How water behaves

Water has a simple molecular structure made up of two atoms of hydrogen and one atom of oxygen. In any single water molecule, the hydrogen atoms will have 'spare' electrons that can bond to the oxygen atom of other water molecules (this is known as hydrogen bonding). This bonding allows water molecules to form a liquid and influences the properties and behaviour of water, such as surface tension and capillary action.

Surface tension

When water molecules bond, those on the surface are pulled inwards by the hydrogen bond. This creates a kind of skin effect, called surface tension, which can be strong enough to resist gravity and allows droplets to cling to building surfaces.

In general, when a water droplet comes into contact with a hydrophobic material (such as gloss-painted weatherboards or glass), where there is no hydrogen bond between the water and the material, the water will tend to run off. 

But when a droplet comes into contact with a hydrophilic material (such as uncoated concrete or unpainted timber), which absorbs water, surface tension will cause the droplet to flatten against and hold on to the material surface. The more the droplet flattens against the material, the greater its chance of resisting gravity and being held on the surface.

Water that is being held on building surfaces by surface tension will still drain down drainage paths on the vertical face of the cladding but may also drain from the vertical surface and cling onto an adjacent horizontal surfaces. Once it is clinging to a horizontal surfaces, it can be blown into a junction where it may penetrate and cause damage.

Creating a sharp transition to an upward slope or surface will make the water drip off at the junction. Surface tension therefore needs to be broken at all vertical to horizontal junctions. This is done with a drip edge, weathergroove, flashing or drip moulding.

Surface tension allows water droplets to cling to building surfaces, even downwards facing ones..

Surface tension allows water droplets to cling to building surfaces, even downwards facing ones.

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Capillary action

Capillary action is where water bonding to two adjacent surfaces is drawn upwards against the force of gravity between the two surfaces. How far the water can be drawn upwards depends on the size of the gap between the surfaces and how hydrophobic or hydrophilic they are. Wind pressure can also act on the water and drive it upwards even further.

Incorporating a capillary break by detailing a gap of 6 mm between surfaces will stop capillary action occurring, as the surfaces will be too far apart for water to bond between them. The incorporation of weathergrooves, seals or hooks/seams on a flashing can also assist, as these will break the contact between the adjacent surfaces. 

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Absorption and wicking

Absorbent or porous materials and surfaces (such as raw fibre-cement, uncoated concrete, weathered coatings and unpainted timber) will absorb moisture.

They can also wick moisture off an adjacent surface, where it can be absorbed. Once water has been absorbed, it will migrate or wick through the material from a warm area to a cold area. It may also be absorbed by other adjacent materials – for example, water may be absorbed by a poorly coated cladding and migrate through to be absorbed by a dry absorbent wall underlay and ultimately by the dry timber framing.

The use of non-absorbent materials or finishes will limit absorption, and the use of capillary breaks or a separation between surfaces (such as a gap at the bottom of the cladding above a waterproof deck) will restrict wicking.

Rapid heating by the sun of surfaces containing moisture can also drive water vapour from absorbed moisture through materials – a process known as solar-driven moisture transfer.

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Condensation

Air contains water vapour, with the amount of vapour present increasing with temperature. As air cools, its ability to hold water vapour is reduced, and the vapour is released and condenses as water. When air is cooled by contact with a cold surface, the released vapour forms as condensation on that surface (for example, the steam created from a hot shower will condense when it comes into contact with the cooler glass of an exterior window).

If condensation occurs, it can be absorbed by materials and can cause material deterioration, so it needs to be managed within wall assemblies. This can be done by ventilation and by incorporating absorbent wall underlays or using cladding materials that have some degree of absorbency on the back of the cladding that will hold the condensation water until it dries again as a result of ventilation.

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Water from cleaning

Building exteriors should only be cleaned with very low-pressure water, as high-pressure water (such as that from water blasting or a high-pressure hose) has the potential to be directed at and driven through gaps in the exterior cladding and up under flashings, where it can enter the roof or wall assembly and be absorbed by components. 

High-pressure water can also damage softer cladding materials and damage or remove protective exterior coatings.

Updated: 9 September 2014