Kumar IronFind the real source
Water often enters at one point and shows up as a damp patch several feet away because it travels along the slab or sheet. Look at the roof from above after rain or run a hose on small areas, starting from the lowest point and working up, with someone watching the ceiling below.
Common causes on an RCC terrace
- Hairline cracks in the slab or screed, often from shrinkage or temperature movement.
- Poor slope, so water ponds instead of running to the drain.
- Choked or high drain outlets.
- Open joint at the parapet and roof corner.
- Gaps around pipes, water tank bases and cables passing through the slab.
- Worn old waterproofing coat that has peeled or become brittle.
- Cracks at expansion joints with no flexible sealant.
Fixes for an RCC terrace
- Clean the surface fully: remove dust, algae, loose plaster and old failed coat.
- Open cracks in a V groove and fill with polymer modified mortar or a flexible crack filler.
- Form a cove of polymer modified mortar at the parapet junction, about 75 mm each way.
- Seal pipe collars and drain mouths with a flexible sealant.
- Correct the slope with a sloped screed toward the drain, about 1 in 100.
- Apply the waterproofing coating in two coats, take it up the parapet at least 150 mm and cure it.
- Do a ponding test for 48 to 72 hours before the rains.
The full step by step method is in our guide on roof waterproofing before monsoon.
Waterproofing products by type
These are generic families. Brands such as Dr. Fixit, Asian Paints, Birla and UltraTech make products in each. Check the data sheet for coverage, coats and curing, and see our waterproofing range for what is in stock.
| Problem | Product type |
|---|---|
| Hairline cracks, general terrace | Polymer modified cementitious coating, or acrylic elastomeric coating |
| Bond coat, repairs, cove | SBR latex mixed in cement slurry or mortar |
| New slab or screed | Integral waterproofing compound added to the mix |
| Moving cracks and joints | Flexible polyurethane or polysulphide sealant |
| Large flat roof, covered finish | Bituminous or APP membrane with a screed on top |
| Sheet roof with small holes | Bituminous tape or sealant, or replace the damaged sheet |
Leaks in sheet roofs
On a GI, GC or fibre cement sheet roof, leaks usually come from loose or missing J bolts and washers, cracked sheets, too little overlap or rusted holes. Use the correct washers with every fastener, give side laps of at least one and a half corrugations and tighten fasteners without crushing the sheet. A damaged sheet is better replaced than patched.
Walls and seepage from the side
If the damp is on a wall below the roof, check the parapet, the sunshade and any exposed pipes first. Water entering at the top of a wall is a roof problem, not a plaster problem. Repaint only after the source is closed and the wall is dry.
When to call a professional
If cracks are wider than about 2 mm, are growing, or show rust stains from the steel inside, ask an engineer to look at the slab before applying any coating. A coating hides a leak, it does not repair structural damage.
Frequently asked questions
- What is the cheapest way to stop a roof leak?
- Clean the surface, fill cracks, seal the joints and apply a coating of the right type in two coats. Fixing the cause with good prep costs less than repeating a thin coat every year.
- Can I waterproof during the rains?
- Only as an emergency patch. Most coatings need a dry surface and dry days to cure, so do the main job before the monsoon.
- Which product is best for terrace waterproofing?
- A polymer modified cementitious or acrylic elastomeric coating is common for exposed terraces. The best choice depends on cracks and movement.
- Why does my roof leak even after waterproofing?
- The usual reasons are missed parapet corners, pipe gaps, poor slope, too thin a coat, or coating over damp and dusty concrete.