# Plaster-Wall Knob-and-Tube Rewiring in Philadelphia

> How knob-and-tube rewiring works through plaster and lath in Philadelphia houses: cable routes, where openings go, why plaster cracks and who patches it.

URL: https://phillyknobandtube.com/plaster-wall-knob-and-tube-rewiring/

Service / Plaster-wall rewiring

New circuits fished through plaster-and-lath walls with planned, mapped openings and a patching scope agreed before the first cut.

Last Updated: October 26, 2026

**2020 NEC** The edition of the National Electrical Code that Philadelphia applies to electrical work

**L&I permit** Pulled by a licensed electrical contractor for rewiring work in the City

**Third-party inspection** Rough-in and final inspections by a licensed electrical inspection agency

[Philly Knob & Tube](https://phillyknobandtube.com/) organizes knob-and-tube rewiring in Philadelphia houses finished with plaster on wood lath, with licensed and insured electrical contractors doing the electrical work. This page is about the physical job: how new cable gets through plaster walls that were never meant to be opened, where the openings go, why plaster sometimes cracks around them, and how the repair is scoped. Plaster-wall knob-and-tube rewiring rarely means tearing walls down, but it does mean some openings, and the quality of the job shows in how those openings are planned and closed.

Plaster is harder to work through than drywall for a simple reason. Drywall is one sheet screwed to the studs; three-coat plaster is a hard, brittle layer locked onto narrow wood strips with gaps between them. A cut in drywall stays where it is put. A cut in plaster can loosen the material around it if it is made carelessly. That difference shapes every decision below.

## What happens on the first visit

The first visit maps routes before it maps openings. The electrician identifies which circuits are still live knob-and-tube and where they go, then looks for the paths a new cable could follow: the open basement ceiling, the attic floor, closets stacked above one another, chases beside chimneys or plumbing stacks, and the space under stair runs. In a rowhouse with an unfinished basement and an accessible attic, many first-floor outlets and top-floor ceiling lights can be reached with little or no wall cutting. The middle floor of a three-story house is usually the hardest, because there is finished space both above and below.

The visit also checks the condition of the plaster itself. Walls that already sound hollow when tapped, show bulges, or have long diagonal cracks above doors are more likely to move when opened. Noting that before work starts protects both sides: the owner knows which walls are fragile, and the patching scope accounts for them.

## Choosing Routes Before Opening Plaster

Most new cable in a plaster house runs vertically inside the wall cavity between two studs, entering from below through the bottom plate or from above through the top plate. An electrician drills up from the basement or down from the attic into the cavity, then feeds cable toward the box location with fish tape, rods or a weighted line. When the cavity is clear from top to bottom, the only opening in the room is the hole for the box.

Cavities are often not clear. Horizontal blocking installed between studs, fire stops at floor lines, a window or door header, a diagonal brace or old plumbing can stop a cable partway. Each obstruction needs a small access hole at its height so the electrician can notch or drill through and keep the cable moving. Long flexible drill bits reduce the number of these holes, though they cannot pass every obstacle. Horizontal runs across a wall are the hardest, because they cross every stud; that is why a route that turns down to the basement and back up is often preferred to one that crosses a room at mid-height.

Ceilings follow their own logic. A light on the top floor can usually be fed from the attic with no ceiling cut at all. A light on a middle floor sits under a finished room, so the choices are to run cable within a single joist bay from a wall, to lift a few floorboards in the room above and drill across the joists there, or to open a narrow strip of ceiling plaster along the route. Lifting boards in a closet or under a rug often costs less in repair than cutting a plaster ceiling, which is harder to patch invisibly than a wall. The old knob-and-tube runs sometimes help: the holes bored through joists for the original conductors can occasionally be reused once the old wire is dead, saving new drilling in places that are hard to reach.

Some walls offer no cavity at all. Where a rowhouse’s party wall or exterior wall is plaster laid directly on brick, there is nothing to fish through. Outlets on those walls are usually relocated to a stud partition, fed from the floor below, or served by a surface raceway. The guide to [replacing knob-and-tube without gutting walls](https://phillyknobandtube.com/guide/replacing-knob-and-tube-without-gutting-walls/) explains which routing options exist and where each one runs out.

