The life of your home depends on it being watertight. This is a challenge because houses have vulnerable areas where water can find its way in. The component used to prevent water leaks at those critical areas is called flashing.
A house is not a submarine
It is important for homeowners to know how flashing works. It may come as a surprise to learn that most flashings do not completely seal-off water like a rubber gasket does. Instead, flashings work more like duck feathers. They simply channel and divert water away from vulnerable areas via gravity.
This shedding approach works fine most of the time. As a bonus this helps with ventilation as air can flow more freely. Unfortunately, instances of very strong winds (or a pressure washer) can force water back under a flashing and thus cause a temporary leak. This can be aggravating but the problem is not chronic.
What this means is that homes can never hope to be 100% water proof like a submarine. But they can be very water resistant.
Good design and flawless installation
Many critical areas require flashing in the average home. See the illustration below. To work properly all these areas need to be individually thought out and well executed. For instance, you may need to modify the design slightly to counteract high wind or driving rain conditions. Flashing is useless and unforgiving if it is either designed or installed improperly. It has to be done right or it will leak.
Flashing material needs to be tough because it is subject to harsh, exposed conditions. In many cases flashing is permanently installed. In those applications it is expected to last the lifetime of the structure and that may be a long time. Roof valleys, roof-to-wall junctions, roof-to-chimney joints, skylights, roof edges, exposed wood, windows or doors, decks and exterior wall junctions are just some of the areas that rely on good flashing.
The right material for the job
Flashing material must often be custom shaped in order to perform well in a given application. Some excellent materials for flashing applications include copper, zinc, aluminum, lead, tin and several types of membrane tapes. Galvanized sheet metal is used frequently but only provides a 20-30 year useful life.
Tile and slate roofs are often flashed with lead because lead is soft and will conform to the odd curvature of the tiles. Any material selected for flashing should match or exceed the durability of the surrounding material.
Good flashing is mandatory for any structure to be considered durable or high quality. It is in fact so important, that when building new, I recommend special supervision be assigned during the installation to ensure that it is done properly.