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Wood Lath Wall System Guide

Layer-by-layer specs and substrate prep for plastering wood lath walls and ceilings with the Earthaus BUILD PHL system — new lath, old lath, and repairs.

This Help Center article is a summary of the official Earthaus Wood Lath Wall System Guide. Download the full Wood Lath Wall System Guide (PDF).

Overview

The Earthaus BUILD Pozzolanic Hydraulic Lime (PHL) Plaster System is standardized for wood lath, an interior-only substrate. Wood lath is narrow wood strips nailed across framing with small gaps between courses. Wet plaster is pressed through the gaps and mushrooms behind the strips, forming keys that mechanically lock the plaster to the wall. Lath-and-plaster was the standard interior wall and ceiling finish in American wood-framed buildings into the 1930s, and lath is still used in new construction, especially for curved walls and ceilings.

This guide covers three situations: plastering new wood lath, replastering existing (old) wood lath that has been stripped of its original plaster, and repairing sections of a lath-and-plaster wall that are damaged down to bare lath.

Key Properties:

  • Specialized formulas made with high-plasticity lime and pumice pozzolan.

  • Excellent workability, adhesion, and durability.

  • Highly breathable, allowing the wall assembly to release moisture.

  • Slow and steady curing supports flexibility during early structural movement.

Interior System (Wood Lath):

Layer

Product

Thickness

Coverage

Scratch Coat (with fiber — required)

PHL Binder

3/8" (1 coat)

65 ft²

Brown Coat

PHL Binder

3/8" (1 coat)

75 ft²

Finish option 1

Sand Finish

1/8" (1 coat)

100 ft²

Finish option 2

Stone Finish

1/16" (1 coat)

250 ft²

Sealer (optional)

Olive Oil Soap Sealer

–

700 ft²

Total system thickness is approximately 7/8", consistent with traditional three-coat plaster on wood lath. Wood lath is not an exterior substrate — the wood lath system is interior only.

Wood Lath Substrate Preparation

  1. Sourcing New Lath — Wood lath is a standard lumber item, sold in bundles at lumber yards and home centers as "wood lath" in the traditional dimension: approximately 1/4" thick x 1-1/2" wide x 4 feet long (the 4-foot length spans framing on 16" centers). Choose kiln-dried softwood (pine, spruce, or fir) that is straight, dry, and free of knots, bark, and pitch. Bundled lath varies in quality, so pick through it, or have a lumber yard rip lath from clear stock for larger projects. Reclaimed lath from architectural salvage is also a sound choice for restoration work when the strips are dense, straight, and clean.

  2. New Wood Lath Installation — Nail lath firmly to framing with 1/4" to 3/8" gaps between courses; the gaps are where the plaster keys form. Butt joints (ends) must meet over framing members and should be gapped slightly to allow expansion. Stagger the butt joints between courses rather than stacking them on the same stud run, so the seams do not line up and telegraph a crack through the finished wall. Soak new laths in water before fixing them: they swell, then shrink tight after nailing, so the plaster's moisture can only expand them back to their fixed position instead of buckling them.

  3. Existing (Old) Wood Lath — Re-nail every loose strip and scour the lath clean of old plaster bits and dust. Old lath is typically bone dry: pre-wet it generously several hours to a day before plastering so the wood swells and settles first, then dampen normally at application time. Skipping the pre-wetting risks warped, buckled lath and a scratch coat that fails to cure properly because the dry wood pulls the water out of the plaster.

  4. Repairs Down to Bare Lath — Clean broken keys and debris from behind the lath and cut the surrounding sound plaster back to clean, square edges. Treat the exposed lath as old lath (re-nail, clean, pre-wet). Rebuild with a fiber-reinforced scratch coat and a brown coat to match the thickness of the surrounding original plaster before applying the finish.

  5. Dampen Before Plastering — Just before applying the scratch coat, mist the lath with clean water in two or three light passes, letting each soak in. The goal is thoroughly damp wood with no droplets or standing water — droplets cause the plaster to slide across the lath instead of gripping it. Avoid over-saturating the wood. Protect floors from misting water and dropped plaster.

  6. Protect Surrounding Areas — Mask windows, doors, trim, and other non-plaster elements.

PHL Binder Installation Over Wood Lath

Fiber Reinforcement (Required):

Fiber in the scratch coat is required over wood lath — it is what keeps the keys from breaking off as the plaster dries and cures. Do not skip it. Add fiber during mixing, using either:

  • Multifilament polypropylene stucco fiber, 1/2" length, available from masonry and stucco suppliers. Dose per the fiber manufacturer's directions, typically 4 to 6 oz per 100 lbs of wet mix.

  • Natural goat or horse hair sold for traditional lime plastering, available from specialty suppliers — approximately 1 lb per 55 lbs of wet plaster, chopped to 1"–2" lengths and teased apart so it distributes evenly. Add natural hair just before application: lime attacks the protein, so haired plaster must not sit mixed and wet.

Scratch Coat:

  • Apply by hand with a hawk and trowel. Hand application is recommended over lath because key formation depends on trowel pressure — each pass must push plaster through the gaps so it mushrooms behind the strips.

  • Thickness: 3/8". No lath should show through the finished scratch coat. Coverage over lath runs lower than over flat substrates because the plaster forming the keys consumes additional material.

  • While wet, scarify 1/4" deep for bonding — and over wood lath, always scratch diagonally to the lath, never horizontally. Horizontal strokes run parallel to the strips and can push the fresh keys off the back of the lath before they have cured.

  • Cure 10 days before applying the brown coat.

Brown Coat:

  • Pre-wet the cured scratch coat before application.

  • Standard thickness: 3/8". All of the shape for the finished surface is achieved in the brown coat, not the thin finish coat.

  • Sponge float to an even, uniform surface with just enough texture to receive the finish coat.

  • Cure 10 days before finish plaster.

Sand Aggregate (Mandatory):

PHL Binder requires local sand aggregate (how to source and select local sand is provided in the PHL Binder Application Guide):

  • Scratch coat: 1 bag PHL Binder + 3 × 5-gal buckets sand (plus fiber).

  • Brown coat: 1 bag PHL Binder + 5 × 5-gal buckets sand.

Applying the Finish Coat

After the brown coat has cured (10 days), pre-wet it and apply your selected finish from the system table above:

Pigment color and optional sealer are listed in the system table above.

Calculate How Much You Need

Estimate the materials for your project with the Earthaus calculators:

Technical Support

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