## Working Around Lath and Trim

Plaster in pre-war Philadelphia houses is commonly three coats over wood lath: a scratch coat pushed through the gaps between strips, a brown coat to build thickness, and a thin finish coat. The plaster that squeezed through the gaps and curled behind each strip, called keys, is what holds the wall up. Breaking keys is what turns a neat opening into a loose patch of wall.

Electricians who do this work regularly cut plaster in ways that limit vibration. The outline is scored first, then cut with a sharp blade or an oscillating tool set to cut rather than hammer, with the lath strips supported or cut cleanly so they do not spring and shake the plaster around them. Box openings are kept to the size of the box. Old-work boxes, which clamp to the plaster or lath from inside the opening, are used because there is usually no stud at the exact location to fasten to.

Placement does a lot of the work. An access hole near the floor can often sit behind the baseboard, so the patch is hidden once the board goes back. A hole at a doorway can sit behind the casing. A box opening at an existing outlet location reuses a spot that already has a cover plate. Some houses also have later metal lath, a layer of mesh over the old plaster, or a ceiling of drywall laid over the original. Each one changes the cutting method and should be noted on the opening map. For houses where the trim and ornament themselves are the priority, the [historic-home replacement](https://phillyknobandtube.com/historic-home-knob-and-tube-replacement/) page covers how finishes are ranked before routes are chosen.

Paint is the other material in the wall. Painted plaster in a pre-1978 house may contain lead in its older layers, so cutting it creates dust that should be contained and cleaned up rather than swept. Ask the contractor how they handle that, especially if young children live in the house.

## Planning Electrical and Finish Repairs

The patching scope belongs in the proposal, not in a conversation after the walls are open. Three arrangements are common. In the first, the electrician fills each opening flush with patching material and leaves it ready for finishing, which means the patch is solid but visible. In the second, a plaster or drywall finisher follows the electrician to fill, skim and match the surrounding surface and prime it for paint. In the third, the owner arranges the finish work, often because a paint or restoration project is already planned. The comparison table on this page sets those options side by side.

What the finisher does depends on the wall. Small box openings are often closed with setting-type compound over a backer fastened to the lath. Larger access holes may get a drywall plug cut to fit and then skimmed so the edges disappear, or a true plaster repair where the owner wants the patch to match original plaster in hardness and texture. Cracks that opened around a cut are usually taped and skimmed as part of the same visit. Painting is almost always a separate line item, because touching up a patch on a wall painted years ago rarely blends, and the honest answer is often to repaint the wall from corner to corner.

There is no single published price for plaster restoration after rewiring. The cost depends on the number and size of openings, how fragile the existing plaster is, whether a plain flush finish or a matched plaster surface is wanted, ceiling height and stairwell access, and whether painting is included. A proposal that lists each opening makes it possible to price that work properly. The guide to [comparing rewiring quotes](https://phillyknobandtube.com/guide/comparing-knob-and-tube-rewiring-quotes/) shows how to line up two proposals that treat patching differently, and the printable quote checklist on the [resources](https://phillyknobandtube.com/resources/) page lists the questions to ask.

## Inspection and Restoration Sequence

Order matters on a plaster job because the openings cannot be closed until the new wiring is inspected. In Philadelphia, rough-in work has to be inspected before it is covered, and the inspection is performed by a licensed third-party electrical inspection agency hired by the owner or the owner’s agent rather than by an L&I inspector. So the sequence runs: openings cut, new cable fished and boxes set, old circuits disconnected, rough-in inspection, then patching, then devices, fixtures and the final inspection.

Planning around that sequence is what keeps the house livable. Openings typically stay open for a few days between the rough-in and the inspection, and that gap is the main reason to protect furniture and keep work areas contained. Plastic sheeting with a zipper door around the work area, covered floors along the path to the basement, and vacuuming with a filtered vacuum as the cutting happens keep dust in one place. Furniture is moved away from the walls being opened, not out of the house. Circuits are turned off one at a time as they are replaced, so the whole house is rarely without power at once.

