Waterproofing Concrete Block Walls - Complete Guide

Understanding Concrete Block Waterproofing Challenges

Concrete block (CMU) walls present unique waterproofing challenges compared to poured concrete. The hollow cores, mortar joints, and porous nature of blocks create multiple pathways for water infiltration. Whether you’re dealing with new construction or retrofitting an old basement, choosing the right waterproofing system is critical.

Why Concrete Block Is Different

Inherent Vulnerabilities:

• Hollow cores collect and hold water
• Mortar joints are weak points
• Block material itself is porous
• Multiple seams create entry points
• Cores can fill with water creating pressure

Traditional Method: Parging with Sand/Cement and Tar

What Is Parging?

Parging is applying a thin coat of mortar (sand and cement mix) to smooth and seal the surface of concrete blocks. It’s often followed by tar or asphalt coating for additional waterproofing.

The Parging Process

Materials:

• Type S mortar or custom parging mix
• Portland cement and sand (1:3 ratio typical)
• Bonding agent for better adhesion
• Asphaltic coating (tar) for waterproofing

Application Steps:

1. Clean block surface thoroughly
2. Dampen blocks (not soaking)
3. Apply scratch coat (1/4” thick)
4. Apply finish coat (1/4” thick)
5. Allow to cure properly
6. Apply tar/asphalt coating

Is Parging + Tar Effective?

Based on building science expertise, parging with tar has significant limitations:

Pros:

• Relatively inexpensive
• Smooths rough surfaces
• Provides some water resistance
• Traditional method many contractors know

Cons:

• Not truly waterproof - only damp-proofing
• Cracks as foundation moves
• Tar deteriorates over time
• Cannot withstand hydrostatic pressure
• Requires reapplication every 10-15 years

Expert Opinion: As Martin Holladay notes, “if the hydrostatic pressure on the exterior of a concrete block wall is great enough, no system of parging or asphalt will keep water out.”

About Adding Ironite

Some contractors suggest adding Ironite (iron oxide) to parging mix. This:

• Can improve workability
• May reduce shrinkage slightly
• Adds rusty color (aesthetic choice)
• Does NOT significantly improve waterproofing

Modern Best Practice: The Dimple Mat System

Why Experts Recommend Dimple Mat

Building science has evolved beyond parging and tar. The modern approach addresses the root cause - water pressure - rather than trying to seal against it.

The Complete System:

1. Functioning footing drain - Most critical element
2. Dimple mat - Creates drainage plane
3. Proper waterproofing - Elastomeric or rubberized asphalt
4. Rigid insulation - Thermal and moisture benefits

How Dimple Mat Works with Block Walls

Installation Process:

1. Excavate to footing level
2. Install or repair footing drains
3. Apply waterproofing to block (optional but recommended)
4. Install dimple mat against wall
5. Add rigid foam insulation (climate-dependent)
6. Backfill carefully with drainage stone

Why It’s Superior:

• Relieves hydrostatic pressure
• Provides air gap for drying
• Protects waterproofing layer
• Works with wall movement
• 50+ year lifespan

Waterproofing New vs Old Concrete Block

New Block Construction

Best System:

1. Damp-proof blocks during construction
2. Install comprehensive drainage
3. Apply elastomeric waterproofing
4. Install dimple mat
5. Add exterior insulation

Cost: $25-40 per square foot Effectiveness: 95-99% when properly installed

Old Block Retrofit

Exterior Approach (Best):

1. Excavate to footings
2. Clean and repair blocks
3. Install new footing drains
4. Apply flexible waterproofing
5. Install dimple mat system

Interior Approach (Alternative):

1. Interior drainage system
2. Sump pump installation
3. Interior waterproofing coating
4. Proper dehumidification

Cost Comparison

Method	Initial Cost	Lifespan	Effectiveness	Total 30-Year Cost
Parging + Tar	$8-12/sq ft	10-15 years	60-70%	$24-36/sq ft
Dimple Mat System	$25-40/sq ft	50+ years	95-99%	$25-40/sq ft
Interior System	$15-25/sq ft	25-30 years	85-90%	$15-25/sq ft

Critical Installation Tips

If You Must Use Parging + Tar:

1. Ensure proper drainage first - No waterproofing works without it
2. Use quality materials - Type S mortar minimum
3. Apply in thin coats - Thick parging cracks more
4. Cure properly - Keep damp for 3 days
5. Use foundation coating - Not roof tar

For Dimple Mat Success:

1. Don’t skip the footing drain - It’s 50% of the solution
2. Overlap sheets properly - 6” minimum
3. Terminate correctly at grade
4. Protect during backfill
5. Use proper fasteners

Common Mistakes to Avoid

1. Believing parging alone waterproofs - It doesn’t
2. Skipping drainage improvements - Fatal error
3. Using roof tar - Wrong product
4. Applying to wet blocks - Poor adhesion
5. Ignoring cracks - Water will find them

Climate Zone Considerations

As noted in the building science discussion, insulation requirements vary:

Zone 3: R-5 minimum (1” foam) Zone 4: R-10 minimum (2” foam) Zones 5-8: R-15 minimum (3” foam)

Always combine waterproofing with proper insulation for best results.

The Bottom Line

For New Construction:

Always use modern dimple mat systems with proper drainage. The small additional cost provides decades of superior protection.

For Existing Buildings:

• If excavating anyway: Install dimple mat system
• If budget-limited: Interior drainage may suffice
• If minor dampness only: Parging + tar can help temporarily

Remember:

As the expert noted: “Most of the cost is getting a backhoe to do the excavation work. Once you’ve made the big investment of excavating, you want to do everything right.”

Professional Installation Recommended

While parging seems simple, proper concrete block waterproofing requires:

• Understanding of hydrostatic pressure
• Knowledge of material compatibility
• Experience with drainage design
• Proper equipment and materials
• Warranty-backed installation

Related Resources

• Exterior Waterproofing Services
• Dimpled Membrane Installation
• Foundation Drainage Solutions
• Concrete Repair Services

Conclusion

While parging with cement and tar is a traditional method, building science has proven that managing water through drainage and depressurization (dimple mat) is far more effective than trying to seal against hydrostatic pressure. For lasting protection of concrete block walls, invest in proper drainage and modern waterproofing systems.

Need professional concrete block waterproofing? Call (437) 545-0067 for expert solutions that last.