Home theater wiring — do it before the drywall

Fix the seating position before anything else

Every speaker location, the screen height, the projector throw and the cable lengths all derive from where people sit. That position is set by the room and the furniture, and it is the one input that cannot be changed later without invalidating the rest.

Watch for: A pre-wire drawn from a layout that has not been confirmed with whoever is furnishing the room. Cable stubs that end up behind a sofa two feet from where they should be are useless and they are already inside the wall.

Deciding the layout and how many positions to provide for

Speaker positions are chosen for the seating and the room, including whether height or overhead channels are wanted — those are in-ceiling and they are effectively impossible to add sensibly afterwards. Provision is normally made for more positions than the current system uses, because the cable is the cheap part.

Watch for: Wiring exactly the system being bought this month. Adding two more cable runs during a pre-wire costs almost nothing; adding them in two years costs the whole access job.

Choosing in-wall rated cable — a code matter, not an upgrade

Cable installed inside a wall or ceiling cavity has to carry the appropriate in-wall rating for its jacket, and cable in a plenum space has a stricter requirement again. This is a fire-safety provision about how the jacket behaves in a fire, and it is not the same as the cable a retailer sells for connecting a speaker across a floor.

Watch for: Ordinary cable pushed into a wall because it was already there. It is a code violation, it is a legitimate finding at inspection or sale, and it is one of the most common things found behind finished walls in this trade.

Planning the video and control runs

Long video runs are not simply longer cables — beyond a certain distance they need a different approach, whether an active cable, a fibre-based one, or an extender over network cable. The route also carries control: infrared, network for the equipment, and a return path for audio from the display.

Watch for: A single video cable pushed into a closed wall with no conduit. If it fails, or a future format needs more than it can carry, the only remedy is opening the wall. A conduit or even a pull string alongside it removes that risk entirely.

Providing power at both ends

A wall-mounted display needs power behind it, and a ceiling projector needs power at the ceiling. Both are electrical work, and both are far cheaper while the wall is open. A recessed outlet behind a display also lets it sit flatter, which is usually the reason the wall mount was chosen.

Watch for: An extension cord run inside a wall cavity to feed a display. It is not permitted, it is common, and it is exactly the kind of thing that turns up during a home inspection.

Terminating and dressing at the plates

Runs land on brush plates or terminated wall plates at each position, with a service loop left inside the cavity so a plate can be pulled forward and worked on. Both ends are labelled, because a bundle of unmarked speaker cable at the equipment end is a genuinely tedious problem later.

Watch for: Cable cut to exact length at the plate. A short tail cannot be re-terminated, and it turns a five-minute repair into opening the wall.

Retrofitting into a finished room, when pre-wire is not possible

Where the walls are closed, the honest options are fishing through the cavity where the construction allows, running in trunking or a moulding channel, going under a floor or through a loft, or accepting wireless for the rear channels. Each has a real trade-off and a competent installer will present all of them rather than only the one they prefer.

Watch for: A retrofit priced without an inspection of the cavity, the floor structure and the space above. Fire blocking and insulation change this job completely and neither is visible from the doorway.

Testing and commissioning before making good

Every run is checked for continuity and polarity before anything is closed, and video and extender links are proved with the actual equipment where possible. Speaker polarity reversed on one channel is easy to do and produces a system that sounds subtly wrong in a way that is very hard to diagnose afterwards.

Watch for: Runs closed up and tested at the end. Finding a fault after the plaster is on is the expensive version of a two-minute problem.