Building Through the Rainy Season in The Gambia

What the rains actually do to a site

The Gambian wet season runs roughly from June to October, with the heaviest rainfall through July, August and September. For any build longer than a few months, part of the programme will fall inside it. The question is which part.

The effects are specific rather than general. Excavations fill and their sides collapse. Access tracks turn to mud and delivery vehicles cannot reach the site. Sand and aggregate stockpiles absorb water, which throws off concrete mixes. Blocks stacked on saturated ground draw moisture up through the stack. Render and paint will not dry properly in sustained high humidity. And an open structure without a roof takes water into every wall it has.

Sequencing: what to bring forward

The single most valuable planning decision is to get the roof on before the rains arrive. A weathertight structure lets internal work continue right through the wet season, which turns four idle months into four productive ones.

Best done in the dry season

• Site clearance, excavation and earthworks
• Foundations and ground beams
• Hardcore filling, compaction and the ground-floor slab
• Roof structure and covering
• External rendering and painting
• Access roads, drainage and external works

Can continue through the rains

• Internal blockwork and partitions
• Electrical and plumbing first fix
• Internal plastering, provided the building is weathertight and ventilated
• Tiling, ceilings and joinery
• Internal painting
• Kitchen and bathroom fit-out

Protecting a site through the wet months

• Cut drainage channels and sumps so water leaves the site rather than sitting in it. Do this before the first rains, not after.
• Cover and raise cement storage. Cement that has absorbed moisture is not usable and it is one of the more expensive things on site.
• Stack blocks off the ground on timber and keep them covered.
• Sheet sand and aggregate stockpiles, or accept that mix water has to be adjusted for the moisture they are carrying.
• Backfill excavations promptly rather than leaving them open between operations.
• Protect fresh concrete from heavy rain for the first several hours after placing — rain falling on a green surface damages it.
• Maintain the access track. Once it fails, deliveries stop and everything else follows.
• Keep reinforcement off the ground and clear of standing water.

Concrete in wet conditions

Concrete needs moisture to cure, so humidity itself is not the enemy. The problems are more specific.

Wet aggregate carries water that is not accounted for in the mix, which quietly raises the water-cement ratio and lowers the strength of the finished concrete. Rain falling directly onto a fresh surface washes cement from the top and leaves a weak, dusty layer. And saturated ground under a foundation changes how the load is carried.

The practical responses are to adjust mix water for the moisture content of the aggregate, to avoid pouring when heavy rain is imminent and cover fresh work when it is not avoidable, and to dewater excavations before placing concrete rather than pouring into standing water.

Designing for the climate, not just around it

The rains are also a design brief, and the decisions that matter are made long before anyone is on site.

Set the finished floor level generously above the surrounding ground. Design the drainage for the intensity of a heavy Gambian downpour rather than for an annual average. Give the roof adequate overhang so water is thrown clear of the walls. Size gutters and downpipes properly, and take the discharge away from the foundations rather than dropping it beside them. Detail the damp-proof course and membrane as structural work, because that is what they are.

Every one of these is close to free at design stage and expensive or impossible to add afterwards.