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Cement Blocks Must They Be Plastered

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Cement Blocks: Must They Be Plastered? A South African Plasterer’s Guide

Introduction

On building sites across South Africa, one question comes up again and again: cement blocks – must they be plastered or can you safely leave them exposed? Home‑owners, developers and even some contractors are often unsure what building standards and good practice actually require, especially when trying to control costs or achieve a particular “off‑shutter” or face‑block look.

In this guide, written from the perspective of a practical plasterer, we unpack when cement block walls should be plastered, when they can be left bare, and what South African regulations and standards say about durability, waterproofing and finishes. We will refer to relevant standards like SANS 10400 and SANS 2001 for good building practice, and draw on information from local suppliers, contractors and industry bodies.

You will learn how plaster affects strength and weather resistance, what happens if you don’t plaster exterior and interior walls, how much plastering costs in 2025/2026 in different regions, and when it is better to call a professional rather than attempt DIY. By the end, you will be able to answer confidently: Cement Blocks – Must They Be Plastered? and make the right decision for your project.

Understanding Cement Block Walls in South Africa

What are concrete masonry units (CMUs)?

In South African standards and technical documents, common “cement blocks” are called concrete masonry units (CMUs) or concrete masonry. The Concrete Manufacturers Association (CMA) explains that CMUs are made from cement, aggregates and water and are used for structural and non‑structural walls in housing and commercial buildings, available in block and brick formats for loadbearing and non‑loadbearing applications, including exposed and plastered work (CMA overview of concrete masonry).

Typical hollow blocks for external walls are 140–190 mm thick, often using Class 1 or Class 2 building units depending on whether the wall is to be plastered or left fair‑faced. Poor‑quality or low‑strength blocks, which are unfortunately common on some small projects, are far more vulnerable if left unprotected.

What does South African regulation require?

South African building work is guided by the National Building Regulations (NBR) and the SANS 10400 series. Part K (Walls) of SANS 10400 sets performance requirements for stability, weather resistance and durability of walls, though it does not literally say “all cement blocks must be plastered”. Instead, it focuses on outcomes: the completed wall must resist rain penetration, wind, and moisture ingress to protect occupants.

The National Home Builders Registration Council (NHBRC), which enforces minimum standards on enrolled homes, refers to SANS 10400 and relevant material standards for masonry and plaster. NHBRC manuals stress correct wall construction, mortar quality, damp‑proofing and finishes as key to avoiding moisture problems and cracking (NHBRC Home Building Manual).

Key implication: standards and regulations don’t force plaster on every cement block wall, but they do require the wall to be durable, water‑resistant and fit for its intended purpose. Plaster is the most common, reliable way to achieve that with ordinary blockwork.

Cement Blocks: Must They Be Plastered on External Walls?

Weather resistance and moisture

Quick Answer

For typical South African houses and boundary walls built with standard hollow cement blocks, plastering and a proper paint or waterproofing system is strongly recommended on external faces, especially in high‑rainfall and coastal regions.

According to guidance on moisture and weather protection in the NHBRC technical requirements, external walls must be constructed and finished in a way that prevents penetration of rainwater and rising damp, including appropriate finishes and flashings (NHBRC moisture and weathering guidance). Bare, porous cement blocks are far more prone to:

  • Rain penetration through mortar joints and block faces
  • Damp patches and efflorescence (white salt deposits)
  • Freeze–thaw damage in cold regions like the Highveld
  • Corrosion of embedded steel (lintels, reinforcement) over time

The CMA also notes that while concrete masonry can be used as exposed masonry, this normally assumes dense, high‑quality units with workmanship suited to facework, and often still includes water‑repellent surface treatments or paints (CMA masonry guide).

Regional considerations: coastal vs inland

Coastal municipalities (e.g. eThekwini, City of Cape Town) typically emphasise enhanced durability due to aggressive marine environments. Building advice from coastal property professionals repeatedly warns that unprotected concrete and masonry deteriorate faster because of salt‑laden moisture and wind‑driven rain. For example, the City of Cape Town’s general building maintenance advice highlights regular painting and sealing of external walls as part of protecting against coastal weathering, even if not specific to plastered vs unplastered (City of Cape Town building maintenance guidance).

