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Plaster and Lath Repairs: Patching Plaster With Drywall, Matching the Thickness, Loose Plaster and Lost Keys, Skim vs Board-Over, and When to Gut the Room
Drywall, Taping & Painting
Plaster and Lath: How to Patch It With Drywall, How to Save It When It Is Loose, and the Point at Which You Board Over the Whole Room
Houses built before about 1955 in Ontario have plaster on wood lath: three coats of lime or gypsum plaster pushed through the gaps between strips of wood and keyed behind them. It is hard, it is heavy, it is quieter and more fire-resistant than drywall, and when it fails it fails in a way drywall never does: the keys break off behind the lath and whole sections go loose and hollow. You can patch plaster with drywall, you can re-secure loose plaster, you can skim the cracks, and you can board over the lot. Here is which one, when, and what each costs.
The four repairs
| Problem | Repair | Ontario cost |
|---|---|---|
| A hole, from a plumber or a doorknob, plaster around it sound | Cut the hole square to the lath, screw a drywall patch to the lath shimmed out to the plaster face (plaster is 5/8 to 3/4 in. thick; 1/2 in. board plus a shim, or 5/8 board), setting compound in the gap, mesh tape, three coats feathered wide. | $350 to $600 a patch |
| Plaster loose and hollow when you knock, but not falling | Plaster washers (perforated discs on screws) through the plaster into the lath or studs every 8 in. across the loose area, then skim over the washers; or the adhesive method, injecting construction adhesive through drilled holes and clamping until set. Both work; the adhesive is neater. | $8 to $12 per sq ft of loose area, plus the skim |
| Cracks everywhere, surface sound | Cut the cracks, mesh tape, and a full skim coat over the wall. Sometimes a fibreglass mesh over the whole wall under the skim (the “Nu-Wal” type systems) for walls that keep cracking. | $4 to $6 per sq ft |
| Loose, cracked, patched, crumbling, or you want flat walls and are not sentimental | Board over: 3/8 or 1/2 in. drywall screwed through the plaster into the studs (find them with a magnet on the lath nails), taped and finished. Casing and baseboard get extension or come off and go back. Or gut it: lath and plaster out, insulation and wiring while the wall is open, new board. | Board over: $3.50 to $4.50 per sq ft. Gut and re-board: $6 to $9 per sq ft including demolition and disposal; plaster is heavy and the dumpster costs. |
When to save it and when not to
Plaster is worth saving when it is mostly sound, when the house has mouldings and details that would be lost in a gut, and when the walls are the character of the place. Washers and a skim coat make a 1920s wall last another fifty years for $5 a square foot. Board-over is the middle road: you keep the plaster’s mass and quiet, lose 1/2 in. of room, and get flat modern walls that take paint properly. Gutting is the answer when the walls need insulation and wiring anyway, when the plaster is falling, or when more than a third of a room is loose; at that point you are paying to save something that is mostly gone, and the open wall is worth more than the plaster.
Two tests before any sanding in a pre-1980 house: plaster and any later joint compound may contain asbestos (Ontario’s regulation, $150 a sample), and paint from before 1978 may contain lead ($40 swab test, or a lab). Wet methods and containment for either; abatement if the plaster is positive and you are gutting. Asbestos in drywall.
Where homeowners go wrong
- Patching a plaster hole with a drywall patch flush to the lath. It sits 1/4 in. below the plaster and shows forever. Shim it out.
- Filling cracks with compound and painting. Plaster cracks move. Tape them or they are back by spring.
- Screwing board-over into the lath. Lath holds nothing. Studs, found with a magnet, and screws long enough to get through plaster, lath and an inch of stud: 2-1/2 in.
- Gutting a plaster room in a dry-sanding frenzy. Test first, wet methods, containment, and a dumpster that is rated for the weight.
- Hiring a drywaller who has never seen plaster. Ask. The ones who have will tell you about the washers.
Questions people ask
Can you patch plaster walls with drywall?
Yes. Cut the hole square to the lath, shim a drywall patch out to the plaster surface, fill the gap with setting compound, mesh tape and three coats feathered wide. It disappears if the thickness matches.
How do you fix loose plaster?
Plaster washers screwed through to the lath or studs every 8 in., or construction adhesive injected behind the plaster and clamped, then a skim coat over the area. Both re-attach plaster whose keys have broken.
Is it better to drywall over plaster or remove it?
Board over when the plaster is mostly attached and you want flat walls without the mess; remove it when you need to insulate or rewire, when it is falling, or when more than a third of the room is loose.
How much does it cost to replace plaster with drywall?
$6 to $9 per square foot in Ontario in 2026 including demolition and disposal, versus $3.50 to $4.50 to board over, versus $4 to $6 to skim coat sound plaster.
Price the room three ways
The repair calculator prices patches and skim coating; the drywall installation calculator prices the room boarded, with the occupied-house factor. The demolition cost page covers the gut.
Sources and further reading
- Asbestos on construction projects (O. Reg. 278/05): Ontario Ministry of Labour. Drywall joint compound and textured ceilings in older houses are named as materials that may contain asbestos; the regulation says what has to happen before anyone sands or scrapes them.
- Gypsum Association construction standards: GA-216 application and finishing (the hanger’s bible: fastener spacing, ceiling framing limits, joint treatment) and GA-600 fire resistance design manual, with the free downloads.
- CGC Inc.: the maker of most of the board and compound in Ontario; the product sheets give thicknesses, span limits for ceilings and the right compound for each coat.