The [living at home during rewiring](https://phillyknobandtube.com/guide/living-at-home-during-knob-and-tube-rewiring/) guide covers daily life during the job in more detail, and the [rewiring process guide](https://phillyknobandtube.com/guide/knob-and-tube-rewiring-process/) walks through every stage from survey to final inspection. If the house is already torn open for a kitchen or bath, the sequence changes; the [renovation replacement](https://phillyknobandtube.com/renovation-knob-and-tube-replacement/) page covers that case.

## Cost Drivers for Plaster-Wall Rewiring

Plaster adds labor in two places: the electrical work, because every route takes longer to find and open, and the repair, because every opening has to be closed and finished. The drivers that move a plaster-wall rewire the most are the number of active knob-and-tube circuits, the number of stories, how open the basement and attic are, finished ceilings on middle floors, the number of devices and rooms, any new circuits required by the work, and the patching and painting scope. Occupied-home protection and uncertainty about how much knob-and-tube is still live also move the price.

For whole-house work, a substantial knob-and-tube replacement in a Philadelphia house is commonly budgeted at $9,000 to $25,000. That is a planning range, not a quote from Philly Knob & Tube; it covers replacing the active knob-and-tube branch circuits, and plaster restoration and painting may be excluded from it, along with panel or service work. A house with difficult access or a matched-plaster finish standard can exceed it. The [Philadelphia cost guide](https://phillyknobandtube.com/guide/knob-and-tube-replacement-cost-philadelphia/) breaks the drivers down, and the [cost estimator](https://phillyknobandtube.com/tools/knob-and-tube-cost-estimator/) shows how access and size shift the figure.

Plaster houses can also be rewired in stages. A [whole-home rewiring](https://phillyknobandtube.com/whole-home-knob-and-tube-rewiring/) replaces every live circuit in one project, while a [partial replacement](https://phillyknobandtube.com/partial-knob-and-tube-replacement/) handles the priority circuits first and documents what remains. Either way the same opening map carries forward. Neighborhood pages under [service areas](https://phillyknobandtube.com/service-areas/) describe the housing in each part of the city, the [guide library](https://phillyknobandtube.com/guide/) covers related questions, the [about](https://phillyknobandtube.com/about/) page explains how projects are arranged, and the [contact](https://phillyknobandtube.com/contact/) page is the place to start a route survey.

## What's included

- A route survey from the basement, attic and closets before any wall is opened
- An opening map that shows each planned cut, its size and why it is needed
- New cable fished through existing wall and ceiling cavities for the circuits in scope
- Old-work boxes set in plaster at existing or agreed device locations
- Floor protection and plastic containment around work areas in occupied rooms
- Rough patching or a clear handoff to a plaster or drywall finisher, stated in writing
- Disconnection of the replaced knob-and-tube circuits at both ends
- The L&I permit, rough-in inspection and final inspection

## Who closes the openings after plaster-wall rewiring

|  | Electrician rough patch | Separate plaster or drywall finisher | Owner arranges finish |
| --- | --- | --- | --- |
| What it covers | Openings filled flush and ready for finishing | Fill, skim, match surface and prime | Whatever the owner's painter or finisher agrees to |
| Surface match | Not intended to be invisible | Closest match to surrounding plaster | Depends on who is hired |
| Scheduling | Done right after the rough-in inspection | Follows inspection; adds a separate visit | Owner coordinates after the electrical work closes out |
| Best fit | Rooms that will be repainted anyway, closets, utility areas | Living rooms, stairwells, original plaster the owner wants matched | Owners already planning a paint or plaster project |

