Beyond Subsidence: The Hidden Causes of Diagonal Cracks in Buildings

An Evidence-Based Guide for Property Owners and Construction Professionals

When diagonal or stepped cracks appear on a building’s walls in Kenya, the immediate assumption is often foundation subsidence—a shift in the position of the foundation. While this is indeed a common cause, limiting crack diagnosis to subsidence alone can lead to misdiagnosis, improper repairs, and recurring problems. This comprehensive guide examines the multiple factors that contribute to diagonal cracking in Kenya’s construction sector, backed by engineering research and local soil conditions.

Understanding Diagonal and Stepped Cracks

Diagonal cracks typically run at angles between 30 and 75 degrees across walls, while stepped cracks follow mortar joints in a staircase pattern. These cracks are wider at one end than the other, with the wider portion indicating the direction of the underlying problem. According to foundation engineering research, the wider end of a diagonal crack typically points toward the area experiencing the most stress or movement.

In masonry walls, diagonal cracks often manifest as stepped patterns because the crack follows the path of least resistance through mortar joints rather than breaking through the bricks themselves. This is particularly common in Kenya’s brick and block construction.

The Primary Suspects: Differential Settlement

Differential settlement remains the most frequently cited cause of diagonal cracks, but understanding its root causes is essential for proper remediation.

1. Soil Conditions Unique to Kenya

Kenya’s diverse soil profile creates significant challenges for foundation stability:

Black Cotton Soil: Found extensively in Nairobi’s Eastlands, parts of Rift Valley, and other regions, black cotton soil exhibits extreme shrink-swell behavior. Research on Nairobi soils shows that these expansive clays have very poor bearing capacity and high plasticity. During Kenya’s dry seasons, the soil shrinks and loses volume; during rainy seasons from March to May and October to December, it expands significantly.

This cyclical movement creates uneven stresses beneath foundations. If one portion of the building sits on black cotton soil while another rests on more stable red soil, differential settlement is inevitable. The soil doesn’t support the structure uniformly, causing one section to sink more than another—manifesting as diagonal cracks pointing toward the settling area.

Red Soils: While generally stable and found in Nairobi highlands, Kiambu, Murang’a, and Nyeri, red soils still require proper compaction. Inadequately compacted red soil can settle under load, particularly if moisture content varies across the building footprint.

Moisture Variation: Studies on Kenyan soil bearing capacity reveal that moisture content dramatically affects bearing capacity. Water infiltration from poor drainage, leaking pipes, or seasonal rainfall can reduce soil strength by up to 50%, causing localized settlement.

2. Foundation Design and Construction Errors

Research on building failures in Kenya identifies several foundation-related issues:

• Inadequate foundation depth: Foundations not reaching below the zone of seasonal moisture variation
• Poor soil compaction: Fill soil that wasn’t properly compacted before construction
• Ignoring soil testing: According to Kenya’s National Building Code (2022), geotechnical investigation is mandatory, yet many developments skip this critical step
• Inappropriate foundation type: Using strip foundations on weak soils that require raft or pile foundations

A structural analysis shows that approximately 98% of collapsed buildings in Kenya have framed structures, with foundation failures frequently cited as a contributing factor.

Beyond Settlement: Alternative Causes of Diagonal Cracks

3. Thermal Movement and Shrinkage

Kenya’s climate, with significant temperature fluctuations between day and night and between seasons, creates thermal stresses in building materials.

Concrete Shrinkage: Freshly poured concrete undergoes substantial volume reduction as it cures. Research indicates that concrete can shrink approximately 1/8 inch in 20 feet. In Kenya’s hot climate, rapid evaporation during curing can accelerate this process, creating shrinkage cracks—many of which appear diagonal, particularly around window and door openings where stress concentrations occur.

Differential Thermal Expansion: Different building materials expand and contract at different rates. When concrete lintels are cast above brick or block walls, the dissimilar thermal properties create stress concentrations. Engineering studies show that diagonal cracks frequently initiate at lintel ends and travel upward through masonry, particularly on sun-exposed walls.

RCC and Masonry Interface: Temperature variations cause differential movement between reinforced concrete columns and masonry infill walls. The concrete frame expands and contracts differently than the brick walls, creating diagonal tension cracks at the interfaces.

4. Structural Loading Issues

Overloading: Adding additional floors to buildings not originally designed for the extra weight is a documented problem in Kenya. When landlords construct upper floors incrementally as rental income accumulates, the original foundation may be inadequate for the increased load, causing settlement and diagonal cracking.

Inadequate Structural Design: Construction without proper engineering supervision can result in undersized structural elements. Beams that deflect excessively under load can create settlement-type conditions in the masonry above, resulting in vertical or diagonal cracks even without actual ground settlement.

