Most failing plaster comes down to one of three causes: water and moisture, structural movement, or earlier repairs done with the wrong materials. Knowing which one you're dealing with matters, because a patch won't last if the problem behind it is still active. Lime Lath handles the plaster work itself, matching and repairing historic lime plaster once any leaks or structural issues have been fixed by the right trade. This guide will help you recognize what's going on behind the cracks before the plaster repair begins.

Water & moisture

Leaking roofs, rising damp, or moisture trapped by later repairs can keep plaster soft, stained, or peeling. Surface work on a wet wall usually fails again.

What you might see:

• Discoloration at ceiling edges or around windows
• Drip or streak staining on painted surfaces
• Efflorescence — white powdery deposits on the surface
• Flaking paint or a bubbling finish coat
• Mold growth on or behind plaster

Until the water source is handled (often by a roofer, plumber, or other trade), a lime repair is fighting the building.

Structural movement

Buildings move through settlement, temperature cycling, and vibration. Crack patterns often hint at what’s happening before anyone opens a wall.

What you might see:

• Long diagonal cracks from window and door corners
• Horizontal cracks along ceiling beams
• Plaster falling in sections rather than crumbling
• Visible bowing or sagging in ceiling planes
• Cracks that reappear after previous patching

When structure is still moving, patching plaster alone won’t stop the pattern. Stabilizing that movement is a different trade from matching historic lime.

Previous poor repairs

Portland cement over lime. Gypsum on the wrong substrate. Hard, impermeable patches that trap moisture and stress the soft original finish.

What you might see:

• A crumbling or powdery surface that used to be solid
• Delamination of the finish coat from the base coat
• Crack patterns that don’t follow structural logic
• Visible patch boundaries where materials meet
• Hard grey patches surrounded by original off-white lime

This is where historic lime patch work often starts: reading the old material, matching binder and sand, and replacing incompatible repairs with a breathable lime system that belongs with the building.

Traditional lime plaster isn't applied in one pass — it's built in three distinct coats, each with its own job, thickness, and curing time. Understanding what's actually inside a wall explains why lime plaster lasts for centuries when it's done right, and why rushing any single stage is where most repairs go wrong.

Scratch Coat
Coarse stuff / first coat — 3/8" to 3/4"

The foundation. A thick, rough application of lime, sand, and natural hair fiber pressed firmly into the lath or substrate. Before it hardens, horizontal lines are scratched into the surface — hence the name — to key the next coat. The hair fiber distributes shrinkage stress as the coat cures, preventing cracking.

Brown Coat
Coarse stuff / second coat — 1/4" to 1/2"

The leveling coat. Applied over the cured scratch coat, the brown coat is worked flat and even, providing the canvas the finish coat will sit on. It must be given time to cure fully — applying the finish coat too soon is one of the most common causes of delamination. Patience here is not optional.

Finish Coat
Setting stuff / top coat — 3/16" to 1/4"

The surface you see and touch. This is where the craft lives. The finish coat is worked to match the texture, sheen, and character of the original. Whether smooth, floated, or textured, it determines whether the repair is visible or invisible. It cures by carbonation over weeks and months — continuing to harden and develop long after the plasterer has left.

Three coats, three purposes, and none of them skippable — which is the whole difference between a repair that lasts and one that doesn't.