How to core drill with an SDS drill

Core bit types

TCT percussion cores have tungsten carbide teeth around the rim and are used with hammer action. They suit brick, block and stone in SDS plus and SDS max drills. They come with an arbor, a pilot drill and an ejector slot to knock the core out.

Diamond cores grind rather than chip and run in rotary only mode on a dedicated core drill, often with water. They cut concrete and engineering brick cleanly but must never be used with hammer action.

Before you drill

Check the wall for cables and pipes on both sides, and check what is directly behind on the outside. Mark the centre on both faces if you can reach, measuring from a common reference such as a window edge. Confirm the core size you need from the fan or vent kit instructions.

Use a powerful SDS drill with a side handle and, ideally, a safety clutch. Large cores put a lot of twisting force on the drill. A small compact SDS drill may struggle with large cores; check the maker's maximum core size in the spec.

Note:A jammed core can twist the drill hard. Use the side handle, brace yourself and keep the cable clear. Wear eye protection, hearing protection and an FFP3 mask for silica dust.

Step by step

This method gives a neat hole with minimal breakout on the far side.

1. 1

   Fit the arbor and pilot drill to the core and lock it into the SDS chuck.

2. 2

   Set rotary only. Start slowly with the pilot drill on your mark until the core teeth touch.

3. 3

   Hold the drill square and let the teeth cut a groove all round, still in rotary only.

4. 4

   Switch to rotary hammer and drill to roughly half the wall thickness, pulling back to clear dust.

5. 5

   Use the pilot hole to start the core from the other side and meet in the middle.

6. 6

   Knock out the core plug and tidy the hole with a chisel.

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Cavity walls and render

Most UK houses built in the last century have cavity walls. Drill the inner leaf, then the outer leaf, and fit a sleeve so the duct crosses the cavity cleanly. Take care not to push debris into the cavity where it can bridge insulation.

On rendered walls, start from the outside face with gentle rotary action to avoid cracking the render around the hole.

When to call in a core drill

Dense concrete, large holes and holes through thick solid walls are much easier with a diamond core drill on a stand. These can be hired, or a professional can cut the hole quickly and cleanly. If the hole passes near a structural beam or lintel, ask a builder or structural engineer first.

If you need an SDS drill for other jobs too, see corded vs cordless SDS drill and hire or buy an SDS drill.

Looking after core bits

Keep the core rim cool by pausing between bursts and clearing dust. Check the TCT teeth for chips; a broken tooth makes the core grab. Store cores with the pilot drill removed so it does not get bent, and grease the SDS shank lightly before use.

Never use a TCT percussion core on reinforced concrete. Hitting rebar can damage the core and twist the drill.

Frequently asked questions

Can a combi drill use a core bit? Small cores with a hex or round arbor are made for combi drills in soft brick, but for 100 mm and larger holes an SDS drill is much better.

Should I use hammer mode for core drilling? Use rotary only to start the cut, then hammer for TCT cores in brick. Never use hammer with diamond cores.

How do I stop the far side blowing out? Drill halfway from each side, using the pilot hole to line up.

What size hole for an extractor fan? Check the fan kit. Many 100 mm fans need a hole around 105 to 117 mm for the duct.