MR Construction
Is old plaster worth repairing?
Yes, old plaster is usually worth repairing when it is still attached to the lath, because three-coat plaster is strong and durable, resists fire and reduces sound transmission, and replacing it is expensive.
Those points come from the National Park Service's Preservation Brief 21 on repairing historic flat plaster, which also notes that plaster is often not as badly damaged as it first appears. A wall covered in cracks can still be structurally sound. The question to answer before spending money is not how bad it looks, but whether it is still holding on.
What is the difference between a crack and failed plaster keys?
A crack is a break in the plaster surface, while failed keys mean the plaster has come loose from the lath behind it, and the second problem is the one that decides between repair and replacement.
On wood lath, wet plaster was pushed through the gaps between the strips and slumped behind them, forming keys that hold the wall in place. When those keys break, the plaster bulges, sounds hollow when tapped, or moves when you press it. Preservation Brief 21 says hairline cracks are not a serious concern when the plaster around them is in good condition, and that a loose ceiling usually means the keys have broken.
- Solid when pressed, fine cracks only: repair and paint
- Flexes, bulges or sounds hollow in a small area: reattach, then repair
- Loose over a large area, crumbling or soft: replace that section
What kind of lath is behind my plaster?
Plaster in older homes sits on one of three bases: wood lath, metal lath or rock lath, which is a gypsum board made as a plaster base.
Preservation Brief 21 describes wood lath as narrow, thin strips with gaps between them, metal lath as having many more openings that hold plaster better, and rock lath as compressed gypsum with a paper facing, in use as early as 1900 and used almost exclusively in residential plastering by the late 1930s. You can often see which one you have inside an outlet box, from the attic or in the basement ceiling. It matters because it changes how loose plaster is reattached and how a patch is built up to the right thickness.
How is loose plaster reattached?
Loose plaster is usually reattached with screws and plaster washers driven through the plaster into the lath or framing, as long as the plaster around the loose area is in reasonably good condition.
Preservation Brief 21 describes this method using flat-head wood screws and plaster washers, followed by fiberglass mesh tape over the cracks and a skim coat over the tape and washers. For cracks that open and close with the seasons, the brief describes widening the crack slightly, bridging it with mesh tape and building it up in coats of setting compound. Simply filling a moving crack with spackle tends to let it come back.
When does a skim coat make sense?
A skim coat makes sense when the plaster is sound but the surface is rough, patched or covered in fine cracks, because a thin coat over the whole wall gives an even surface for paint without demolition.
Loose areas are secured and larger cracks taped first. Then the whole wall or ceiling gets a thin coat of compound or veneer plaster and is smoothed for primer. It costs more than painting over the cracks and much less disruption than tearing out the wall. For a ceiling that is too cracked to patch but not water damaged, the brief also describes covering it with furring strips and new lath and plastering over it, which it notes creates much less dust than demolition.
When should plaster be replaced with drywall?
Plaster should be replaced when it is loose over a large area, crumbling, or badly damaged by long-term moisture, because repairs over a failing or wet base will not last.
When a section comes out, the new wall has to finish flush with the plaster next to it and keep the same reveal at the window and door trim. Preservation Brief 21 lists typical plaster thickness on wood lath at about 3/4 to 7/8 of an inch, which is thicker than a single sheet of common drywall, so new board is often shimmed or doubled to match. The brief itself favors veneer plaster over drywall for historic replacement work, since it better matches a hand-troweled finish. For most homes that aren't historic restorations, drywall blended carefully into the old plaster is the common choice.
What drives the cost of plaster repair versus replacement?
The cost of plaster work is driven mostly by how much area is loose, how much has to come out, and how much finishing it takes to make old and new surfaces look like one wall.
Repairing a few cracks is a small job. Reattaching a sagging ceiling, skim coating a whole room, or tearing out walls and hauling away heavy plaster debris are much larger ones. Demolition also means more dust control, more protection of floors and trim, and sometimes work on lath, insulation or wiring that's exposed once the wall is open. Ask for a price that separates repair from replacement so you can compare them on the same wall.
- Square footage of loose or damaged plaster
- Repair and skim versus full tear-out and disposal
- Ceilings versus walls, and room height
- Dust containment, especially if lead paint may be present
- Trim that has to come off and go back on
In Wakefield and nearby towns
Plaster walls are common in the older houses found across Wakefield, Melrose, Winchester, Medford and the other towns MR Construction Revolutions serves. The company repairs plaster walls and ceilings, skim coats them, and blends in drywall where a section can't be saved. At a free in-home estimate, you can press along the cracks together and see which parts are sound before deciding what to spend.
Questions homeowners also ask
Sometimes. Laminating thin drywall over sound plaster avoids demolition dust, but it adds thickness, so outlets, window and door trim and baseboards have to be adjusted to keep the reveal right. The plaster underneath has to be firmly attached first, or the new board is fastened to a wall that is still moving.
It can be. Preservation Brief 21 warns that plaster demolition dust may contain coal soot, lead from old paint, and possibly asbestos, which was used in the mid-twentieth century as an additive. The EPA lists patching compounds and textured paints on walls and ceilings among products that may contain asbestos, and says you generally can't tell by looking. Have suspect materials tested by an accredited professional before demolition.
Usually because the plaster around them is moving. Preservation Brief 21 notes that horizontal cracks are often caused by wood lath expanding and contracting with humidity, and that diagonal cracks can point to settling. Taping the crack with mesh and setting compound after reattaching any loose plaster holds up far better than filler alone.
Plaster generally does. Preservation Brief 21 notes that three-coat plaster reduces sound transmission. That is one reason some homeowners keep plaster on shared walls and bedroom walls even when they replace damaged sections elsewhere.
They can. Most old plaster walls have decades of paint on them, and in homes built before 1978 that paint may contain lead. The EPA's Renovation, Repair and Painting rule requires paid contractors who disturb paint in pre-1978 housing to be certified and use lead-safe practices, so ask how dust will be contained.