What's behind your wall before you drill

Lag bolts go two and a half inches into a stud. Most of the time there's nothing in the way. The times there is, it's expensive — so it's worth two minutes to read the wall first.

Electrical cable

House wiring usually runs vertically along the side of a stud to switches and outlets, and horizontally through holes drilled in the middle of the studs, often around outlet height or about four feet up.

• An outlet or switch directly above or below your drilling spot means a cable is probably stapled to that stud.
• Cables are supposed to sit back from the stud's face, and where they pass close, a steel nail plate should protect them. A bolt that stops dead on metal after half an inch may have hit one. Don't force it.

Plumbing

Supply and drain pipes live in "wet walls". Be careful if the other side of the wall is a kitchen, bathroom or laundry, or if there's a bathroom directly above. Vent pipes go straight up through the wall to the roof.

Ducts and returns

A supply register in the floor or ceiling near the wall, or a return-air grille on it, suggests ductwork in the cavity. Sheet-metal ducts sound hollow and tinny when tapped.

Gas lines

Next to a gas fireplace there's a gas line somewhere in or below the chase, usually low and to one side near the valve or key. Above the mantel is normally clear, but check for the shutoff location first.

Chimneys and flues

Over a fireplace, a metal flue runs up inside the chase. Framing must keep clearance from it, which is why there's often solid wood left and right but nothing in the middle. Bolt into the framing, never toward the flue.

Low-voltage wiring

Coax, network and alarm wires can run anywhere. Hitting one won't hurt you, but it can take out your internet.

Tools that help

• A stud finder with AC detection warns about live cable — treat it as a hint.
• A small inspection hole where the bracket will cover it lets you look or feel inside.
• A borescope camera through that hole shows the cavity properly.

If you do hit something

Stop. For a cable, switch off the breaker and call an electrician. For a pipe, shut off the water at the main. Don't pull the screw out of a pipe until the water is off — it's often plugging the hole.

We check for all of this before drilling on every job. Labor starts at $94.