What is Capillary Action?

Capillary action is when water travels in a narrow space due to the forces of cohesion and adhesion against the force of gravity. Its more likely in warmer damp and humid conditions. To understand how it occurs we need to look at the science. In a nutshell, water will flow on its own against gravity in a small space.

For example, if you place a clear straw into a glass of water, you will see the water level inside the straw is higher. This happens due to the attraction of the water molecules to the molecules of the solid straw. Likewise, if you dip a paper towel into water it will soak up the liquid. Again, this is because water molecules like to stick to each other and flow between materials that are closely packed together.

capilary action diagram
Capillary action causes the water in the thinnest tube to rise to a higher level than it does in the wider tubes.

In this post we’re going to focus primarily on the affects of capillary action in steel roofs and cladding.

What is Capillary Action in a Steel Roof?

Capillary action occurs on steel roof sheets when water travels in the narrow spaces (capillaries) between where they overlap. Capillary action is sometimes referred to as capillary draw. Rubber or mastic seals are applied when the roof is first constructed in between the roof sheets, on each side of the fixings.

Over time these seals will degrade. This is due to constant movement in the roof system caused by external weather conditions like wind, heat, the weight of water or the build-up of snow. Water molecules like to stick together. Water in tight spaces, such as the gap between steel roof sheets, will flow in any direction, even defying gravity. This is caused by the forces of adhesion, cohesion and surface tension.

Water will always travel into a roof space by capillary action when its allowed. The process increases in warmer more humid environments. However it decreases when temperatures are cooler. With global temperatures increasing and climates becoming more humid, capillary action in untreated steel roofs will increase. Accumulating moisture in a steel roof will eventually cause rust and leaks.

capillary action steel roof sheets
In the diagram above you can see how water will flow downwards until it reaches the gap where the untreated roof sheets overlap. Capillary action will now draw the water upwards into the tight space. This is due to the cohesive forces of the water molecules that are attracted to the opposing solid molecules of the steel roof sheet. Surface tension is produced by the proximity of the overlapping roof sheets driving the water upwards.

The Science of capillary action explained in more detail

Leonardo da Vinci first observed and recorded capillary action in the late 15th century. A century later Irish chemist, Robert Boyle performed experiments to investigate it further. There are three main forces at play in capillary action. They are adhesion, cohesion and surface tension.

  • Adhesion happens when particles that are not the same are attracted to each other, read more here.
  • Cohesion is when molecules that are the same cling to each other, for example a water droplet exists because the water molecules inside it stick together. This video perfectly demonstrates cohesion.
  • Surface tension is what allows objects that are more dense to float on a liquid. For example a strand of hair, a metal filing or an insect can float on water even though they are more dense. Its the same process that happens when water is compelled to shrink into small spaces, like a straw.

Capillary Action in Steel Cladding

Although not as common, its still a problem when the right conditions exist. When wind and rain are combined this adds an additional pumping dynamic to the process. Moisture penetrates the space between the cladding overlaps. Just as it does with roof sheets, it settles and triggers the corrosion process or rusting.

Benefits of Capillary Action

Without capillary action trees and plants wouldn’t grow, our tears wouldn’t clean our eyes and towels and paper cloths wouldn’t work. Think about putting flowers in a vase of water. The stems use capillary action to draw the moisture upwards. Without it the flowers would wilt and die much sooner.

How to prevent Capillary Action in Steel Roofs

Applying an external seal of the gaps where the roof sheets overlap will prevent water ingress. This will effectively stop the water from getting into the gaps between the steel roof sheets. Capillary action can no longer occur externally.

capillary action prevention on a steel roof
In the illustration above you can see how treatment of the roof sheet overlaps seals the gaps and prevents capillary action. This will stop leaks and has the added benefit of preventing cut edge corrosion.


Just like every other aspect of building maintenance, vigilance is vital when dealing with a steel roof system. Just because a roof might be relatively new, doesn’t mean it cannot leak. If you see dripping water inside a building, particularly in the roof, investigate it. Water ingress by capillary action is easy to stop. A corroded roof system is not as easy to repair.

Get in touch

If you would like to speak to a member of our sales team to discuss your steel roof maintenance requirements please call 01606 554040 or head over to our contact page and fill out the short form, we’re looking forward to hearing from you.

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