Why find the source before patching?
- Water finds another path at C or D
- The wall stays damp; efflorescence and rust keep worsening
- It looks fixed for months, then returns
Step 1: Keep a leak log (free and critical)
Step 2: Eliminate zones (outside-in, bottom-up)
- Bathroom / kitchen directly above: suspect upstairs pipes or waterproofing
- Balcony / roof above: rain-only → roof waterproofing
- Concealed pipes in the ceiling void: joints on older conversions
- AC drain: drip under the unit → check detached drain hose
- External face: worse after rain → cracks or aged window sealant
- Indoor pipe routes: fixed stain locations vs pipe drawings
- Back of bathroom wall: failed waterproofing or pipe joints
- Ground-floor damp: rising moisture or groundwater
- Corner stains: parapet / balcony drain blockage pushing water into the wall
Step 3: Simple tests
- **Shut-off test**: Close the main inlet. If leaking stops → pipe issue; if not → waterproofing / external water.
- **Drainage test**: Run bathroom water and watch the ceiling below.
- **Hose test**: Flush a suspected external wall area from outside and watch indoors.
When not to DIY
- Deep in-wall leak, hollow knock sound, growing area—hire instrument tracing
- Falling concrete / exposed rebar—structural assessment immediately
- Suspected soil pipe (yellow water, odour)—licensed plumber first
- Scaffold needed for external walls—do not climb
- Common-area building pipes—notify management; do not DIY