Capillary action is where the surface tension of the water can pull the water into small gaps. Any gaps in your building that are less than 5mm (1/4″) could allow water into your building. Capillary action is extremely powerful, and can even overcome gravity, allowing water to creep up a detail and into your building. The good news is that it’s extremely easy to solve. All you need is a gap that is wider than 5 mm (1/4″). If you look at good detailing you will notice that there are often gaps at the bottom of flashings that will prevent water from creeping up the detail.