Module 1 · Introduction to Damp

1.2 Why Damp Occurs in Buildings

7 min read

Damp is never spontaneous. It requires water to get in or be generated, and it requires something about the building to prevent that water getting back out. Beginners tend to hunt only for the source. Experienced surveyors look at both halves, because the second half is very often the one that can actually be fixed.

Learning objectives

By the end of this lesson you will be able to:

Separate moisture sources from the conditions that let moisture accumulate.
Describe the main routes moisture takes through a building.
Explain why two identical houses can behave completely differently.
Identify the design and condition factors that raise a property's damp risk.

Key terms

Moisture source: Where the water originates — an activity, the weather, the ground, or a failed service.
Moisture pathway: The route water takes through the fabric, as liquid, as vapour, or by capillary action.
Retention: Moisture staying in the fabric because it cannot evaporate or drain away fast enough.
Ventilation: The exchange of moist internal air for drier external air. The building's main drying mechanism.

Think of a building as having a moisture budget. Water comes in from occupancy, weather, ground contact and services. Water leaves through ventilation, evaporation, drainage and absorption into materials that later dry out. When the outgoings match the incomings, the building stays dry regardless of how much water is passing through it.

Damp is a deficit in that budget. It follows that you can produce damp two ways: by increasing what comes in, or by reducing what goes out. Almost every case you will investigate is one, the other, or both at once — and cases where the source is entirely normal and only the drying has failed are extremely common in modern, well-sealed housing.

Where the moisture comes from

There are five sources, and a competent inspection considers all of them before ruling any out:

Occupancy — cooking, showering, washing, drying laundry indoors, and simply breathing. The largest single source in most homes.
Weather — driving rain on exposed elevations, standing water at the base of walls, snow lying against a threshold.
The ground — permanent moisture in the subsoil, in contact with foundations, solid floors and the base of walls.
Services — supply pipes, waste pipes, heating circuits, appliance connections, and roof drainage that has failed.
Construction — water built into the fabric of a new or recently altered building that has not yet dried out.

How moisture moves

Water does not stay where it enters, which is why the damp you can see is often nowhere near the defect that caused it. It moves by three mechanisms:

As vapour — carried by air, moving from warm to cold and from wet to dry, until it meets a surface cold enough to condense on.
As liquid under gravity — running down a cavity, along a joist, behind a render, and emerging at the first horizontal obstruction.
By capillary action — drawn through the fine pores of masonry, mortar and plaster, in any direction including upward.

The practical consequence: water follows the path of least resistance, and it will happily travel several metres before it shows itself. A ceiling stain in a back bedroom can be produced by a defect at the front of the roof.

Why moisture stays — the retention half

Moisture becomes visible damp when it cannot leave. Retention is usually the more tractable half of the problem, and it is where the genuinely useful recommendations live. Moisture is retained when:

Ventilation is poor — extractor fans missing, broken, or never switched on; trickle vents shut; air bricks blocked.
The building is cold, or heated in short bursts, so surfaces never get above dew point for long.
Materials are saturated and have no dry side to evaporate towards.
Impermeable finishes — vinyl wallpaper, cement render, non-breathable paint — have sealed moisture into a wall that needs to breathe.
Furniture, insulation or storage is pressed against a cold external wall, killing the air movement across it.

This is why two identical terraced houses, side by side, can behave completely differently. One is heated steadily and ventilated; the other is heated for two hours a day with the windows shut and washing on the radiators. Same fabric, entirely different outcome.

Condition and design factors that raise the risk

Some features do not cause damp on their own but reliably make it more likely. Note them during an inspection even when they are not implicated in the current problem:

Cold spots — uninsulated lintels, structural columns, window reveals, external corners.
Blocked, leaking or undersized gutters and downpipes.
External ground levels raised close to or above the damp-proof course.
Cracked render, failed pointing, perished sealant around openings.
Single-skin construction, or solid walls with impermeable internal finishes.
Bathrooms and kitchens with no mechanical extraction, or extraction ducted into a loft rather than outside.

Common mistakes to avoid

Finding one plausible source and stopping, when two or three are contributing at once.
Treating ventilation and heating as the occupier's problem rather than as part of the diagnosis.
Assuming the defect is next to the damp, when moisture routinely travels before it emerges.
Overlooking recent works — a new render, a new floor, a filled cavity — that changed how the wall dries.
Recording the source but never asking why the building failed to shed the water.

Lesson summary

Damp requires both a source and a retention condition. Sources are occupancy, weather, ground, services and construction water. Retention comes from poor ventilation, low temperatures, saturated or sealed materials, and blocked airflow. Because moisture travels as vapour, as liquid and by capillary action, the visible symptom is frequently some distance from the defect that caused it.

In Lesson 1.3 we introduce the four mechanisms that account for almost every damp case you will meet.