Common Mistake

Leaving cheap, low‑density cement blocks unplastered on a boundary or house wall to “save money” in areas like Durban or Cape Town. Within a few seasons, hairline cracks, damp marks and salt deposits appear, and you end up paying more for remedial plastering and waterproofing.

When Can Cement Block Walls Be Left Unplastered?

Face blocks and fair‑faced masonry

Blocks designed to be left exposed (often called face blocks or fair‑faced units) are denser, less porous and manufactured with a better surface finish. The CMA notes that concrete masonry can be used for attractive exposed walls where the units and workmanship are of facework quality, emphasising correct mortar jointing, control joints and weatherproof design (CMA exposed masonry guidance).

Where such units are used, plaster is not mandatory, provided:

  • The designer has detailed proper flashings, copings and damp‑proof courses
  • The blocks meet appropriate strength and absorption requirements
  • A suitable water‑repellent coating or breathable sealer is applied where necessary

Internal walls in dry environments

For internal non‑habitable or low‑risk spaces (e.g. garages, storerooms, some industrial buildings), unplastered blockwork is often acceptable, especially if the client is comfortable with an “industrial” look. These walls should still comply with SANS 10400 Part K (stability) and Part T (fire), which may require specific wall thicknesses and fire‑resistance ratings.

Internal bathroom and shower walls, however, are more sensitive. NHBRC and moisture‑control guidance recommend durable, water‑resistant finishes in wet areas (NHBRC moisture guidance). In practice, that usually means plaster plus tiles, or a proprietary waterproof lining system.

How Plaster Protects Cement Blocks (and When It Doesn’t)

Functions of plaster in South African construction

The Association of South African Quantity Surveyors (ASAQS), in explaining wall finishes, notes that plaster provides an even surface, protects masonry, and forms a base for paint or other coatings (ASAQS finishes overview). More technically, SANS‑based specifications for plaster (for example, SANS 2001‑EM1 series covering earthworks and masonry) describe plaster as a cement‑based render that:

  • Improves weather resistance and reduces water penetration
  • Provides impact protection to softer or weaker masonry
  • Reduces air leakage (improving energy efficiency and comfort)
  • Gives a uniform substrate for paint or specialised coatings

When plaster can fail

Plaster is not a magic fix. If not designed and applied correctly, it can crack and detach, allowing water in behind the layer. Common causes include:

  • Poor substrate – weak, dusty or under‑strength cement blocks
  • Wrong mix (too strong, too much cement, or incorrect sand grading)
  • Lack of curing, especially in hot, dry inland summers
  • No control joints in long walls

According to South African plastering contractors like Tal (tile & construction products), bond between plaster and substrate is critical, and using the right mix, surface preparation and curing regime is essential to prevent debonding and cracking (Tal guidance on cementitious renders). When these basics are ignored, a plastered wall may perform worse than a correctly detailed exposed face‑block wall.

Cement Blocks Must They Be Plastered? Decision Guide

Practical decision checklist

Use this quick checklist to decide if your cement block wall should be plastered.

Question If YES, plaster is usually… If NO, plaster is usually…
Is it an external wall exposed to rain and wind? Recommended Optional
Located in high rainfall or coastal area (KZN coast, Cape Town, etc.)? Highly recommended Optional
Built with ordinary hollow building blocks (not face blocks)? Recommended Only if aesthetic demands
Is it a habitable room (bedroom, lounge, kitchen)? Recommended (for finish) Acceptable in specials only
Is the wall part of a bathroom/shower area? Strongly recommended Not advisable
Are there known damp problems on site (high water table, poor drainage)? Strongly recommended High risk if left bare
Do you want a smooth painted finish? Required N/A
Is long‑term maintenance ability limited (rental, low‑care property)? Recommended Higher risk if bare

Key Takeaway

In mainstream residential and small commercial work in South Africa, external cement block walls should almost always be plastered and painted, unless they were deliberately designed and built as high‑quality exposed masonry.

Cost of Plastering Cement Block Walls in South Africa (2025/2026)

Typical labour and material rates

Project‑pricing resources aimed at South Africa give useful ballpark figures for plastering costs. The consumer platform ProCompare cites average wall plastering rates (labour plus materials) in the range of roughly R120 – R250 per m² depending on complexity and finish level (ProCompare wall plastering cost guide).

