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

Overview

The Earthaus BUILD Pozzolanic Hydraulic Lime (PHL) Plaster System is standardized for cob construction, while retaining flexibility for project-specific modifications.

Key Properties:

Formulated with high-plasticity lime and pumice pozzolan.
Excellent workability, adhesion, and durability.
Highly breathable, allowing cob to release moisture.
Slow, steady curing supports structural flexibility and lowers cracking probability.

Exterior System (Cob Walls):

Layer	Product	Thickness	Coverage
Scratch Coat	PHL Binder	3/8" (1 coat)	65 ft²
Brown Coat	PHL Binder	3/8" (1 coat)	75 ft²
Finish Option 1	Lime Stucco Coarse	1/4" (1 coat)	50 ft²
Finish Option 2	Lime Stucco Fine	1/8" (1 coat)	100 ft²
Sealer (optional)	Breathable Sealer	–	TBD

Interior System (Cob Walls):

Layer	Product	Thickness	Coverage
Scratch Coat	PHL Binder	1/4" (1 coat)	100 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

Cob Substrate Preparation

Inspect the Walls — Check for cracks, holes, or irregularities. Remove dust and contaminants. Ensure a uniformly rough texture for mechanical grip.
Fully Dry Cob — Walls must be completely dry and cured before plastering.
Dampen Walls Before Plastering — Pre-dampen with several light mist coats; the goal is walls darkened but not saturated.
Plaster Stops & Lath — Install plaster stops where plaster ends. Protect wood with felt paper plus expanded metal lath. Recommended: embed fiberglass reinforcing mesh in the brown coat to reduce cracking.
Protect Openings — Mask windows, doors, and other non-plaster elements.

PHL Binder Installation Over Cob

Scratch Coat:

Thickness: 3/8". Score horizontally 1/4" deep while wet.
Must fully cover the cob substrate. Cure 10 days before the brown coat.

Brown Coat:

Thickness: 3/8". Cure 10 days before applying the finish.

Sand Aggregate (Mandatory):

Scratch coat: 1 bag PHL Binder + 3 × 5-gal buckets sand.
Brown coat: 1 bag PHL Binder + 5 × 5-gal buckets sand.

Applying the Finish Coat

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

Exterior finishes (Lime Stucco Coarse or Fine): see the Lime Stucco — Application Guide.
Interior finishes (Sand or Stone): see the Interior Sand & Stone Finishes application guide.

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:

PHL Binder (scratch & brown coats): PHL Binder Calculator — select your wall system in the calculator.
Exterior finish: Lime Stucco Fine Calculator or Lime Stucco Coarse Calculator.
Interior finish: BUILD Sand Finish Calculator or BUILD Stone Calculator.

Technical Support

Product questions: email support@earthausplaster.com
Technical consultation: Earthaus technical consultation