Patching Plaster and Lath

Plaster was the wall surface in nearly all buildings until the 1930s or ’40s, and the National Park Service makes a strong case for keeping it: three-coat plaster is unmatched in strength, replacing it is expensive, and it’s often much less damaged than it first appears ¹. Before you tear out a wall, learn to repair it.

How plaster holds

In a house from around 1900, the plaster was usually applied in three coats over wood lath, narrow wooden strips nailed across the studs with small gaps between them. The first coat was pushed hard enough that it squeezed through the gaps and slumped down behind the lath, forming keys that lock the plaster on ¹. Two thick base coats (the scratch and brown coats, about 3/8 inch each) and a thin finish coat make a wall about 3/4 to 7/8 inch thick on wood lath ¹.

Early plaster was lime (with sand and animal hair); by the early 1900s plasterers were switching to gypsum, which sets in minutes rather than months but stands up to water less well ¹. Once cured, you usually can’t tell them apart ¹.

Before sanding or chipping, remember that the paint on old plaster probably contains lead. Mist the surface, contain the dust, and clean up with a HEPA vacuum ².

Reading cracks

The pattern tells you the cause ¹:

• Diagonal cracks running from the corners of doors and windows: structural stress or settling.
• Horizontal cracks along lath lines: wood lath swelling and shrinking with humidity, or loosened lath nails.
• Fine networks of cracks in the finish coat: a problem with how the plaster was originally applied.
• Bubbling, crumbling, or efflorescence: moisture. Find and stop the water before any repair.

Fixing cracks

• Hairline cracks in otherwise sound plaster aren’t serious; just fill them with patching material ¹.
• Cracks that reopen with the seasons: widen the crack slightly with a pointed tool, fill it, and bridge it with fiberglass mesh tape pressed into quick-setting joint compound. When that dries, add a second coat to cover the tape, feathered at the edges, then a third coat to level it, sanded lightly ¹.
• Big structural cracks: fix the structure first. Then the plaster is cut back about 6 inches on each side down to the lath, metal lath is applied, and the gap is replastered. If a crack is growing, call a structural engineer ¹.

Patching holes

Small holes, under about 4 inches, that go through the finish and brown coats get two layers: a base coat scraped back below the surface, then, once it has set, a finish layer flush with the wall. One-coat patches tend to dry concave and show through paint ¹.

Larger holes down to the lath ¹:

1. Clean out all loose plaster and re-nail any loose lath.
2. Mist the old lath so it doesn’t twist when wet plaster hits it, or better, brush on a bonding agent.
3. Fasten expanded metal lath (diamond mesh) over the wood lath with tie wire or lath nails, for stronger keys.
4. Apply three coats, stepping each one to lap onto the old plaster around the hole, and follow any waviness in the wall so the patch doesn’t look flat.

For homeowners, the Park Service suggests all-purpose joint compound for fine cracks, quick-setting compound to bed tape and fill larger holes (it shrinks less but is hard to sand), and ready-mixed perlited base-coat plaster for the base coats ¹.

Loose ceilings

Cracks are cosmetic; a sagging ceiling isn’t. It means the keys have broken, and gravity will eventually bring chunks down ¹. If the plaster around the loose area is sound, it can be pulled back up to the lath with flat-head wood screws and plaster washers, thin perforated discs that spread the screw’s grip. Then the cracks are taped and the washers skim-coated over ¹. Where you can reach the back of the lath from the attic, bowed plaster can be pushed back with a padded brace and re-keyed with plaster or adhesive ¹. A ceiling too far gone can be furred, relathed, and replastered right over the old one ¹.

Try it

Walk through the house with a flashlight held flat against the walls and ceilings; low-angle light shows cracks and bulges. Press gently on any bulge: if it moves or crunches, the keys are broken there. Make a list: hairline cracks, moving cracks, holes, and loose spots. Most of it is weekend work.