Similarly, building cost guides used by contractors and QS practices in 2024 show comparable ranges for 12–15 mm internal plaster and 15–20 mm external plaster, although exact rates depend on region, scale and specification.

For 2025/2026, considering inflation and industry feedback, you can reasonably expect:

  • Gauteng (Johannesburg, Pretoria):
  • Western Cape (Cape Town, Winelands):
    • Internal: ± R160 – R240/m²
    • External: ± R180 – R280/m² (higher on coastal/complex jobs)
  • KwaZulu‑Natal (Durban, North Coast):
    • Internal: ± R140 – R220/m²
    • External: ± R170 – R260/m²

These ranges assume:

  • Standard cement‑sand plaster, 12–20 mm
  • Reasonably accessible residential work
  • Excluding scaffolding beyond single‑storey height and special finishes
  • Including basic materials (cement, sand, bonding liquids where needed)

Factors that push costs up or down

Pro Tip

When comparing quotes, insist that contractors specify plaster thickness, number of coats, mix ratios and whether paint/primer is included. A cheaper quote may simply be for a thin, single‑coat “skim” that will not protect your cement blocks adequately.

DIY vs Professional Plastering on Cement Block Walls

When DIY can be realistic

If you have basic building experience, good tools and time, DIY plastering can work for:

  • Small internal feature walls in non‑critical areas
  • Garages or storerooms where appearance is not critical
  • Patch repairs on existing plastered walls

Tiling and construction product suppliers like Tal and cement producers such as Afrisam provide basic guidance on plaster mixes and application, for example typical 1:4 to 1:6 cement:sand mixes depending on conditions (Afrisam plaster mix guidance).

However, achieving a consistently flat, crack‑free finish on a large external wall is harder than it looks.

When to call a professional plasterer

You should strongly consider hiring an experienced plastering contractor when:

  • The wall is external and critical for keeping water out
  • The project involves a new house, addition or significant renovation
  • You are in a coastal or high‑rainfall region
  • You need a high‑quality painted finish (no waves or hollows)
  • The wall has structural cracks or movement joints that must be detailed correctly

Professional contractors familiar with SANS‑aligned spec sheets (for example, SANS 2001 series and SANS 10400 requirements for walls and moisture) will understand how to detail corners, joints, reveals and damp‑proof courses properly.

To find contractors, South African homeowners often use directories like The Master Builders Association member listings or regional online platforms. For example, Master Builders Association Western Cape lists vetted contractors and specialists (MBA WC contractor search).

Common Mistake

Letting the bricklayer “quickly do the plastering” at the end of the job without checking their plastering experience. Plaster is a finishing trade; a poor plaster job can ruin the appearance and performance of otherwise well‑built walls.

Practical South African Scenarios

Example 1: Boundary wall in Johannesburg

A homeowner in Randburg builds a 1,8 m high boundary wall in standard hollow concrete blocks. The site is on a slope with some stormwater running along the wall base.

  • Risk factors: rain splash‑back, stormwater, porous blocks.
  • Best practice: plaster both sides of the wall, add a capping to the top, and apply a quality exterior paint system. This aligns with general moisture‑protection guidance from NHBRC and SANS 10400 aims for durable walls.

Leaving the wall bare would almost certainly lead to damp patches and cracking over time.

Example 2: Warehouse internal walls in Durban

A small warehouse in Durban uses cement block internal partitions between storage bays. The owner prefers a low‑maintenance, robust finish and is not concerned with a decorative look.

  • Risk factors: coastal humidity but mostly internal and sheltered.
  • Best practice: blocks can be left unplastered, but applying a breathable sealer or paint to reduce dusting and surface abrasion is advisable. Plaster is optional here, provided structural and fire‑rating requirements are met.

Example 3: Face‑block house in Cape Town

An architect specifies high‑quality fair‑faced concrete blocks for a contemporary home on the Atlantic Seaboard. The blocks are dense, precision‑made and laid by masons skilled in facework.

