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  • Writer's pictureNicholas Tapia

The Unique Behavior of Water

The unique behavior or water accounts for many of the reasons why roofs are designed and installed the way they are. So, let's learn more about water!

The Properties of Water

Typically, water flows downhill. That's the direction of gravity in case you're wondering.

But water doesn't always go straight down. Have you ever tried to pour water from a glass and you ended up making a big mess? Whether or not the water pour easily depends on the glass shape, physical composition, and the tilt of the glass. Fluid physics are complex!

In roofing, we care a lot about how water flows. Like in the glass example, this isn't always straight forward. Keep in mind these two facts about water:

  • Water is attracted to water. (This is called cohesion.)

  • Water is attracted to other substances. (This is called adhesion.)

These two terms can be *boiled* down to the idea that water isn't just wet, it is STICKY!

This behavior of water is due to the properties of the polarity of its molecules and, specifically, to their ability to form hydrogen bonds with each other and with other molecules.

Hydrogen bonds

If you'd like to read more about the physical properties of water, check out these sources:

The Dripping Edge and the Behavior of Water

So, how does this relate to roofing? One area where this is very observable is where the water runs off the edge of the roof. This area is called an eave. If your house has gutters, eaves are typically where there are installed. The unique behavior or water is the reason shingles are installed at a slight overhang. It is also the reason we often install drip edge flashing. Drip edge flashing is a type of metal flashing that is installed at the edge of a roof to direct water away from the underlying structure.

In this wonderfully clear demonstration by This Old House, we can see how proper overhang installation of shingles and the use of drip edge flashing make a tremendous difference. If these components are not installed correctly, your house could rot!


In conclusion, understanding the unique behavior of water is crucial for designing and installing roofs that can withstand the elements. The ability of water to form hydrogen bonds and its attraction to itself and other substances can have a significant impact on how water flows on and off a roof. Proper installation of shingles and drip edge flashing at the eaves can make a tremendous difference in preventing water damage and prolonging the life of a roof. By taking these factors into account, homeowners can ensure that their roofs are built to last and can protect their homes from the damaging effects of water.


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