Core drilling explained

Core drilling bores a precise, round hole through hardened concrete — the method behind everything from a 2-inch plumbing vent to a 12-inch HVAC duct penetration. Here's what it is, how it's done, and what to expect if you need one.

What core drilling is

Core drilling cuts a clean, round hole through a concrete slab, wall, or footing, typically to pass something through it: plumbing pipe, electrical conduit, HVAC ductwork, or to set an anchor, dowel, or railing post. Unlike a saw cut, which follows a line, a core drill removes a full cylindrical "core" of concrete in one pass, leaving a hole with clean, consistent walls.

How it works

A diamond-tipped core bit — a hollow steel tube with diamond segments brazed to the cutting edge — is mounted on a rig anchored to the concrete for stability and spun at high speed against the surface. Nearly all core drilling is done wet: water flows through the bit to cool the diamonds and flush cuttings out as slurry, rather than airborne dust. That's what lets crews core indoors, near finished spaces, without a dust cloud.

Typical diameters

Standard core bits run roughly 1 inch to 12+ inches in diameter, which covers the large majority of residential and commercial jobs — plumbing and electrical penetrations, HVAC runs, anchor bolts, and dowel holes for rebar connections. Larger specialty rigs can core openings up to around 60 inches for stairwells, elevator shafts, or major utility penetrations, though that's a different scale of equipment and not every shop carries it.

Horizontal, vertical & overhead

Rigs can be set up to core straight down through a floor or slab, horizontally through a wall, or overhead through a ceiling — each orientation needs a different anchoring approach to keep the bit steady and the water/slurry contained, which is part of why this is specialist equipment rather than a rental-counter tool for most hole sizes.

Homeowner vs. contractor jobs

A homeowner might need a single core hole for a new hose bib, a dryer vent, or a conduit run. Contractors call in core drilling far more often — mechanical, electrical, and plumbing subs routinely need multiple penetrations coordinated around a pour schedule or retrofit. Either way, the process and the equipment are the same; it's mostly a question of scale and scheduling.

Why reputable shops scan before they drill

Concrete slabs and walls often hide rebar, electrical conduit, or post-tension cables — steel cables under extreme tension that hold certain slabs together. Coring blind into one of those is dangerous and expensive to fix. That's why reputable operators run a GPR (ground-penetrating radar) scan over the drilling area first, to map what's inside the concrete before the bit ever touches it. See our GPR scanning guide for how that scan works and why it matters even on a single-hole job.

Find a core drilling specialist near you