Grand Designs Australia

Understanding the types and signs of dampness

Knowing more about damp problems can reduce worry and expense. Thankfully, most problems can be cured or minimised by simple remedial work but a few will need substantial outlays. In this factsheet, we outline the main types of dampness and the signs to look out for.

Types of dampness

Rising damp – moves upwards vertically


Rising damp occurs at the bases of walls. Water accumulating there has a tendency to "wick up" through the capillaries that are present in the walls, be they brick, block or most stone; and through the mortar in which they are laid. Damp-proof courses are there to block this upward movement of moisture but sometimes they are ineffective.

The Building Code of Australia, to which all new construction must comply, requires damp-proof courses (dpcs) to be placed through the full thickness of the base of walls below floor level to form an impervious layer that keeps rising dampness out of the interior of the house. (SAA Masonry Code, AS3700.)

Falling Damp – moves downwards vertically


This refers mainly to leaking water carriers around the home such as roofs, pipers and gutters. If these are located near a wall and are leaking, this can also increase rising damp.

Horizontal Damp – spreads horizontally


Various defects can cause water to move horizontally through a wall at any height and create a damp patch.

Condensation Dampness – caused by condensation


Moist air which is inside a house will condense into its liquid state if it touches cold windows, walls, or the underside of metal roof sheeting.

Signs of damp and what to look for


If the wall or ceiling is damp, problems can be hard to detect, but these start to show up as things become drier. The three most common signs are:

1. Surface Stains


Water moving through bricks and blocks may dissolve some of the alkaline salts from the mortar. The salts can then react with the tannins in timber, wallpaper or the like to produce stains that are usually brown. They can be unsightly, but do not cause damage.

Some clay bricks with vanadium salts stain brown or purple after being cleaned with spirits of salts (i.e. hydrochloric acid, which is often applied to remove mortar splashes.). Such stains usually disappear with a scrubbed-on application of diluted caustic soda (eg: dishwashing detergent) but the stains can sometimes be stubborn and require specialised treatment.

2. Lifted Surface Finishes


As a rising damp wall dries, the water will be drawn to the surface and find itself trapped under the paint film or other surface finish. The evaporating water lifts the film in bubbles that will eventually break to leave blisters of the sort shown in the image below. Wallpaper or other applied finishes, including timber panels, can be similarly damaged.

3. Efflorescence and Fretting


Where there is a continuous supply of water rising up a wall, it will contain dissolved salts and when that water dries out at the surface the salts will crystallise. If the crystals form on the surface of the wall as a white furry coating, it will be suffering from non-damaging efflorescence, but if the crystallisation occurs within the bricks or mortar, the forming crystals can exert pressure that causes the surface to break down and fret away.

In most of Australia this phenomenon is called salt attack, but the South Australian name for it: salt damp, is often heard. With rising damp, there is a continuous supply of water so salt attack damage worsens over time, but it usually reaches a stage where crystallisation occurs only on the surface as efflorescence and fretting stops.

Now that you know about the main types of dampness and the signs to look out for, read our factsheets on the causes and the cures of rising damp, falling damp, horizontal damp and condensation dampness.

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