# Replacing knob and tube wiring in San Francisco plaster walls

> Replacing knob and tube wiring in plaster walls without removing the plaster: fishing routes, small access openings and patching as its own quoted line.

URL: https://fogcitysfknobandtube.com/plaster-wall-rewiring/

**Service**

New circuits fished through lath-and-plaster walls from the attic, basement and garage, with small openings that are patched as an itemized line.

Last Updated: October 30, 2026

**Where**

San Francisco and the East Bay: Berkeley, Oakland, Alameda and Piedmont.

**Local rules**

- An electrical permit from DBI before new wiring goes in, then a pre-cover and a final inspection.San Francisco Electrical Code 89.127
- A separate permit for each dwelling unit, except in single-family (R-3) buildings.DBI fee schedule
- Only owner-occupants of a single-family home may pull their own electrical permit.SF Department of Building Inspection
- No insulation around concealed knob and tube without a licensed electrical contractor's certification.California Electrical Code 394.12
- Up to $2,000 toward active knob and tube for income-qualified PG&E customers in an EASE Home project.BayREN

[Fog City SF Knob and Tube](https://fogcitysfknobandtube.com/) replaces knob and tube wiring in lath-and-plaster houses across San Francisco and the East Bay without stripping the walls. The company plans the routes, manages the job and closes the openings; the electrical work is carried out by licensed and insured electrical contractors. Plaster patching is included in every quote as its own itemized line.

Plaster is a reason owners put a rewire off. The fear is a house opened stud to stud for months. The method works differently: new cable travels through the cavities the old wiring already used, entering from the attic, the basement or the garage, and the walls are cut only where a run cannot get past an obstruction. How many openings a house needs comes down to its access, not its age, which is why the plan starts with a test of every circuit and a route for each one. For the wider choice between [whole-house rewiring](https://fogcitysfknobandtube.com/whole-house-rewiring/) and replacing only what is still live, see [partial knob and tube replacement](https://fogcitysfknobandtube.com/partial-knob-and-tube-replacement/).

## Can you rewire a house without removing plaster?

Yes. Where the cavities can be reached from above or below, the plaster stays where it is. New cable is fished through existing stud and joist bays, and small openings are cut only at boxes, fixtures and the points where blocking, a turn or a floor stops the fish tape. Each opening is patched afterward. What cannot be promised is zero openings, because every new box is cut into the wall.

The method is old and well understood. A fish tape or fiberglass rods carry the cable along a cavity, long flexible bits drill through a top or bottom plate from the attic or basement, and an inspection camera looks into a bay before anything is cut. In a house with an open attic and an unfinished basement or garage ceiling, many wall circuits can be reached that way with only the box openings. The middle floor of a two-story house, or a ceiling with living space both above and below, is where extra openings concentrate.

## What happens on the first visit

The first visit tests every circuit and records what each one runs on: knob and tube, cloth-insulated cable, later cable, or a mix joined by splices. Then each circuit gets a route. The visit checks whether the attic is walkable, whether the basement or garage ceiling is open or finished, which walls stack between floors, and where closets or a stacked bathroom could carry a riser.

The finishes are noted as carefully as the wiring: plaster condition, existing cracks, picture rails, crown moulding, ceiling medallions, original baseboards and built-ins. Each one changes where an opening can go. The result is a written scope and price that lists each opening by room and location, and who closes it. In DBI’s electrical permit data, 187 of the 2,023 knob and tube permits filed from January 2016 to October 2026 mention a basement, crawl space or garage, the routes this plan relies on most. A [knob and tube inspection](https://fogcitysfknobandtube.com/knob-and-tube-inspection/) covers the testing on its own when no work is planned yet.

