“Flat concrete roofs in Kenya suffer heavily from standing rainwater and intense tropical sunshine. Here is the field-tested method our engineers use with torch-on APP membranes.”
Why Flat Concrete Roofs Leak in Kenya
In Nairobi, Mombasa, and across East Africa, flat concrete roofs are widely chosen for residential homes, commercial plazas, and apartment buildings. However, flat roofs take the full brunt of weather: alternating between heavy monsoon rain and scorching equatorial UV sun.
When concrete expands in daytime heat and contracts at night, hairline cracks form. Without an elastomeric membrane, standing water finds every crack and seeps into gypsum ceilings and electrical points.
Step 1: Proper Surface Cleaning & Slope Inspection
Never apply waterproofing membrane on dusty, damp, or oily concrete. The slab must be power-washed and inspected for correct slope (minimum 1:80 fall toward drainage gutters or scupper pipes).
- Fill honeycombs and cracks wider than 2mm with non-shrink structural repair mortar.
- Install a 50mm x 50mm cement mortar angle fillet (chamfer) along all 90° parapet wall corners to prevent sharp folds in the membrane.
- Allow new concrete to cure for at least 21 days until moisture content is below 5%.
"Pro Tip from Solphem Engineers: If you apply membrane on damp concrete, trapped moisture turns to steam under the midday sun and blows bubbles under your roof within 3 months."
Step 2: Applying the Bitumen Penetration Primer
Apply a uniform coat of Solphem Bitumen Primer D41 across the entire slab and at least 300mm up the parapet walls. The primer seals microscopic concrete pores and ensures a permanent bond with the torch-on sheet.
- Coverage rate: 0.25 to 0.30 litres per square meter.
- Drying time: 4 to 6 hours depending on ambient temperature and wind.
Step 3: Torching the 4mm APP Membrane
Unroll the Solphem 4mm APP Bituminous Membrane and align it starting from the lowest drainage point working uphill. Using a standard propane gas torch:
- Heat the underside burn-off polyethylene film until the bitumen becomes glossy and begins to flow.
- Slowly unroll the membrane forward while pressing firmly with a roller to release air pockets.
- Maintain a 100mm side overlap and 150mm end overlap between adjacent rolls.
- Use a heated rounded trowel to smooth out the bitumen bleed at all seam edges for a watertight weld.
Step 4: The 24-Hour Water Ponding Test
Before handing over the roof or tiling, plug all drainage outlets and fill the roof with water to a depth of 50mm. Leave the water for 24 to 48 hours. Inspect the ceiling below for any dampness. Once verified 100% watertight, release the water.
Need Materials or Installation Support?
Solphem Ltd delivers certified torch-on membranes, primers, and application tools directly to your building site in Nairobi and across Kenya. Reach out to our technical team today for quantity calculation or quote.
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