El Niño is coming. What kind of buildings survive it?

On 18th August 2026, the Tanzania Meteorological Authority issued a forecast warning of a very strong El Niño season running from October 2026 through January 2027. The warning covers Arusha, Kilimanjaro, Manyara, the northern coast, Dar es Salaam, Tanga, and the Lake Victoria basin. Above-normal rainfall, flooding, strong winds, and landslides are all cited as expected risks.

The question for property owners and developers is not whether El Niño will arrive. It is whether their buildings are designed to handle it. This article sets out the five structural and design factors that determine whether a building survives an El Niño season with minimal damage.

Aftermath of El Nino rains, Tanzania.

1. Drainage design and site water management

The most common cause of building damage during heavy rainfall is not the rain itself but the failure of drainage systems to handle peak water volumes. Buildings on sites with inadequate drainage design experience water ingress at foundation level, flooding of ground-floor spaces, and erosion of subgrade soils that undermines foundation stability.

A correctly designed site drainage system accounts for the peak rainfall intensity for the region, not average annual rainfall figures. In Arusha and the northern highlands, peak rainfall events during El Niño seasons significantly exceed normal seasonal averages. Drainage channels, soakways, and discharge points must be sized for these peak scenarios.

Buildings with flat or near-flat roofs require internal rainwater outlets with overflow provisions.

A single blocked outlet on a flat roof during a peak rainfall event can result in roof loading that exceeds the structural design capacity, with potentially catastrophic consequences. This is a design and maintenance issue simultaneously.

If your property has surface drainage that has never been assessed for peak flow capacity, now is the correct time to do it , before October.

Proper Drainage Design

2. Foundation type and soil conditions

Soil behavior changes significantly when saturated. Clay soils expand when wet and contract when dry, generating movement in any structure founded on them. Black cotton soil, which is common in parts of northern Tanzania, is particularly problematic: it has high shrink-swell potential and low bearing capacity when saturated.

Buildings founded on strip or pad footings in problematic soil types without adequate reinforcement or founding depth are vulnerable to differential settlement during periods of prolonged heavy rainfall. Differential settlement produces cracking in walls and slabs that starts as cosmetic damage and progresses to structural failure if left unaddressed.

Buildings with raft foundations or deep strip foundations founded below the zone of soil movement are significantly more resilient. Buildings on slopes with retaining structures must also have those structures assessed for stability under saturated soil conditions, where the lateral pressure on retaining walls increases substantially.

If your building is on a slope, adjacent to a retaining wall, or founded on a site that was not subject to a geotechnical investigation before construction, it warrants inspection before the El Nino season begins.

3. Roof structure and wind uplift resistance

The TMA forecast specifically cites strong winds as a risk during the 2026 El Nino season. Wind uplift on roofs is the primary mechanism by which roofing is lost during storm events. It is not the wind pushing the roof down that causes failure. It is the pressure differential between the underside and the top surface of the roof that generates a lifting force.

Roofs most vulnerable to wind uplift are those with wide overhangs and inadequate fixing connections between the roof structure and the wall plate. Corrugated metal sheet roofing fixed with insufficient fasteners at inadequate spacings is the most common failure mode seen in Tanzania after storm events. The sheets lift at the edges, the wind gets underneath, and the roof is progressively lost from the perimeter inward.

A building with a pitched roof, correctly fixed purlins, adequate rafter-to-wall plate strapping, and properly spaced sheet fasteners at 150mm maximum centres at edges and 300mm at internal zones will perform substantially better in high wind conditions than one where any of these elements were cut back during construction.

If your roof structure has not been inspected since construction, get it checked before October.

Roof structures

4. Wall construction and water penetration resistance

Masonry walls without adequate external render, or with render that has cracked and allowed moisture ingress, are vulnerable during sustained heavy rainfall. Water penetrates the render cracks, saturates the masonry, and in severe cases causes the render to delaminate or the masonry to spall.

At ground level, the critical detail is the damp-proof course: a horizontal barrier in the wall construction that prevents ground moisture from rising into the wall by capillary action. Buildings without a functional damp-proof course show rising damp staining on internal wall surfaces within the first metre above floor level. During a prolonged wet season, this moisture ingress accelerates and can compromise internal finishes, timber door frames, and floor screeds.

External wall surfaces should be inspected for cracking, delaminating render, or gaps at window and door reveals before the rainy season. These are straightforward maintenance interventions when addressed before water ingress has occurred. They are significantly more expensive to address after sustained wetting has damaged the wall fabric.

5. Site positioning and flood risk

Buildings positioned in natural drainage channels, low-lying areas adjacent to rivers or streams, or on sites with no overland flow outlet are at direct risk of flooding during peak rainfall events. This is a site planning issue that cannot be resolved through building design alone once the structure is in place.

For existing buildings in flood-risk positions, the options are limited to temporary flood barriers at openings, raising electrical installations and critical equipment above predicted flood levels, and ensuring contents and documentation can be moved quickly if a flood warning is issued.

For buildings under design or in planning, site positioning relative to flood risk is one of the most consequential decisions made at inception. A site analysis that includes assessment of the natural drainage pattern, proximity to seasonal watercourses, and historical flood extent for the plot is not an optional extra. It is the minimum due diligence required before a foundation is designed.

If you are currently planning a project on a site you have not had formally assessed for flood risk, that assessment should happen before design begins, particularly given the current El Nino forecast.

The 2026 El Nino season will test every building in northern and coastal Tanzania. The structures that come through it with minimal damage will be the ones where drainage, foundations, roof fixing, wall waterproofing, and site positioning were all correctly addressed at the design and construction stage.

If you have concerns about an existing property or are planning a new project and want to understand your site's exposure, talk to our team before October.

Get your site assessed before El Nino season.

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