Many property owners assume that concrete blocks must be waterproof. After all, they appear solid, dense, and impermeable when you look at a finished wall or foundation. This logical assumption leads to a crucial question: are the concrete blocks used for foundations and walls truly waterproof?
Are Standard Concrete Blocks Waterproof?
No. Standard concrete blocks are not waterproof. Despite their solid appearance and structural strength, concrete blocks are naturally porous materials that allow water to penetrate through them. Untreated concrete block walls will eventually allow moisture, dampness, and even standing water to seep through over time.
Understanding this reality is essential for anyone working with concrete blocks and planning effective moisture protection for their structures. The good news is that concrete blocks can be made waterproof through appropriate treatment — the key is knowing which methods to apply and when.
Why Are Concrete Blocks Porous? The Science Explained
The porosity of concrete blocks stems from their composition and manufacturing process. Concrete blocks consist of cement, sand, and aggregates mixed with water. During curing, chemical reactions bind these components together, but the process inevitably creates a network of tiny pores and capillaries throughout the material. These microscopic pathways form as excess water evaporates and as the hydration process leaves small voids within the block structure.
This internal network allows water to move through the block via capillary action — the same principle that allows a sponge to absorb water. When water contacts one side of a concrete block, it can gradually work its way through these capillaries to emerge on the opposite side.
The mortar joints between blocks present additional vulnerabilities. These joints often contain even more pores than the blocks themselves and can develop hairline cracks over time, creating direct pathways for water ingress. This is why leaking concrete block foundation issues commonly originate at joint locations.
For below-ground applications, choosing the right product specification from the outset is the most effective defence. Procon 24/7’s interlocking concrete blocks are specified for demanding structural and retaining applications — call our team to discuss the right product for your project.
How Can You Make Concrete Blocks Waterproof?
Whilst concrete blocks aren’t inherently waterproof, they can be made highly resistant to moisture through proper treatment.
Step 1 — Surface preparation: No waterproofing system performs effectively on poorly prepared surfaces. Thoroughly clean the concrete block walls to remove dirt, efflorescence, loose mortar, and any existing coatings. Repair visible cracks, holes, or damaged mortar joints before proceeding.
Step 2 — Choose the right waterproofing method:
- Waterproof sealants and paints: Acrylic-latex and elastomeric coatings create a flexible, waterproof film on the block surface. These penetrate into surface pores whilst forming a protective barrier that prevents water infiltration. Suitable for internal applications or areas with low water pressure.
- Cementitious coatings: Cement-based slurries applied like thick plaster, bonding chemically with the concrete block substrate. These cure to form an integral waterproof barrier and can withstand significant hydrostatic pressure — often the best choice for basement applications.
- Exterior waterproofing membranes: For heavy-duty foundation applications, sheet or liquid-applied membrane systems stop water before it reaches the blocks — eliminating the source of moisture problems rather than merely managing them.
Does It Matter Whether You Waterproof From Inside or Outside?
Yes — significantly. Waterproofing professionals distinguish between “positive side” (exterior) and “negative side” (interior) approaches.
Exterior waterproofing always provides superior protection because it prevents water from entering the block structure itself. When water cannot penetrate the blocks, freeze-thaw damage, efflorescence, and structural deterioration are eliminated at their source.
Interior waterproofing — whilst more accessible and cost-effective — manages water that has already entered the block structure. This approach can control moisture problems but doesn’t prevent the underlying water infiltration that causes long-term structural issues. For complex foundation systems, professional assessment helps determine the most appropriate strategy based on site conditions, budget, and performance requirements.
If you’re specifying concrete for a new build with below-ground elements, our commercial concrete service and domestic concrete service can advise on waterproof admixture options that reduce permeability at the mix stage — the most effective first line of defence.
Frequently Asked Questions About Concrete Block Waterproofing
Can I use regular paint to waterproof concrete blocks?
Regular paint will not waterproof concrete blocks effectively. Standard paints lack the thickness, flexibility, and adhesion properties required to seal the porous surface permanently. Use specifically designed waterproofing coatings that bridge pores and resist hydrostatic pressure.
How long does waterproofing treatment last on concrete blocks?
Quality waterproofing systems typically last 10–25 years, depending on product type, application quality, and environmental conditions. Elastomeric coatings may require reapplication every 5–10 years, whilst cementitious systems often provide longer-lasting protection.
Are cinder blocks as porous as concrete blocks?
Yes — cinder blocks are similarly porous to standard concrete blocks. In fact, traditional cinder blocks made with coal cinders can be even more porous than modern concrete blocks. Both materials require waterproofing treatment for moisture-sensitive applications.
Should I waterproof concrete blocks from inside or outside?
Exterior waterproofing is always preferable when accessible, as it prevents water from entering the blocks entirely. Interior waterproofing should only be used when exterior access is impossible or as a supplementary measure to exterior systems.
Will water pass through a concrete block wall?
Yes. Without treatment, water passes through concrete blocks and the mortar joints between them by capillary action. Sealing the surface or specifying a low-permeability mix stops water before it reaches the inside face.
