Where water gets in, and where it goes

The roof is not the only way in.

Water that gets in does not fall straight down. It runs along rafters and beams and emerges indoors somewhere else entirely. The stain is not necessarily beneath the hole, which is the single reason leaks are difficult to trace and why the same leak gets repaired twice.

Balconies deserve their own paragraph

In practice this is where problems begin most often, for three reasons that compound each other.

A balcony is designed to hold water. It has almost no fall and relies on a drain. Block the drain with leaves and the water stays there.

The waterproofing wears out. With the resin systems commonly used, the top coat is generally inspected and renewed at intervals of around five years and the waterproof layer itself at around ten, though this varies with the system, the exposure and the workmanship. Unlike a roof, it is not a component that quietly lasts for decades.

There is a room underneath. Below a second-floor balcony or a roof terrace is usually living space, so water that gets through goes directly into that room's ceiling void.

People viewing an older house worry about the roof. Almost nobody asks when the balcony waterproofing was last renewed. It is one of the most useful questions available before an offer.

Why it stays hidden: the leak is not continuous

A leak wets when it rains and dries when it stops. After a run of clear days the stain fades and the trace becomes faint. Somebody visiting for the first time in months is almost always looking at the dried state.

For the timber, that cycle is the damage. Wood-decay fungi become active once timber holds moisture above roughly twenty per cent, and each wetting advances the deterioration inside a little further while the surface continues to look ordinary.

Timber well into decay can look entirely normal. What identifies it is whether it gives when pressed, whether it sounds dull when tapped, and whether it smells musty. A photograph of the surface captures none of those.

An empty house decays without any leak

In an occupied building, air moves. Windows open, extractors run, heating and cooling cycle. In a closed-up house none of that happens, and moisture simply sits.

• Damp accumulates below the floor. Subfloor boards take on water and lose strength. The floor sinks or gives underfoot, fastest around the bathroom and washroom.
• Mould spreads. Condensation forms in the sealed space and black mould covers wallpaper and boarding, which then lifts and peels.
• Joinery distorts. Timber swells and doors and sliding screens become hard to move.

Mould favours unlit rooms, the backs of furniture and the undersides of rugs, where air does not move and surfaces meet. Walking through a room and glancing around will not find it. Moving the furniture will.

Opening every door is a free diagnostic

This needs no tools, no cost and no expertise, and it is the most useful five minutes available in an empty house. Open and close every door and every sliding screen.

A door that has become hard to close, a lock that no longer turns easily, a gap that has opened at the head or the side. These are rarely just about the door. Any of them can indicate:

• joinery swollen by moisture
• uneven settlement of the building, meaning ground or foundation
• distortion from deterioration in the floor framing or the sill
• structural timber lost to termites

The doors work as a sensor that reports structural movement in a form a person can notice. On any visit, check every one of them alongside running the taps and opening the windows. It catches a great deal, early, for nothing.

Then the chain starts

Damp timber attracts termites. The two long-established species in Japan depend on moisture, so a leak or underfloor damp is not merely damage in itself. It is an advertisement. Decay and termite damage are routinely found in the same place, because the first created the conditions for the second.

There is a paper consequence as well, and it catches people out. Termite treatment warranties commonly exclude damage where water entered through a leak. So an unrepaired leak can disable the warranty before any termite has arrived. Which species a warranty actually names, and what else voids it, is set out in the termite your five-year warranty does not cover.

The cost moves in steps

The same single leak produces all four outcomes. The only variable is when somebody found it. And from abroad, the first three stages are seen by nobody, which means the stage you are first told about tends to be the fourth.

Where to look

The last row is the most informative and the least often done. Those two spaces hold the most honest record of how the house has been treated, and they never appear in a listing photograph. What to look for in each is covered in the two rooms nobody opens during a viewing.