap Hole in a Ceramic or Granite Sink

To undertake this job, we recommend the following tools and personal protective equipment for your safety and for best results.

1. Safety Eyewear
2. Safety Gloves
3. Safety Mask
4. Cordless or Standard Drill
5. 35mm Diamond tip hole saw or a preferred size for your hole
6. Hammer with flat and rounded side
7. Centre punch
8. File
9. Water spray or similar
10. Saw horse or similar

First remove the sink from it's original packaging and inspect the sink, ensure there are no visible signs of an faults before you consider knocking through any tap holes you will not be able to return your sink after you have modified your sink but both ceramic and granite sink undergo strict quality control before it leaves the factory so chances of receiving a faulty sink is extremely unlikely.

It is possible and more common for ceramic and granite sink to show signs of hairline cracks, or chipping on the underside of the sink. These are purely superficial, and in no way impact the performance of the product. Ceramic and granite sinks are complex to manufacture, so as a result are produced using fine materials and some small cracks can appear when the sink is fired in the kiln (part of the manufacturing process)

If you are any way concerned, you should take a picture of the part and forward your images to your supplier for their advise.

Place the ceramic or granite kitchen sink upside down on a saw horse or similar where a drill can pass through without damaging the surface below, ensuring you place some cardboard or towel under the sink to ensure it from been scratched or marked.

First, observe all the available tap hole knocking out areas and decide which holes are to be knocked through. Most sinks have various tap hole knocking out positions which allow you to install your kitchen sink mixer tap, automatic waste switch, soap dispenser, waste disposal air switch or similar to be fitted to the sink, so it is very important you check all the available options.

Once you have observed all the possible tap hole knocking out positions, you will then need to check how your tap holes areas are initially prepared. Typically there are 2 types of tap hole preparation at the factory as below.

1. Indented tap hole marking, where the tap hole positions are located within the manufacturing process
2. Scorched tap hole marking, where the tap hole knocking out position has been scorched at the factory

Before you knock through the tap hole, the required tap hole will needs to be fully weakened so the possibility of cracking or smashing the sink during the knocking through process is totally minimised. Many inexperienced fitters skip this section as many manufacturers instructions miss out this section expecting all professionals to know this part of the installation. By not carrying out this part of the installation usually results in cracking, smashing or damaging the sink, so ensure this section is not skipped.

After identifying the position of the tap hole, using a diamond tip 35mm hole saw suitable for ceramic or granite kitchen sink or the same size hole saw of your scorch mark. Using a drill, carefully and slowly using the water spray gun to keep dust levels minimum, drill very carefully and with medium speed, the indented area or the scorched area of the underside of your sink.

Do not drill all the way through the sink! Continue to drill and check the sink until the sink is approximately 3mm away from drilling all the way through. (Hint! For ceramic sink, you will feel and hear a difference in sound and feel as the drill comes closer to the glaze as the material becomes harder as it reaches closer to the glaze area of the sink) Once the sink is approximately 3mm in thickness, it is then ready for knocking the tap hole through.

Once you have prepared the sink for it's tap hole to be knocked through, turn the sink around and place it back on the saw horse or similar so the sink is now facing the correct way for installation.

Before you knock through the hole, ensure the sink is fully supported, it is good practice to ensure the area to be knocked through is fully supported, minimising any risk of cracking or damaging the sink. There are many illustration that show some fitters knocking through the tap hole without properly securing the back of the sink, this may be fine for experienced fitters but we do not recommend punching the hole in this way.

Once the sink is supported and secure, locate the area to be punched thru to create the tap hole, many sinks have a sticker placed where the tap hole center punch is to be used to punch through a hole, but before you commit to knocking through, please ensure the sticker has been placed in the center of the hole at the factory, do not knock through without checking, the sticker is there for guidance only and you should personally ensure the punch location is central to the hole to avoid any risk of damaging the sink.

Once all the above tasks has been completed, use a center punch and hammer to knock through the tap hole, once through, chip away the sink to form a tap hole.The tap holes are usually 35mm but some taps require 40mm so check to ensure you have the adequate size hole. For a professional finish, use a file to smooth away edges until you create a smooth circle.