5. Poor Construction Practices

According to Kenya’s Building Code Section 32, materials must be “of suitable nature and quality” and “applied, used or fixed properly.” However, enforcement challenges lead to:

• Insufficient concrete curing: Concrete not watered for the required 21 days leads to weak, crack-prone structures
• Improper construction sequencing: Building walls before columns, or constructing elements at different times with inconsistent concrete mixes
• Use of substandard materials: Salty sand in concrete mixes, poor-grade reinforcement steel, or uncured masonry units that continue shrinking after placement

6. Environmental and External Factors

Tree Roots: Large trees planted too close to buildings can extract moisture from soil beneath foundations, creating voids and causing settlement. This is particularly problematic with black cotton soil, which shrinks significantly when dehydrated.

Excavation Damage: Nearby construction, particularly deep excavations for basements or trenches for utilities, can destabilize adjacent foundations by removing lateral soil support or changing groundwater flow patterns.

Vegetation and Drainage: Inadequate plinth protection and poor drainage around building perimeters allow water to infiltrate foundation zones unevenly. In areas with black cotton soil, this creates differential expansion and settlement cycles.

7. Seismic Activity

While Kenya is not in a high-seismic zone, minor earth tremors do occur. These can create diagonal shear cracks in unreinforced masonry walls, particularly at corners and around openings where stress concentrations are highest.

Diagnostic Approach: Identifying the True Cause

Proper crack diagnosis requires systematic investigation:

1. Crack Pattern Analysis

• Cracks wider at top suggest settlement at that end
• Cracks wider at bottom may indicate heaving or slab deflection
• Location near corners often indicates settlement; location above lintels suggests thermal movement or lintel shrinkage

2. Crack Width Monitoring Mark crack ends with dated reference points to determine if cracks are active (growing) or stable. Active cracks wider than 3mm (1/8 inch) typically require professional structural assessment.

3. Site Investigation

• Examine drainage patterns and moisture accumulation
• Check for nearby trees, excavations, or plumbing leaks
• Assess soil type and local geology
• Look for signs of differential settlement: sloping floors, sticking doors, gaps between walls and floors

4. Professional Assessment Kenya’s complex soil conditions and construction practices make professional geotechnical and structural engineering assessment essential for cracks exceeding hairline width or showing progressive growth.

Prevention Strategies for Kenyan Context

Soil Management

• Mandatory soil testing: Conduct geotechnical investigation including borehole tests, standard penetration tests, and moisture profile studies
• Appropriate foundation selection: Use raft or under-reamed pile foundations on black cotton soil; strip foundations only on stable soils
• Soil improvement: For expansive soils, consider soil replacement, lime stabilization, or cement stabilization

Design and Construction Excellence

• Adequate foundation depth: Minimum 1200mm on black cotton soil, 600-900mm on stable red soils
• Proper drainage: Install French drains, perimeter channels, and 2-meter-wide flexible waterproof aprons around buildings
• Correct construction sequencing: Build skeletal frame first, then infill walls
• Quality materials and curing: Use specified grade materials and ensure proper 21-day concrete curing

Expansion Joint Installation

For long walls (over 20 meters) or buildings on variable soil conditions, install vertical expansion joints to accommodate thermal movement and differential settlement without cracking.

Regular Maintenance

• Keep planting at least 2 meters from building perimeter
• Maintain drainage systems to prevent water accumulation
• Monitor and repair plumbing leaks promptly
• Conduct annual inspections for early crack detection

Repair Approaches

Repair methodology must address the underlying cause:

For Settlement Issues:

• Foundation underpinning using helical or push piers
• Soil stabilization through grouting or chemical treatment
• Drainage improvement to control moisture

For Thermal/Shrinkage Cracks:

• Flexible crack filling with appropriate sealants
• Installation of expansion joints for future movement
• Reinforcement with mesh at crack-prone locations

For Structural Overloading:

• Structural strengthening of deficient elements
• Foundation augmentation if additional loads are permanent
• In extreme cases, removal of unauthorized additions

Conclusion

While differential settlement from foundation movement is a legitimate concern for diagonal and stepped cracks in Kenyan buildings, it represents only one point on a spectrum of possible causes. Kenya’s unique combination of expansive soils, climatic conditions, construction practices, and regulatory enforcement challenges creates multiple pathways to diagonal cracking.

Effective diagnosis requires understanding these interconnected factors rather than defaulting to subsidence as the sole explanation. For property owners, the key takeaway is clear: diagonal cracks warrant professional investigation by qualified structural engineers and geotechnical specialists familiar with Kenyan soil conditions and construction context.

The investment in proper soil testing before construction and professional assessment when cracks appear is invariably less costly than misdiagnosed repairs that address symptoms rather than causes. In Kenya’s dynamic construction sector, informed decision-making based on evidence rather than assumptions protects both structural integrity and financial investment.

About Makaobora.com

Makaobora.com is committed to providing evidence-based construction guidance for Kenya’s built environment, helping property owners and construction professionals make informed decisions based on local conditions and international best practices.

References

This article synthesizes information from peer-reviewed research on foundation engineering, Kenya’s National Building Code (2022), geotechnical studies of Nairobi soils, and structural engineering principles specific to East African construction contexts.