What Is Cement Plastering?

Cement plastering is one of the most important finishing processes in building construction. It creates a strong, smooth, and protective surface over masonry or concrete walls, preparing them for paint, wallpaper, or other finishes. The quality of plastering has a direct effect on the building’s appearance, durability, and resistance to moisture. Poorly executed plastering can lead to cracks, damp patches, and an uneven surface, which can compromise both aesthetics and performance.

This guide explains cement plastering in-depth, covering the materials used, thickness requirements, ratios, internal and external plaster guidelines, application methods, curing practices, and best standards followed in modern construction.

Understanding Cement Plastering

Cement plastering is the process of covering walls and ceilings with a mixture of cement, sand, and water. This layer protects the underlying masonry and provides a level surface for finishing work. Plastering serves both functional and aesthetic purposes.

Core Functions of Cement Plastering

• Provides a smooth, even surface

• Enhances appearance

• Protects walls from moisture and weather

• Improves durability by sealing pores

• Prevents cracks, dust accumulation, and surface defects

• Increases wall thickness for sound and heat insulation

Plastering is usually done after masonry works but before flooring, false ceilings, or interior finishing.

Materials Used in Cement Plastering

Cement plaster mixes typically include:

1. Cement

Ordinary Portland Cement (OPC) 43 or 53 grade or PPC is commonly used. Cement must be:

• Fresh

• Free of lumps

• Stored properly in dry spaces

2. Sand

Sand plays a major role in plaster quality. It should be:

• Well-graded

• Clean

• Free from clay, silt, and organic impurities

Medium sand is preferred for external plastering, while fine sand is used internally for smoother finishes.

3. Water

Water must be potable, free from chemicals and salts, as impurities can weaken plaster and cause efflorescence.

4. Additives

Sometimes, admixtures like waterproofing compounds, plasticizers, or bonding agents are added to:

• Improve adhesion

• Reduce permeability

• Enhance workability

• Prevent shrinkage cracks

Cement Plaster Ratio

The plaster mix ratio affects strength, durability, and smoothness.

Common cement plaster ratios include:

1:4 – Cement : Sand

Used for external plastering due to higher strength and durability.

1:5 – Cement : Sand

Used for internal wall plastering in most residential and commercial buildings.

1:6 – Cement : Sand

Used when masonry is already strong and smooth; also used for ceilings.

Finish Coat (Optional)

A thin layer of 1:3 or 1:4 plaster may be applied over the base coat to improve surface texture.

The exact ratio depends on wall conditions, thickness required, and structural recommendations.

Plastering Thickness

Thickness varies based on whether the wall is internal or external, its alignment, and desired surface finish.

Standard Plaster Thicknesses

• Internal plaster thickness: 12 mm to 15 mm

• External plaster thickness: 15 mm to 20 mm

• Ceiling plaster thickness: 10 mm

• Two-coat plaster system: 18 to 20 mm total

The thickness should be uniform across all surfaces for better aesthetics and structural integrity.

Internal Plaster Thickness Explained

Internal walls require a smooth finish for paint and interior décor. Plastering inside a building must be carefully controlled to avoid cracks and ensure uniformity.

Common Internal Thickness

12 mm is standard for most internal walls.

When to Use 15–18 mm Internal Plaster

• To correct uneven masonry

• On hollow block or AAC block walls

• When pipes or conduits are concealed

• When finishing requires extra smoothness

Why Internal Plaster Thickness Matters

• Too thin: cracks easily and does not cover masonry joints

• Too thick: leads to shrinkage cracking and poor bonding

Internal plaster must provide a firm substrate for putty, primer, and paint application.

External Plaster Thickness Explained

External walls face harsh weather conditions. They must resist rain, dirt, solar radiation, and temperature variations. The plaster must therefore be thicker, stronger, and more durable.

Standard External Thickness

15 mm to 20 mm is ideal for exterior surfaces.

Two-Coat System for External Plaster

Most buildings use:

1. Base coat: 12–15 mm

2. Finish coat: 6–8 mm

This layered system provides:

• Better bonding

• Enhanced weather resistance

• Reduced surface cracks

• Longer durability

External Plaster Must Resist:

• Heavy rainfall

• High winds

• Thermal expansion and contraction

• Moisture ingress

• Pollution and dirt

Proper curing is essential to prevent surface cracking on external walls.

Steps in the Cement Plastering Process

Cement plastering involves several carefully executed steps to ensure a durable and uniform finish.

1. Surface Preparation

The wall must be cleaned and prepared:

• Remove loose mortar, dust, and grease

• Wet the surface to reduce suction

• Fill large cracks or holes

• Fix mesh over joints (brick to concrete joints)

This improves adhesion and prevents cracks.

2. Marking the Thickness

Dabs or dots of cement mortar are placed on the wall to determine the level. Vertical and horizontal lines are drawn using:

• Plumb bob

• Spirit level

• Straight edge

This ensures a uniform thickness of plaster.

3. Applying the Base Coat

The first coat of plaster is thrown against the wall with force so that it adheres properly. It is then leveled using wooden or aluminum screeds.

Best Practices

• Maintain the mix ratio as per the design

• Keep continuous mixing to avoid inconsistency

• Do not apply too much thickness in one go

4. Applying the Second or Finish Coat

After the base coat sets slightly, the finish coat is applied. This coat:

• Levels imperfections

• Provides the final smooth texture

• Prepares the wall for painting

The finishing coat is usually thinner, around 5–8 mm.

5. Curing

Curing is critical for strength and crack prevention.

Standard Curing Duration

• Internal walls: 7 days

• External walls: 10 to 14 days

Curing methods include:

• Sprinkling water

• Using wet gunny bags

• Applying curing compounds (in some cases)

Proper curing ensures long-term durability.

Types of Cement Plaster Finishes

The finish depends on the project requirements.

1. Smooth Finish

Achieved by troweling. Common for internal walls.

2. Sand-Faced Finish

A rough texture created using sponge and fine sand. Ideal for external surfaces.

3. Rough-Cast and Pebble-Dash

Decorative finishes used for aesthetics and weather protection.

4. Textured Finish

Created using tools, trowels, or rollers.

Common Plastering Problems and Prevention

1. Cracking

Causes:

• Rapid drying

• Extra water in mix

• Poor curing

• Excessive plaster thickness

Prevention: Maintain ratio, proper curing, and mesh at joints.

2. Dampness

Causes:

• Poor waterproofing

• Cracks in plaster

• Leaks in masonry

Prevention: Add waterproofing compound, use thicker external plaster.

3. Hollow Sound

Causes:

• Poor bonding

• Dust on surface

• Dried wall during application

Prevention: Wet walls adequately before plastering.

External vs Internal Plaster: Key Differences

Why Cement Plastering Matters in Construction

Good plastering:

• Enhances structural protection

• Prevents moisture ingress

• Improves building aesthetics

• Prepares walls for final finished layers

• Increases property lifespan

Poor plastering, on the other hand, leads to early repairs, peeling paint, and cracks.

Final Thoughts

Cement plastering is more than just creating a smooth wall surface. It is a technical process that demands the right mix ratios, proper thickness, skilled workmanship, and careful curing. Internal and external walls have different requirements, and the plastering technique must suit the intended function and environmental conditions.

When done properly, plastering ensures strength, protection, durability, and a clean finish for further treatments. Whether you are supervising site work, learning construction basics, or executing plastering on a project, understanding these principles helps achieve long-lasting and high-quality results.