The Five Questions to Ask About a System That Is Not Yours

Why this matters

Most of the damage one trade does to another's system is not done by careless people. It is done by good techs who were concentrating on their own work and treated the pipe, wire or duct in the way as scenery. The insulation crew that buries a vent, the drywaller who screws through a supply line, the siding crew that pulls a meter base loose: each one goes home thinking the job went well, and the callback lands on whoever touched the house last. Five questions, asked before you work near anything you did not install, catch nearly all of it.

The five questions

Ask them in this order, because each answer limits the next.

1. What is it? Name the system and, if you can, the part. "A gas line" is a start; "a half-inch black steel drop to the dryer, with a shutoff valve" is what lets you answer the rest. If you cannot name it, that is an answer too, and it changes question 4.
2. Where does it run? Systems are networks, not objects. The thing you can see came from somewhere and goes somewhere, often inside the cavity you are about to close, cut or fill. Trace it as far as you can without opening anything.
3. What can I damage? Name the specific failure your work could cause: a puncture, a crushed run, a pulled joint, a buried access point, a blocked opening, heat, vibration, overspray, a lost label. Be literal. "I could hurt it" is not an answer; "my 2-1/2 inch screw could reach it" is.
4. What must I not touch? Draw the line between what any occupant may operate and what only the owning trade may open. The line is drawn below.
5. Who do I call? Name the trade, the utility, or the emergency service, and decide when: now, before you continue, or on the handoff at the end of the day.

The order matters most between 3 and 4. A tech who jumps straight to "do not touch it" protects the system from his hands and still damages it with his work: he never touches the duct boot, and his blown insulation buries it.

Where recognizing ends and repairing begins

This shelf teaches recognition and protection. It never teaches another trade's repair, and the reason is not only liability.

You may operate what an occupant may operate. A fixture shutoff valve, a breaker you are switching off for your own work, a thermostat, an appliance gas shutoff closed on your way out when you smell gas at that appliance and the valve is directly in reach, a manifold port valve, a water main in an active leak. These are designed for people with no license.

You may not open, alter or reassemble the system itself. That means a panel dead front, a meter base, a gas meter seal, a refrigerant service port, a draft hood, a vent connector, a trap, a sealed telecom box, a sprinkler head. You may not reconnect what you disconnected, even if you watched it come apart and it looks obvious.

The edge case is the one you created. You pulled a duct joint loose, you nicked a line, you knocked a vent off its collar. The temptation is to fix it before anyone notices. Do not. Your repair hides the damage from the trade that has to warrant the system, and an amateur gas or vent repair can fail in a way that kills someone. Report it the same day, in writing.

Licensing law adds a second reason. Most states license some combination of electrical, plumbing, gas and mechanical work, and doing licensed work without the license can expose the shop to fines and make the work uninsurable; the scope and penalties vary by state and sometimes by city. This is not legal advice, and your state contractor board or your attorney has the real answer for your trade.

Why protecting it is part of your job

Three practical reasons, none of them about being nice.

• You will be blamed whether or not you did it. The last trade in the house owns every new problem in the customer's mind. A photo of the condition before you started is cheap insurance.
• Your work can be undone by theirs. If you bury a junction box under new tile, the electrician who later has to open it opens your tile.
• Your general liability policy may not see it the way you do. Damage to a system you were working near is often exactly what a claim is about. Ask your agent how your policy treats it; do not assume.

Worked example: one attic, three systems

An insulation crew is booked to bring an attic in a 1970s ranch house up to a deeper blown cellulose layer, about 14 inches over the existing 4. Before blowing, the lead walks the attic with the five questions. He finds three systems that are not his.

System 1: a round metal pipe rising from the hall closet below through the roof.

