01 · Materials
What waterproofing is made of.
Each material suits different surfaces and conditions. Lifespans are typical figures for correctly applied, maintained work; actual life depends on exposure and upkeep.
APP modified bitumen membrane
10–15 years
A 3–4 mm sheet of bitumen modified with plastomeric polymer, reinforced with polyester, torch-applied in overlapping rolls.
- Best for
- Exposed terraces, flat roofs, podiums. Handles heat and sun well.
- Watch for
- Needs a skilled torch crew and careful detailing at drains and parapets. Best finished with a protective screed or reflective coat.
SBS modified bitumen membrane
10–15 years
Similar sheet with an elastomeric polymer. More stretch and flexibility than APP; available torch-on or self-adhesive.
- Best for
- Roofs with structural movement, podiums, foundations.
- Watch for
- Less UV-resistant than APP when left exposed; protect it.
Self-adhesive membrane
10+ years when protected
A cold-applied, peel-and-stick bituminous sheet. No flame needed.
- Best for
- Occupied buildings, timber or sensitive areas where a torch is unsafe, basements.
- Watch for
- Surface must be dry, primed and clean or it will not bond.
HDPE / TPO sheet
15–20+ years
Single-ply plastic membranes. HDPE is pre-applied below slabs and behind basement walls; TPO is heat-welded on roofs.
- Best for
- New basements and foundations (HDPE), large flat roofs and tanks (TPO).
- Watch for
- Must be installed during construction for blind-side basement work.
Crystalline waterproofing
Life of the concrete
A cement-based slurry or admixture whose chemicals react with water and lime in concrete, growing crystals that block the pores.
- Best for
- Water tanks, basements, lift pits, sumps. Works from the inside (negative side) too.
- Watch for
- Only for concrete. Does not bridge moving cracks.
Polyurethane (PU) liquid membrane
8–12 years
A liquid that cures into a seamless rubbery film with high stretch. Aliphatic grades resist sunlight; aromatic grades need a UV topcoat.
- Best for
- Terraces with many pipes and corners, balconies, planters, roof details.
- Watch for
- Needs correct thickness (measured in microns) and a dry surface.
Acrylic elastomeric coating
4–6 years
A water-based, flexible, UV-stable coating applied in two or three coats, often with a fibre mesh at cracks.
- Best for
- Exposed terraces with light foot traffic, external walls, quick refurbishments.
- Watch for
- Thinner than a membrane. Plan to recoat every few years.
Cementitious coating
Life of the tiling
A two-part polymer-modified cement slurry, brushed on in two coats. Flexible grades bridge hairline cracks.
- Best for
- Bathrooms and wet areas under tiles, water tanks, balconies, sunken slabs.
- Watch for
- Not for exposed roofs. Must be covered by tiles or screed.
Injection grouting
Varies by resin
Resins pumped into cracks under pressure. PU foam stops running water; epoxy bonds cracks structurally; acrylic gel seals behind walls.
- Best for
- Live leaks in basements and tanks, structural cracks, construction joints.
- Watch for
- Treats the crack, not the whole surface. Usually combined with a coating.
Chemical DPC (damp-proof course)
20+ years
A silane/siloxane cream injected into the mortar line near floor level, forming a water-repellent barrier against rising damp.
- Best for
- Ground-floor walls with rising damp, older buildings.
- Watch for
- Old salty plaster must be replaced with a breathable render afterwards.
Flexible PU or polysulphide sealants that fill movement joints and gaps around frames.
- Best for
- Expansion joints, window frames, pipe entries, coping joints.
- Watch for
- Need the right joint width and backer rod. Rigid fillers crack.
02 · Methods
How each area is done properly.
Terraces & flat roofs
The most common source of top-floor leaks.
- Inspect and remove failed coatings, loose screed and broken tiles.
- Repair cracks; rebuild hollow areas.
- Correct the slope so water reaches the outlets (about 1 in 100).
- Form rounded coving where the slab meets the parapet.
- Seal outlets and pipe penetrations with collars.
- Prime, then apply the membrane or liquid system, turned up the parapet at least 300 mm.
- Flood test for 48–72 hours.