## How new cable gets through a plaster wall

A stud cavity in an older house is rarely a clean tube from floor to ceiling. Many walls have a horizontal block partway up, and floors and ceilings interrupt every vertical run. A run is planned around those interruptions. An exterior wall on the top floor is usually easy: drill down through the top plate from the attic and drop the cable to a new box. A first-floor wall over a garage is similar from below. Where the attic is insulated, the old runs up there are dealt with first; [knob and tube under attic insulation](https://fogcitysfknobandtube.com/knob-and-tube-attic-insulation/) explains why. A wall on a middle floor may need one small opening at the blocking and another where the cable turns into the ceiling.

Ceilings are harder than walls because the joists run one way and the light fixture often sits across them. Where there is no attic over a room, a ceiling run may follow a single joist bay to an outside wall, or it may need a narrow opening at each joist it crosses. That is why the route plan names each ceiling path separately. Old runs that are retired along the way are disconnected at both ends; San Francisco’s electrical code requires unused conductors to be removed or identified and terminated, which [knob and tube removal](https://fogcitysfknobandtube.com/knob-and-tube-removal/) covers.

## Plaster and lath vs drywall

Plaster behaves differently from drywall when it is cut. Traditional plaster is applied in coats over strips of wood lath, and it holds to the wall because the first coat squeezed between the strips and hardened behind them. Break that grip carelessly and the plaster around the opening loosens and cracks. So openings are scored and cut, not knocked out, and the lath is cut cleanly so the patch has something firm to bond to.

Drywall is faster on both ends: it cuts cleanly and patches with a backed piece and joint compound. Many San Francisco houses have both. A 1920s house may have original plaster in the front rooms and drywall wherever a later kitchen or bathroom remodel opened the walls, and SF Planning’s Sunset District context statement notes that by the late 1930s large merchant builders organized crews for sheetrock as well as framing. Where a house turns out to have drywall in some rooms, those routes cost less to open and close. The access plan follows the actual wall material room by room, not an assumption about the year.

## Picture rails, mouldings and original trim

Older houses carry finishes that cannot be replaced from a hardware store, and those finishes decide where openings go. Openings stay clear of ceiling medallions, cornices and decorative plaster. Where a baseboard can come off and go back, the cable may run behind it, so the patching hides under original trim. A picture rail can sometimes cover a horizontal run at the top of a wall. Built-in cabinets and paneled wainscot are worked around rather than through. Houses with the most ornament usually need the most planning; [Victorian and Edwardian rewiring](https://fogcitysfknobandtube.com/victorian-edwardian-rewiring/) covers the building forms behind those finishes.

## Patching is a line in the quote

Every opening is patched, and the patching appears in the quote as its own itemized line. Writing it down settles a question homeowners report causing friction after a rewire: who closes the holes, and how well. Homeowners report bids that leave the openings for the owner’s own plasterer; here, closing them is part of the scope.

Patching waits for the inspector. San Francisco requires a [pre-cover inspection](https://fogcitysfknobandtube.com/guide/san-francisco-electrical-permit/) before any wiring is covered or concealed, and DBI can order uninspected work exposed at the owner’s expense. Once the pre-cover sign-off is in, each opening is backed, filled and brought flush with the surrounding wall. The quote states that finish. Paint is a separate choice, so it is either written into the scope or planned with the owner’s painter. The [guide to patching after a rewire](https://fogcitysfknobandtube.com/guide/do-electricians-patch-walls/) explains the difference between a rough patch, a finished patch and paint, and what each means in a bid.

## Lead-safe openings in pre-1979 plaster

An old plaster wall may carry decades of paint. DBI’s lead safety guide presumes buildings built before 1979 contain lead-based paint unless a certified inspector shows otherwise, and it requires neighbors to be notified at least three business days before covered work, a posted “Lead Work in Progress” sign, dust containment, and no uncontained dry sanding or scraping. The EPA’s Renovation, Repair and Painting rule requires firms paid to disturb paint in homes built before 1978 to be certified and to use lead-safe work practices, and it expressly covers electrical work. Which rules apply to a given job depends on the building and whether it is rented, so that is confirmed per project. The [lead and asbestos guide](https://fogcitysfknobandtube.com/guide/lead-and-asbestos-during-rewiring/) covers what containment adds to the schedule.

