Which drill bit for Indian sandstone and porcelain paving?
Indian sandstone looks soft but it is abrasive and layered, and a standard tungsten-carbide masonry bit on hammer action will chip the surface, wander off the mark and often split the slab along a bedding plane. Porcelain paving is the opposite problem: it is so hard and dense that a masonry bit simply polishes the surface. In both cases the right tool is a diamond drill bit used without hammer action.
The short answer
- Indian sandstone, limestone, slate: a dry diamond drill bit with a hex or round shank in a cordless drill on rotary (no hammer). The Marcrist PG750X hex tile drill bit handles 5mm to 14mm fixing holes cleanly.
- Porcelain paving and vitrified tiles: a diamond drill bit made for porcelain. Dry, from an angle grinder with the M14 Marcrist PG850, or from a drill with a diamond tile drill bit set. For larger outlets use a wet-drilling bit such as the PG350.
- Anything 30mm and over (drainage, post fixings, lighting): a small diamond core drill bit rather than a drill bit. See the core drill bits range.
Why masonry bits fail on sandstone
A masonry bit relies on hammer action to smash the material ahead of the tip. Sandstone is made of cemented grains in layers, so each hammer blow shocks the slab and the exit side of the hole spalls or the slab cracks. A diamond bit grinds instead of hammering: it runs on rotary only, at moderate speed, and produces a clean round hole with no shock to the stone.
Dry or wet?
For fixing holes up to about 14mm in sandstone, dry diamond bits are the practical choice: they work in a cordless drill on site and do not need a water feed. Keep the bit moving in and out of the hole every few seconds so dust clears and the tip stays cool. Porcelain is harder on the diamond, so either use a bit designed for dry porcelain drilling, or drill wet with a water feed or cooling gel for larger diameters and long runs of holes.
How to drill without cracking the slab
- Switch the drill to rotary only. Never use hammer action with a diamond bit.
- Support the slab fully on a flat bed. Drilling an unsupported corner is the most common cause of cracks.
- Start the hole at an angle of about 45 degrees to create a crescent, then bring the drill upright. A tile drill guide makes this easier on glazed porcelain.
- Use light pressure and let the diamond do the work. Forcing the bit heats it and shortens its life.
- Ease off as the bit breaks through so the back of the slab does not spall.
Which size?
Most paving fixings, handrail brackets and light fittings need 6mm, 8mm or 10mm holes, which is why a tile drill bit set is the best value for the first job. For waste outlets, cable ducts and post sockets from 32mm to 152mm, use a diamond core drill bit with a pilot or a guide. If you are unsure which bit suits your slab, the tile and porcelain drill bits collection is grouped by material and shank type.