Roof Drain Outlet Waterproofing in Ghana 2026: The Detail That Leaks
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Most flat-roof leaks in Ghana that are not at the parapet are at the drain outlet. The fix is a specific detail: cut the outlet flush with or slightly below the slab, reinforce a collar of fleece around it, and carry the polyurethane membrane down into the pipe — not just up to its edge. Before any of that, clear the outlet: a blocked outlet causes more ponding than a badly laid slab.
Why outlets fail
- Two materials meet. PVC or metal pipe expands and contracts differently from concrete. Under a 20°C daily swing, the joint opens and closes every day.
- Shrinkage cracks. Concrete poured around a pipe cracks radially as it cures.
- The outlet sits high. If the pipe rim stands a few millimetres above the slab, water ponds around it and sits on the most vulnerable joint.
- The coating stops at the rim. A membrane that ends at the pipe edge has nothing to hold it down. Water gets under the edge and lifts it.
- Undersizing. A single 75 mm outlet on a large roof cannot clear a storm of 100 mm per hour. Water backs up over upstands.
First: the free fixes
Clear leaves, sand and Harmattan dust from outlets and the pipe below. Pour a bucket of water and watch it drain. If the outlet sits proud of the slab, it can often be cut down. Only if ponding persists after this is a tapered screed worth pricing. Saying this loses us some screed jobs; it also saves clients a lot of money.
The detail that holds
- Cut the outlet flush or 2-3 mm below the finished slab level.
- Repair the concrete around the pipe; fill any radial cracks.
- Prime the concrete and abrade the pipe surface so the membrane can key.
- First coat of polyurethane around the outlet, about 300 mm out and down inside the pipe.
- Embed fleece into the wet coat, cut in petals down into the pipe.
- Saturate the fleece and apply the main two-coat membrane over the whole roof, overlapping the collar.
- Fit a leaf guard so the outlet stays clear.
Outlet options compared
| Detail | Typical result | Verdict |
|---|---|---|
| Coating stops at pipe rim | Edge lifts within 1-2 rainy seasons | Avoid |
| Sealant bead around pipe, coating over | Holds until sealant hardens in UV | Short-term only |
| Fleece collar carried into pipe | Bridges cracks up to ~3 mm; stays bonded | Recommended |
| Proprietary outlet with clamping ring | Excellent where budget and access allow | Good for new builds |
How many outlets?
As a practical guide in Ghanaian storm conditions, a 100 mm outlet serves roughly 40-60 m² of flat roof. More important than the exact number: never rely on one outlet. Give every roof area either two outlets or an outlet plus a through-parapet overflow set slightly higher, so a single blockage cannot flood the roof. On commercial roofs, have the engineer confirm sizing against the design storm.
Gutters and outlets together
Concrete gutters feed outlets, and the two fail together. On our Tema Community 10 job we lined the full gutter channel with membrane and carried it into each outlet, rather than patching the leaking corners.
Cost and material
- Fleece-reinforced detail at outlets: GHS 180-280/m² of reinforced area (a few m² per outlet).
- Rest of the roof: 2-coat PU at GHS 80-130/m². No fleece needed on a sound roof field — it roughly doubles cost there and does not extend the 20-year design life.
- DIY material: about 0.4 kg/m² for two coats. 5kg (GHS 500) gives 10-20 m² finished (20-40 m² per coat); 10kg (GHS 1,000) gives 20-40 m² finished (40-80 m² per coat).
Temporary hold for a leaking outlet: Waterproof Leakage Spray (GHS 200) around a dry pipe rim, or Leak Seal Tape (GHS 250) on a flat joint.
Warranty and design life
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.
Have your outlets checked
We check every outlet, water-test it and quote the detail separately from the roof field. Book an inspection or call 0555 548 001.