- Finish with a protective screed, tiles or UV-reflective topcoat.
Bathrooms & wet areas
Leaks show on the ceiling below or on the wall behind.
- Remove tiles and bedding down to the slab.
- Treat pipe entries and the floor drain with sealing collars.
- Apply a flexible cementitious or PU coating: the full floor, walls up to 300 mm, and shower walls to about 1.8 m.
- Reinforce corners with tape or mesh.
- Pond test for 24–48 hours before re-tiling.
- Re-tile with epoxy grout in the shower area.
Basements & lift pits
Water pushes in under pressure from the soil.
- New construction: wrap the outside with HDPE or bituminous membrane and a drainage board before backfilling.
- Existing basements: stop live leaks with PU injection.
- Treat the inside face with crystalline or cementitious tanking.
- Seal construction joints and pipe entries.
- Add a sump and drainage channel where water still collects.
Water tanks
Underground and overhead tanks hold drinking water.
- Drain, clean and inspect the tank.
- Seal cracks and honeycombed concrete; inject live leaks.
- Apply a non-toxic crystalline or cementitious system rated for potable water.
- Cure fully and flush before refilling.
External walls & façades
Driven monsoon rain enters through cracks, hollow plaster and window joints.
- Tap-test and break out hollow plaster; re-plaster with a waterproof mix.
- Fill and bridge cracks with polymer mortar and mesh.
- Seal window frames, sills and AC openings with flexible sealant.
- Apply an elastomeric weather-shield paint, or a clear silane/siloxane repellent on exposed stone and concrete.
Metal sheet roofs (galvanised patra)
Leaks come from fasteners, laps and rust.
- Clean and treat rusted areas with a rust converter and primer.
- Seal every screw head and the sheet laps.
- Apply membrane strips over laps and ridges, or a full elastomeric roof coating.
- Clear gutters and fix broken sheets.
Rising damp
Peeling paint and salt marks along the bottom of ground-floor walls.
- Remove salt-damaged plaster up to about 1 m above the damp line.
- Inject a chemical DPC into the mortar course near floor level.
- Apply a salt-neutralising wash.
- Re-plaster with a breathable, salt-resistant render; paint only once dry.
03 · Choosing
Which system for which area.
A starting point. The right answer for your building comes from a site inspection.
| Area | Usually best | Alternative |
|---|
| Exposed terrace, heavy sun | APP membrane with protective screed | Aliphatic PU membrane |
| Terrace with many pipes and corners | PU liquid membrane | Acrylic elastomeric with fibre mesh |
| Bathroom, balcony, sunken slab | Flexible cementitious coating | PU coating under tiles |
| Basement, existing building | Injection + crystalline tanking | Cementitious tanking with drainage |
| Basement, new building | HDPE or bituminous membrane outside | Crystalline admixture in the concrete |
| Drinking water tank | Potable-grade crystalline | Potable-grade cementitious |
| External walls | Elastomeric weather-shield paint | Silane/siloxane clear repellent |
| Metal sheet roof | Fastener sealing + lap membrane strips | Full elastomeric roof coating |
| Ground-floor damp walls | Chemical DPC + breathable re-plaster | Tanking on the inside face |
04 · Mistakes
Why waterproofing fails.
Painting over dampWaterproof paint on a wet wall traps moisture. It blisters within a season.
Ignoring the slopeIf water ponds on a terrace, even a good membrane is under constant attack. Fix the slope first.
Forgetting the detailsMost leaks start at drains, parapet corners, pipe entries and door thresholds, not in the open area.
Treating the wrong sideCoating a basement from inside without stopping live leaks just moves the water.
Skipping the flood testA 48-hour water test is the only proof the work is sound before tiles go back on.
Choosing on price per square footA single thin coat costs less today and twice as much after the next monsoon.
05 · Maintenance
Before every monsoon.
- Clear terrace outlets, gutters and rainwater pipes of leaves and debris.
- Look for cracks, blisters or peeling on the terrace and parapets.
- Check sealant around windows, AC units and pipe entries.
- Look at top-floor ceilings and external walls for fresh stains.
- Check water tank lids and overflow pipes.
- Book an inspection in March to May, so any work cures before June.