## What plaster adds to the cost

For a whole-house knob and tube replacement in a San Francisco single-family home, a typical range for the electrical scope is $20,000 to $50,000 or more, with permits. Plaster pushes a house toward the upper part of that range when many runs need openings, and toward the lower part when the attic, basement and garage carry most of them. Other contractors’ quotes may or may not include patching, which is one reason two bids for the same house can sit far apart.

The labor behind the patching is also priced locally. Bureau of Labor Statistics wage data puts the mean wage for plasterers in the San Francisco metro at $37.07 an hour, against $29.83 nationally. The [San Francisco cost guide](https://fogcitysfknobandtube.com/guide/knob-and-tube-replacement-cost-san-francisco/) breaks down every part of the price, the [cost estimator](https://fogcitysfknobandtube.com/tools/knob-and-tube-cost-estimator/) gives a first number without a call, and the [rewiring quote checklist](https://fogcitysfknobandtube.com/guide/rewiring-quote-checklist/) lists what to compare.

## Living in the house while the walls are open

In a single-family house, a plaster-wall rewire can often be done with the household at home. Work moves room by room, each room is covered and closed off while it is cut, and the dust is contained and cleaned the same day. Some rooms lose power for part of a day while their circuits change over, and the [rewiring timeline guide](https://fogcitysfknobandtube.com/guide/how-long-does-it-take-to-replace-knob-and-tube/) shows how long each stage usually runs. In a building of flats, power-off windows are scheduled and announced to residents in advance. The [guide to living at home during a rewire](https://fogcitysfknobandtube.com/guide/living-at-home-during-rewiring/) covers the day-to-day.

**Scope**

## What's included

- A test of every circuit, marking which run on knob and tube and which on later wiring
- A route plan for each circuit that names the path, the openings it needs and where they fall
- Floor and furniture protection and dust control in each room before cutting
- Openings cut in plaster by scoring and cutting, not by breaking, at boxes, fixtures and the tops and bottoms of cavities
- New grounded cable fished through existing cavities, with new boxes cut in where the old devices sat
- Old runs disconnected at both ends, removed where reachable, otherwise identified and terminated
- DBI permit, pre-cover and final inspections, a written circuit record and a labeled panel directory
- Patching of every opening, itemized in the quote, with the finish written down

**Choosing the scope**

## Three ways new cable gets into a plaster wall

| Compare | Fished through an open path | Small access openings | Opening a full wall or ceiling |
| --- | --- | --- | --- |
| How it works | Cable is dropped from the attic or pushed up from the basement or garage into a stud cavity | A few small cuts let cable pass blocking, corners or joists the fish tape cannot reach | A section of plaster is removed to expose framing |
| Where it fits | Exterior and interior walls with a clear cavity above or below | Middle floors, ceilings, cavities with fire blocking or a turn | Plaster already failing, a remodel opening the room anyway, or a run with no other path |
| What is disturbed | Only the box location itself | The box plus a few openings per run | The whole surface |
| Patching | Around the new box | Each opening, itemized in the quote | Replaced as a wall section, usually beyond a patch |
| Typical outcome in San Francisco | Top-floor and garage-level circuits | Most circuits in a two-story house or a stack of flats | Rare outside a remodel |

