Why buildings leak.

Waterproofing is the item most often trimmed when a budget tightens, because it is invisible when it works and the consequence arrives after everyone has left. It is also the source of most complaints in the first two monsoons of a new building.

Water finds what was skipped

Water does not enter a building through the middle of a slab. It enters at junctions - where a slab meets a parapet, where a pipe passes through, where two materials meet and move at different rates. Every one of those junctions is a detail somebody either resolved or did not, and the list of them is short enough to walk through before work starts.

The cost of doing these properly is a fraction of the cost of chasing them later, because chasing them later means opening finished work to reach them.

The terrace

The largest exposed surface on the building and the most common source of complaints on the top floor.

1. Slope, first Water has to be able to leave. The terrace needs a fall towards the outlets, set in the screed and checked by pouring water on it rather than by eye. A terrace that ponds will find a way through whatever membrane is under it, given enough seasons.
2. The parapet junction Where the terrace meets the parapet wall is the single most common leak point. The waterproofing has to turn up the parapet and terminate in a groove, not stop at the corner.
3. Outlets and pipes Every penetration is a detail. Rainwater outlets need to be set below the finished level so water reaches them, and sealed where they pass through.
4. Protection A membrane left exposed to sun and foot traffic degrades. Whatever protects it - screed, tile, a protective layer - is part of the system, not a finish applied over it.

Sunken slabs and bathrooms

A sunken slab holds the plumbing below floor level and fills with water if anything leaks into it. It is a tank under a bathroom, and it is treated as one.

• The sunken portion is waterproofed and ponded - filled with water and left for a day or more - before filling, to establish that it actually holds.
• Plumbing is tested under pressure before the sunken area is closed. A joint that weeps is trivial to fix at this point and needs a demolished floor later.
• Waterproofing turns up the wall to above the expected splash height, not just across the floor.
• The fill material matters. Waterlogged fill keeps the slab wet indefinitely and works through to the ceiling below.

A leaking bathroom shows up as a damp patch on the ceiling of the room underneath, which is someone else's room. That is why it generates complaints out of proportion to the size of the defect.

The lift pit and tanks

The lift pit sits below the water table on many sites and is waterproofed from the outside where possible, because external waterproofing resists water pressure while internal coatings are pushed off by it.

Underground tanks have the opposite problem in the same place: water must not get in, and stored water must not get out into the soil around the structure. Both faces matter, and construction joints in the tank walls are where both tend to fail. Ponding the tank before it is put into service is the test that settles it.

The plinth and external walls

Moisture rises through masonry from the ground unless something stops it, which is what a damp proof course at plinth level does. Where it is missing or discontinuous, damp appears on internal walls at low level and is usually misdiagnosed as a plumbing leak.

External walls take driving rain during the monsoon. What keeps it out is sound plaster, sealed junctions at window and door frames, and a functioning external paint system. Window junctions in particular leak more often than the wall does, because that is where three materials meet.

When each one happens

Each of these has a moment in the programme when it is straightforward, and after that moment it is not.

• Lift pit and underground tanks - before backfill. Once the earth is back, the external face is unreachable.
• Damp proof course at plinth - during the plinth, and never again.
• Sunken slabs - after plumbing is tested, before filling.
• Terrace - after the structure, before the terrace finish.
• External walls - with the plaster and paint system, as one decision rather than three.

This is the argument for treating waterproofing as a sequence rather than an item: it is not one trade that arrives once, it is a series of details each of which has exactly one convenient moment.