• What is it: a double-wall metal gas vent, the label on the pipe reads Type B. It carries flue gas from the water heater.
• Where it runs: from the closet ceiling straight up through the roof. No horizontal run in the attic.
• What can I damage: cellulose packed against it. The pipe's own label states a clearance to combustibles, commonly 1 inch for this type; cellulose is combustible.
• Must not touch: the pipe, its joints, the firestop where it passes the ceiling.
• Who to call: nobody, if he builds a noncombustible dam that holds the stated clearance and rises above the final insulation depth. He photographs the label and the dam.

System 2: six round can lights in the hall and kitchen ceilings.

• What is it: recessed light fixtures. Four carry a label reading IC (rated for insulation contact); two are unlabeled and look older.
• Where it runs: each has a cable leaving it toward a junction box on a joist.
• What can I damage: an unrated can buried in insulation overheats. NEC 410.116 (2023 edition, as adopted) requires a 3 inch clearance from insulation for a can that is not IC rated, and no insulation above it.
• Must not touch: the fixtures and their wiring.
• Who to call: the electrician, for the two unlabeled cans. In the meantime the crew dams them at 3 inches and leaves them open above, which is the protective action available to them.

System 3: porcelain knobs and two separate wires running along the joists near the attic hatch.

• What is it: knob and tube wiring, original to an earlier part of the house or left from a remodel. He cannot tell whether it is live.
• Where it runs: from the hatch area toward the bedrooms, about 22 feet of visible run.
• What can I damage: this wiring relies on open air around it to shed heat. NEC 394.12 (2023 edition, as adopted) does not permit it in hollow spaces where it would be surrounded by loose or blown insulation, and a few jurisdictions amend that rule or allow it after an electrician's evaluation.
• Must not touch: the wires, the knobs, any splice.
• Who to call: the electrician, now, before blowing. This one stops the job for that part of the attic.

The tally. Three systems found. One is protected by the crew's own dam and needs no call. Two need the electrician: the two unrated cans can be dammed and blown around today, and the knob and tube area cannot be blown at all until the electrician answers. The lead calls the office, the office reschedules the bedroom half of the attic, and the crew blows the remainder that afternoon. The customer is told in plain words why half the attic is waiting.

What a crew without the questions does. It blows the whole attic. The B-vent sits in cellulose, the old cans run hot inside it, and the knob and tube wiring is buried. Nothing fails that day. That is the problem.

What changes the answer

You cannot name it. If question 1 comes back blank, treat question 4 as "touch nothing near it" and question 5 as "call before continuing." An unknown pipe could be a dead abandoned line or a live gas line; you do not get to guess. See related: How to Tell Which Trade Owns a Pipe, Wire or Duct You Found.

It is a hazard right now. A gas smell, a hiss, a scorched or arcing electrical part, water near live wiring: the five questions wait. Act first. For gas, everyone leaves immediately, nobody operates a switch, a light or a phone inside, and the call goes to the gas utility or 911 from outside and well away from the house. For water near live electrical parts, stay out of the water and keep everyone else out, and do not reach for a breaker in a wet area; call the electrician and, if anything is smoking, 911.

The customer asks you to fix it. "You're already here, can you just reconnect that?" is the moment the line gets crossed. Decline, say who should do it, and write down that you said so.

How to verify you got this right

• Before you start, each system near your work has a name, a route and a stated risk, even if only in your head. A crew lead should be able to say them out loud.
• Before you close anything up, walk back to every system you identified and look at it again. Is the access panel still reachable, the label still visible, the opening still open, the clearance still held?
• At handoff, anything you reported is written down with a photo, so the next trade does not have to rediscover it. See related: How to Hand Off a Problem You Found in Another Trade's System.
• When you call, use the owning trade's words. See related: Describing a Problem in Another Trade's Own Words.

References

• NEC (NFPA 70), 2023 edition, Articles 394 and 410, as adopted by your jurisdiction, which binds the electrical installation and reaches other trades through the inspection.
• Manufacturer labels on the specific vent pipe and light fixture, which set the clearances the crew must hold.
• See related: Working in Older Homes: A Cross-Trade Field Guide
• See related: Where Ducts, Pipes and Wires May Not Go