  • Risk factors: severe coastal exposure, salt spray.
  • Best practice: no plaster, but detailed flashings, deep overhangs, a high‑performance water‑repellent sealer and careful control‑joint layout. This follows the CMA’s concept of exposed concrete masonry as a designed system, not just “leaving cheap blocks unplastered” (CMA exposed masonry guidance).

FAQ: Cement Blocks and Plastering in South Africa

Q: Do the South African National Building Regulations say cement block walls must be plastered?

A: The National Building Regulations, implemented through SANS 10400, do not literally state that all cement block walls must be plastered. Instead, they require walls to be stable, durable and resistant to moisture and weather. Plaster is the most common way to achieve those performance requirements, especially for external walls. NHBRC guidance on moisture protection reinforces the need for appropriate finishes to prevent water penetration (NHBRC moisture guidance).

Q: Can I leave my new cement block boundary wall unplastered to save costs?

A: You can, but it is usually a false economy. Ordinary hollow blocks are porous and, over time, unplastered walls tend to show damp marks, cracks and efflorescence, particularly in higher‑rainfall regions. Repairing such problems later can cost more than plastering from the start. Current guides indicate external plastering costs of about R170 – R260/m², which is modest compared to the overall wall cost and long‑term durability benefits (ProCompare plastering costs).

Q: Are face blocks or fair‑faced concrete blocks okay to leave unplastered?

A: Yes, if they are specifically manufactured and installed as facework or fair‑faced units. The Concrete Manufacturers Association confirms that concrete masonry can be used for exposed walls, provided the units have appropriate quality and the detailing allows for weatherproofing and movement control (CMA masonry guide). Often, a water‑repellent sealer or paint is still applied to enhance durability, especially in coastal conditions.

Q: Is internal plastering on cement block walls just for aesthetics, or does it have a technical purpose?

A: Internal plastering is partly aesthetic, providing a flat, paintable surface, but it also improves performance. Plaster reduces air leakage, offers some sound insulation, and protects blocks from knocks and abrasion in high‑traffic areas. Technical guides from organisations like ASAQS highlight plaster’s protective role as part of the wall system (ASAQS wall finishes overview). In bathrooms and wet areas, plaster (plus proper waterproofing and tiles) is critical to manage moisture.

Q: What plaster mix should be used on cement block walls in South Africa?

A: Common practice is a cement‑sand plaster with a mix like 1:4 to 1:6 by volume, depending on exposure and sand quality. Cement producers such as Afrisam recommend using clean, well‑graded sand, proper curing and, where needed, bonding liquids to ensure adhesion (Afrisam plastering guidance). External walls in harsh conditions may require richer mixes and two‑coat applications. Always follow manufacturer and project specifications.

Q: How thick should plaster be on external cement block walls?

A: For typical South African residential work, external plaster is generally 15–20 mm thick, often in one or two coats. Thicker plaster helps to even out block irregularities and improves weather resistance, but excessive thickness on weak or poorly prepared substrates can increase the risk of cracking. Contractor and QS pricing guides that underlie ranges like R170 – R260/m² assume these standard thicknesses for external work (ProCompare plastering costs).

Q: Is it worth paying extra for a professional plasterer instead of DIY?

A: For small internal jobs, DIY can be reasonable. But for external walls, coastal locations, or where you want a high‑quality finish, a professional plasterer is usually worth the cost. Poor plastering can trap moisture, crack, or pull away from the wall, undermining the protection you wanted in the first place. Reputable contractors often follow SANS‑based specifications and understand local conditions. Using member directories such as Master Builders Association lists helps in finding vetted professionals (MBA WC find‑a‑member).

Conclusion

  • In South Africa, cement block walls are not legally forced to be plastered, but standards like SANS 10400 and NHBRC guidance effectively make plastering the safest choice for most external and many internal walls.
  • For ordinary hollow blocks, especially in coastal or high‑rainfall regions, plaster plus paint is strongly recommended to protect against moisture and long‑term deterioration.
  • Face blocks and certain internal walls can be left unplastered if they are correctly specified, built and, where needed, sealed.
  • Considering realistic 2025/2026 costs, plastering is a relatively modest investment compared to the cost of future damp repairs and aesthetic issues.

For professional plastering services in your area, contact an experienced local team for a free consultation and quote, and use this guide to discuss whether your cement blocks must they be plastered for best long‑term performance.

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