How do I drill into stone, granite or slate?

Stone covers everything from soft limestone you can scratch with a coin to granite that will blunt carbide in seconds, and the drilling method is completely different at the two ends. Getting it wrong does not just fail to make a hole — percussion in hard or laminated stone splits it, and the split runs where the stone wants rather than where you drilled.

Safety

Step by step

1. Work out how hard the stone is.Try to scratch an inconspicuous area with a steel nail. If it marks easily it is soft — sandstone, limestone, chalk. If the nail slides off, it is hard, and you are in diamond territory.
2. Look for bedding and lamination.Slate, some sandstones and many paving stones have visible layers. Drilling across the layers is fine; percussion parallel to them splits the stone along a bed. Slate in particular will delaminate.
3. Use carbide with light hammer on soft stone.An ordinary sharp masonry bit, gear 2, and hammer on the lightest useful setting. Start rotary only until the bit has a seat, then engage percussion.
4. Use diamond with no hammer on hard stone.A diamond-tipped or vacuum-brazed bit, hammer off, gear 1, and steady moderate pressure. Diamond abrades; it does not chip, and percussion destroys both the bit and the stone.
5. Keep hard stone wet.Water cools the diamond, carries away the swarf and suppresses the dust. A spray bottle, a sponge, or a putty dam around the hole filled with water. Dry diamond drilling in granite fails in under a minute.
6. Support the stone.A loose slab, sill or hearth must be fully bedded on a flat surface, not spanning a gap. Unsupported stone flexes at the moment of breakthrough and cracks straight across.
7. Ease off at breakthrough.The last few millimetres are where stone spalls out of the back face. Reduce pressure, and where the back face matters, back the stone up with a scrap board clamped tight.
8. Fix with resin, not expansion.Once the hole is made, an expanding plug or bolt puts outward force on brittle stone and can split it. Chemical anchor resin loads the stone in shear rather than expansion and is the appropriate fixing.

Tips

• Engineered quartz worktops and porcelain slabs behave like very hard stone: diamond, wet, no percussion, and support underneath. Drilling one in place is a genuine risk of a very expensive crack.
• For a hearth, a sill or a stone worktop, a hole close to an edge is much more likely to split the stone than one in the middle. Move the fixing if you can.
• On old lime-mortared stone walls, the joints are often much easier to drill than the stone and, unlike modern brickwork, are sometimes the intended place for a fixing. What the joint will hold is a separate question.

Common mistakes

• Using hammer action on granite or slate — Percussion in hard or laminated stone starts a crack rather than a hole, and the crack follows the stone's own weaknesses across the whole piece.
• Drilling a slab that is not fully supported — The stone flexes at breakthrough and cracks. It is the single most common way a hearth or sill is ruined.
• Dry drilling with a diamond bit — The bond holding the diamond overheats and breaks down, and an expensive bit is ruined in one hole.

Questions people ask