Can You Waterproof a Wet Roof in Ghana? 2026 Moisture Test Guide
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No — a polyurethane membrane must go onto a surface-dry roof. Moisture trapped in the concrete turns to vapour when the sun heats the slab and pushes the membrane up into blisters. Before coating, run a simple overnight plastic-sheet test; if moisture shows under the sheet, wait. For leaks that cannot wait, use a stop-gap that is designed for wet conditions and apply the proper membrane later.
Why wet concrete ruins a good membrane
Concrete is porous. After rain, water sits in the pores and in any screed on top. When the membrane goes over it, that water is sealed in. Then the sun arrives. Ghanaian roof surfaces reach 75°C, and the trapped water expands as vapour with nowhere to go except up. The result is blistering, then cracking at the blisters, then leaks — on a roof that has just been paid for. It is one of the most common reasons behind coatings peeling off within the first year.
The confusing part: polyurethane does need moisture — from the air — to cure. It does not want moisture underneath it.
The plastic-sheet test
- Cut a clear plastic sheet about 50 cm × 50 cm.
- Tape all four edges tightly to the slab with strong tape.
- Leave it overnight, or for most of a sunny day.
- Lift it. If you see droplets on the underside, or the concrete under the sheet is darker than around it, the slab is too wet.
- Repeat in the lowest spot, the shadiest spot and near any area that ponded.
It costs almost nothing and is the check we run before coating in the rainy seasons. Ask any contractor whether they will do it.
What affects drying time
| Factor | Dries faster | Dries slower |
|---|---|---|
| Drainage | Clear outlets, no ponding | Water sat for hours or days |
| Screed | Thin, sound, well bonded | Thick, hollow or cracked screed holding water |
| Exposure | Full sun, breeze | Shaded by tanks, parapets, trees |
| Weather | Dry season, Harmattan | Humid, overcast minor-rain days |
| Old coatings | Removed | Failed coating trapping water beneath it |
There is no honest single number for "days after rain". A sound slab in full sun can pass the test quickly; a roof with a hollow screed can hold water for weeks. Testing beats guessing. If the screed is hollow and saturated, it may need cutting out rather than waiting.
If the leak cannot wait
| Situation | Product | Price |
|---|---|---|
| Water pushing through a hole or crack in concrete | Plugging King (sets under water) | GHS 100 |
| Crack or joint you can dry for about an hour | Leak Seal Tape, butyl, 10 cm × 5 m | GHS 250 |
| Small awkward area, bolts, laps | Waterproof Leakage Spray | GHS 200 |
These are holding repairs. They stop the drip; they do not waterproof the roof.
When the roof is dry: the proper system
- Clean off dust, salt and loose material; prime.
- Fleece-reinforce parapets, gutters, outlets, penetrations and cracks (bridges ~3 mm vs ~0.3 mm unreinforced). Not the whole sound roof field — that roughly doubles cost for no gain, and fleece does not extend the 20-year design life.
- Apply two coats at about 0.4 kg/m² in total. 10kg (GHS 1,000): 40-80 m² per coat, 20-40 m² finished. 5kg (GHS 500): 20-40 m² per coat, 10-20 m² finished.
- Check the forecast: each coat needs a rain-free period to cure.
Installed: 2-coat PU GHS 80-130/m²; fleece-reinforced GHS 180-280/m². Cover: 3-year workmanship warranty, 3-year manufacturer material warranty, 20-year design life. The warranty is the period we return and fix it at our cost; the design life is how long the membrane is engineered to keep performing.
Book an inspection
We test the slab before we quote a date, and we will tell you if it needs to dry first. Book an inspection or call 0555 548 001.