▽ +09.00Problem · Roofs
Water is leaking through the roof
Damp patches on the top-floor ceiling, or water dripping after rain or after the roof is washed.
What you see
- Damp patches or discolouration on top-floor ceilings, especially near corners and light fittings
- Peeling or blistering paint, or plaster coming away
- White salt deposits (efflorescence) on walls or ceilings
- Water standing on the roof after rain: the falls are not leading it to the drains
- Waterproofing lifting at parapets, corners and drains, or around AC bases and pipes
- Rust stains or cracks in exposed concrete
Possible causes
Failed details
Most leaks start at parapets, drains, corners and penetrations rather than in the open roof.
Ponding water
Poor falls keep water on the membrane, where it finds every weak lap.
A damaged or aged membrane
UV, heat, traffic or later works such as new AC units and fixings damage the layer.
Blocked drains
Dust and debris block the outlets, so water rises above the upstands.
Cracked protection or tiles
Water gets in and travels under the finish until it finds a weak point.
How it is diagnosed
The diagnosis confirms the cause before any repair is chosen.
Map the stains
Mark them on a roof plan; a leak often shows far from where the water gets in.
Inspect the details
Parapets, drains, corners, penetrations and AC bases — photographed.
Check falls and drains
Wet the roof and watch where water stands and how fast the drains run.
Test zone by zone
A controlled flood or spray test, zone by zone, narrows down the source (ASTM D5957 is the usual guide).
Check the membrane
Small inspection openings where needed, to see the layer and the moisture beneath it.
- Membrane
- Clamping ring
- Strainer
- Outlet pipe
Possible solutions
The fix follows the diagnosis; these are the common options and their systems.
Local repair of details
When the membrane is sound and the leak comes from a detail: rebuild the detail with a compatible system, often liquid PU.
Systems
Re-waterproofing the roof
When the layer is aged or has failed widely: strip to a sound base and apply a new system with all its details.
Correcting the falls
Where water ponds: a lightweight screed to falls before the new waterproofing.
Systems
Treating tiled roofs without removal
Only when the tiles are sound: a clear PU coating over repaired joints.
Systems
Upgrading to an inverted roof
When thermal insulation is needed too and the heights allow it.
Systems
What not to do
- Sealing cracks with random products: they can trap water or hide its source
- Adding a new layer over a wet roof without checking moisture and compatibility
- Drilling or fixing AC bases through the waterproofing without detailing
- Waiting for the rainy season to find the leak
General educational content; final values and requirements follow the manufacturer’s TDS and the approved project specification.
Common questions
Why does the stain appear far from the leak?
Water travels under the tiles, the screed and along the slab before it finds a way down.
What should I check before the rainy season?
Clear the drains and outlets, remove dust and debris, look over the corners, penetrations and waterproofing edges, and photograph anything unusual.
Why does the roof leak only in heavy rain?
Because water ponds and rises above the waterproofing terminations at parapets and doors when the drains are blocked or the falls are poor, or wind drives the rain through the parapet capping and the facade.
Technical inspection
Need a diagnosis for your case?
Send photos, a description and the location; our engineers identify the cause and the fix after an inspection.