## Related resources

- Decision Guide [Can Knob-and-Tube Wiring Be Replaced Without Gutting Walls?](https://phillyknobandtube.com/guide/replacing-knob-and-tube-without-gutting-walls/)Most rewires in plaster houses are done by fishing cable through existing cavities, not by tearing out walls. Here is how routes are found, where small openings still happen, and how to pin down expectations before work starts.
- Learn [What changes when rewiring a historic Philadelphia home?](https://phillyknobandtube.com/guide/knob-and-tube-in-historic-philadelphia-homes/)Philadelphia's housing is old, attached and often designated. Here is how that history affects where knob and tube hides, who reviews the work and how period finishes are protected.
- Decision Guide [Can You Live at Home During Knob-and-Tube Rewiring?](https://phillyknobandtube.com/guide/living-at-home-during-knob-and-tube-rewiring/)Many households stay put through a rewire, and some should not. What the days actually feel like, which parts of the house go dark and when, and how phasing changes the answer.
- Decision Guide [How to compare knob and tube rewiring quotes](https://phillyknobandtube.com/guide/comparing-knob-and-tube-rewiring-quotes/)Two rewiring quotes for the same house can differ by thousands because they describe different jobs. Here is how to put them on the same footing before price decides anything.
- Learn [How Does Knob-and-Tube Rewiring Work?](https://phillyknobandtube.com/guide/knob-and-tube-rewiring-process/)The technical sequence behind a rewire, from tracing which old circuits are live to the moment the last original run is cut loose at the panel and the inspection agency signs off.
- Cost Guide [How Much Does Knob-and-Tube Replacement Cost in Philadelphia?](https://phillyknobandtube.com/guide/knob-and-tube-replacement-cost-philadelphia/)A planning range for whole-home work, a national range for limited jobs, Philadelphia's published permit fees, and the parts of a price that only a property-specific scope can answer.

## What a Replacement Involves

1. Survey Document the property, inspect visible routes, and map what appears to be active.
2. Define Agree on exactly which circuits, access openings and associated repairs belong in the scope.
3. Replace Install the planned replacement wiring through accessible routes and necessary openings.
4. Close out Complete required inspections and document the electrical scope actually performed.

## What this work looks like

Rectangular opening in an old plaster wall showing the thickness of the plaster over horizontal wood lath

Narrow rowhouse hallway prepared for plaster patching with a bench under plastic, drop cloths, a bucket, a trowel and a small wall opening

Smooth painted plaster wall with a blended patch beside dark original wood trim in a sunny Philadelphia room

New white outlet above a dark baseboard next to a fresh unpainted square plaster patch

Before

After

## Plaster-wall rewiring questions

### How many holes does rewiring a plaster wall usually take?

There is no fixed count. A wall with open basement space below and attic space above may need only the box opening itself. A wall interrupted by horizontal blocking, a header or a doorway may need one or two access holes along the route. A good proposal shows each expected opening on a map and says which ones are contingent on what the electrician finds inside the cavity.

### Why does plaster crack beyond the hole that was cut?

Old plaster holds to wood lath through keys, the plaster that squeezed between the strips when it was applied. Cutting and drilling vibrates the lath and can break keys around the opening, so the plaster nearby loosens or cracks. Hand tools, scoring and sharp blades reduce it. Walls that were already loose or bulging before the work are the most likely to crack further.

### Is plaster repair included in a knob-and-tube rewiring quote?

Sometimes, and the wording matters. Many electrical proposals include only a rough patch that fills each opening flush, with skim coating, surface matching and painting left to a finisher or the owner. Ask the proposal to state who closes the openings, to what standard, and whether painting is included, so two quotes can be compared on the same terms.

### Can I stay in the house while the plaster walls are being opened?

Many owners do, with planning. Work usually moves room by room, with floors covered, plastic sheeting around the work area and dust collected as cutting happens. Circuits are switched off while they are being replaced, so some rooms lose power for part of a day. People with respiratory concerns, and households with young children in a pre-1978 house, should discuss dust and paint handling before work starts.

### Can new outlets be added in a plaster room during rewiring?

Yes. Adding receptacles is often easiest at the same time as replacing the old circuits, because the routes are already being opened and mapped. Each new location still needs a path for cable and an opening for the box, so it adds to the opening map. Placing new outlets on interior stud walls rather than plaster-on-brick walls usually keeps the work simpler.

## Related services

### Historic homes

Replace live knob-and-tube in an older Philadelphia house with a plan that decides, room by room, which period finishes are touched and which are left alone.

### Whole-home rewiring

Replace every active knob-and-tube branch circuit in the house, with the permit, inspections and a written record of what changed.

### During a renovation

Replace knob-and-tube that a remodel uncovers while the walls are open, with the electrical permit, rough-in inspection and other trades sequenced around it.