Before hiring crews with jackhammers to tear up the roof, you need to be 100% certain that water is entering through the roof surface and not from a side wall, pipe, or parapet. The detection process is purely investigative work, split into two phases.
Don't look for huge holes. Water needs a crack smaller than a millimetre to cause damage. Go up on a dry day and check the 3 most common "victims" in order:
Check meticulously around the solar water-heater mounts, bathroom vent pipes, and antenna cables. Where metal or plastic penetrates the membrane (or tiles), is there polyurethane sealant? Has it dried out and cracked open?
Right-angle junctions are the "Achilles heels." Look for cracks in the skirting joint. Also inspect the top of the parapet wall (the coping). If the coping has cracks, water enters the wall, runs vertically down, and emerges on your ceiling!
Are there blisters in the bitumen sheet? Is grout missing between tiles?
If the visual inspection doesn't reveal anything obvious - or if you found 5-6 suspicious spots but don't know which one is guilty - it's time to intentionally "drown" the roof.
The technician plugs the drain outlets airtight, usually with inflatable test balloons or mechanical plugs that tighten with a screw inside the pipe.
We open the hose and fill the roof with water, turning it into a shallow pool. The water level must NOT exceed 5-8 cm and must stay below the height the membrane turns up on the parapets.
We leave the roof flooded. If there's a hole, the enormous hydrostatic pressure will force water through every defect in the membrane.
What does the Water Test tell us?
If the living-room stain starts dripping heavily, then absolutely certainly the problem is in the horizontal roofing membrane. The waterproofing has been punctured.
If 48 hours pass with a full roof and a dry living room, the roof is fine. Water is entering from somewhere higher - vertical parapet walls, cracked copings, or damaged pipework in wind-driven rain.
The living-room stain has driven us crazy. The contractor says: "The bitumen sheets are gone - full tear-off, €3,000."
We listen to the contractor. We strip 100 m² and lay fresh membranes. Winter comes, a storm hits with strong northerlies, and the stain reappears! What happened? Water was entering through a crack in the external, vertical wall - not through the roof at all! €3,000 wasted.
We call an engineer. He plugs the drains and fills the roof with 5 cm of water. We wait 2 days. The roof is flooded but the living room is bone-dry. The engineer drains the roof, then locally hoses the vertical parapet wall for 30 minutes. Bingo! The room starts dripping. We've found the real culprit, and with €50 of elastomeric paint we fix the problem.
Bottom Line: Never proceed to expensive repairs or tear-offs unless you've proven, beyond any doubt, exactly where the water enters. The Water Test is the cheapest, most reliable and "toughest" test to see whether your horizontal membrane holds or has failed.
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