**Read next**

## Related resources

- [Decision GuideDo electricians patch walls after rewiring?Who closes the openings after a rewire, what the word patch can mean in a quote, and why plaster, lead-safe rules and the inspection schedule shape the answer.](https://fogcitysfknobandtube.com/guide/do-electricians-patch-walls/)
- [Decision GuideCan you live in a house while rewiring is under way?How an occupied rewire is sequenced, what a day without power in some rooms looks like, and when it is better to be somewhere else.](https://fogcitysfknobandtube.com/guide/living-at-home-during-rewiring/)
- [LearnDoes knob and tube wiring have asbestos?What the wiring itself is made of, which materials around it deserve a test before anyone cuts, and how lead-safe rules change the way openings are made and patched.](https://fogcitysfknobandtube.com/guide/lead-and-asbestos-during-rewiring/)
- [Decision GuideWhat should a rewiring quote include?The lines that make two knob and tube rewiring quotes comparable, what each one tells you, and the gaps that turn into surprises later.](https://fogcitysfknobandtube.com/guide/rewiring-quote-checklist/)
- [Cost GuideHow much does knob and tube replacement cost in San Francisco?The ranges, what moves a house within them, and the parts of the price that come from public figures.](https://fogcitysfknobandtube.com/guide/knob-and-tube-replacement-cost-san-francisco/)

**How a job runs**

## From the first call to the Green Tag.

### First call

Tell us what started this: an insurer's letter, an inspection report, a remodel, an attic you want to insulate.

The age of the house, the number of units and any deadline shape the next step.

### Walk-through

Circuits are tested to separate live knob and tube from abandoned runs, and attic, basement and garage access is checked.

Plaster, trim and finishes are noted, and you get a written scope and price.

### The work

The permit is filed with DBI. New cable is fished along the planned routes and openings are kept small.

The pre-cover inspection happens before anything is closed.

### Patching, inspection and the Green Tag

Once the pre-cover inspection passes, the openings are patched. Plaster patching is included and shown as its own line in the quote.

After the final inspection and the Green Tag, old runs are removed or terminated and the panel directory is updated.

**On the job**

## What this work looks like

A pencil-marked rectangle partly cut into a plaster wall at outlet height, with an oscillating tool on the drop cloth below

A steel fish tape running from its reel into a small opening near the top of a plaster wall

A freshly patched rectangle of white plaster near a baseboard, sanded smooth and waiting for paint

Close view inside a small plaster wall opening showing the wood lath strips and plaster keys

**Questions**

## Plaster-wall rewiring: questions

### Can you rewire a house without removing plaster?

Yes, where the stud and joist cavities can be reached. The plaster stays on the walls. New cable is fished through the existing stud and joist cavities from the attic, basement or garage, and small openings are cut only where a run cannot get past blocking, a corner or a floor. Each opening is patched afterward, and that patching is its own line in the quote.

### Why does every new outlet still need a small opening in the plaster?

Because each outlet, switch and light needs a box, and cutting in the box is itself an opening. Runs between floors, around fire blocking or across ceilings usually need a few more. A quote that promises no openings at all is either leaving old wiring in place or has not traced the routes yet.

### Do plaster walls make a knob and tube replacement cost more?

Usually, yes. Plaster on wood lath is slower to cut cleanly than drywall, it cracks if it is broken out instead of scored, and it takes longer to patch. The share of circuits that can be fished from the attic, basement or garage matters more, because each run reached that way needs no extra openings.

### Is lead paint a concern when plaster is opened for new wiring?

It can be. DBI presumes buildings built before 1979 contain lead paint unless a certified inspector shows otherwise, and the EPA requires firms paid to disturb paint in pre-1978 homes to be certified and follow lead-safe practices. Openings are cut with containment and cleanup planned in, not with dry sanding.

### Does patched plaster come back ready for paint?

Patching is included, as its own line in the quote, and the line says how each opening is closed and to what finish. Paint is a separate decision: if you want the patched areas painted, ask for it to be written into the scope or plan it with your own painter, so nothing is assumed.

**Related services**

## Other work on the same wiring

### Whole-house rewiring

Every old branch circuit replaced, new circuits added where the house needs them, and the panel sized for the load.

### Victorian and Edwardian rewiring

Knob and tube replaced in pre-1906 Victorians and post-fire Edwardian flats, routed around ornament and stacked units.

### Partial knob and tube replacement

The circuits still running on knob and tube replaced from the panel out, with a written record of exactly what remains.